---
title: "Compost Blanket Erosion Calculator: Slope Math, Washout Thresholds, and Material Volumes"
canonical: "https://theyieldgrid.com/compost-blanket-erosion-calculator/"
model_id: "tyg-2769"
model_version: "1.0.0"
last_reviewed: "2026-08-24T10:37:31"
reviewed_by: "Umer Hayiat"
---

# Compost Blanket Erosion Calculator: Slope Math, Washout Thresholds, and Material Volumes

> Canonical calculator: [https://theyieldgrid.com/compost-blanket-erosion-calculator/](https://theyieldgrid.com/compost-blanket-erosion-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Compost Blanket Erosion Calculator: Slope Math, Washout Thresholds, and Material Volumes The physics of mulch on a slope are unforgiving. When slope steepness and water energy exceed the friction capacity of your material, the result is not partial erosion: it is total loss. Standard shredded bark mulch is buoyant and has almost no shear resistance on grades above 4:1 (25%). Even heavier compost blankets reach a hard friction limit at 3:1 (33%), the point where gravity reliably overcomes surface adhesion. Understanding that threshold is not optional; it is the difference between a functioning erosion control system and a street-clogging cleanup job after the first rain.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Slope Run (Horizontal Length) | `mulchblanket_run` | number | feet | 0.1 to | No |
| Slope Rise (Vertical Height) | `mulchblanket_rise` | number | feet | 0 to | No |
| Slope Width | `mulchblanket_width` | number | feet | 0.1 to | No |
| Target Blanket Depth | `mulchblanket_depth` | number | inches | 0.5 to 6 | No |
| Type of Flow | `mulchblanket_flow` | select |  | — Select flow type — = ``; Sheet Flow = `sheet`; Concentrated Flow = `concentrated` | No |

## Outputs

| Output ID | Default state |
|---|---|
| `mulchblanket_results` | — cubic yards of compost needed — 0% Slope Steepness 100%+ Safe (33%) Metric Value Material Volume 0% Quick Slope Reference Table Run (ft) Rise (ft) Slope % Ratio Risk Level 30 3 10% 10:1 Low 30 6 20% 5:1 Low 30 7.5 25% 4:1 Moderate 30 10 33% 3:1 High Caution 30 15 50% 2:1 Washout Risk 30 20 67% 1.5:1 Severe 30 30 100% 1:1 Extreme How This Calculator Works Step 1 — Calculate Slope Percentage: Slope % = (Rise ÷ Run) × 100 This converts the vertical rise and hori |
| `mulchblanket_out_primary` | — cubic yards of compost needed |

## Formula and method

This diagram highlights the critical 3:1 slope ratio where gravity overcomes the friction of loose compost. Show the calculation steps Step 1: Slope Percentage Slope (%) = (Rise / Run) x 100 This is the foundational measurement. All safety thresholds, product recommendations, and material limits in this tool are indexed to slope percentage. Round to one decimal place for display; the safety threshold logic uses the unrounded value internally. Step 2: Slope Ratio Ratio = (Run / Rise) : 1 The ratio format is the construction and grading industry standard. A 3:1 slope has 3 horizontal feet for every 1 vertical foot. Ratios are rounded to one decimal place. On flat or near-flat ground (rise near zero), the ratio approaches infinity and is displayed as “Flat.” Step 3: Slope Angle in Degrees Angle = arctan(Rise / Run), converted from radians to degrees This is a supplemental output for reference. Slope angle in degrees is less commonly used in field erosion control than slope percentage, but it is useful when cross-referencing geotech reports or seed germination guides that specify angle. Step 4: Cubic Yards of Material CY = (Run x Width x (Depth / 12)) / 27 Depth is divided by 12 to convert inches to feet, then the three dimensions are multiplied to get cubic feet. Dividing by 27 converts cubic feet to cubic yards. This uses the plan-view area (Run x Width), which is the horizontal footprint, not the actual slope surface area. For slopes below 30%, the difference is minor (less than 5%). For steeper slopes, surface area can be meaningfully larger; advanced users can substitute slope length for run in this formula if surface-area accuracy is required. Step 5: Washout Safety Check If Slope is greater than 33% AND flow type is Concentrated Flow, the washout alert fires. If slope is greater than 33% with sheet flow only, the high-risk warning fires without the full washout flag. The 33% threshold (3:1 ratio) is derived from the friction limit of bulk compost on compacted soil: above this grade, gravitational force along the slope face reliably exceeds the material’s resistance to sliding or being displaced by moving water. Assumptions and Limits All calculations use horizontal run, not slope surface length. Entering slope surface length instead of horizontal run will understate slope steepness. Compost density is assumed at approximately 800 to 1,000 lbs per cubic yard (standard bulk screened compost). Finer or wetter composts can be heavier; very dry or coarse material can be lighter. The volume calculation assumes uniform depth across the entire area. Irregular terrain, berms, or concave slopes will produce variable actual depth and may require more material than calculated. The 3:1 (33%) washout threshold applies to loose-applied compost blankets on compacted or graded slopes. Freshly tilled or disturbed soil has lower cohesion and the effective safe threshold may be lower. This calculator does not account for soil erodibility class (K-factor), design storm intensity, or return period. Sites subject to regulatory stormwater permits require engineered erosion and sediment control plans. Vegetation establishment is not modeled. A compost blanket is a temporary measure. Permanent erosion control requires deep-rooted plant cover; the tool’s material estimate covers the application phase only. For slopes where the difference between horizontal run and slope surface length is significant (grades above 50%), substitute slope length for run in the formula for a surface-accurate volume estimate.

## Verified worked examples

### Scenario 1: Gentle Backyard Slope After Landscaping

Slope Run: 40 ft Slope Rise: 8 ft Slope Width: 15 ft Target Blanket Depth: 1.5 in Type of Flow: Sheet Flow Result: Slope = 20% (5:1 ratio). Volume = (40 x 15 x 0.125) / 27 = 2.78 cubic yards. Risk zone: Safe. At 20%, this slope is comfortably below the caution threshold. A standard 1.5-inch compost blanket applied uniformly will provide adequate erosion control. No netting is required at this grade with sheet flow, though staking the perimeter edges is still a best practice on freshly graded soil.

### Scenario 2: Moderate Landscape Berm with Uphill Impervious Surface

Slope Run: 25 ft Slope Rise: 7 ft Slope Width: 20 ft Target Blanket Depth: 2 in Type of Flow: Concentrated Flow Result: Slope = 28% (3.6:1 ratio). Volume = (25 x 20 x 0.167) / 27 = 3.09 cubic yards. Risk zone: Caution. The slope percentage alone would pass a basic sheet-flow check, but the presence of concentrated flow pushes this project into caution territory. At 28% with channeled water, shear stress in the flow path is high enough to undercut loose compost within a single storm event. Jute netting with 12-inch stakes across the full width, combined with redirecting the concentrated flow path using a berm or diversion swale, is the recommended approach before applying material.

### Scenario 3: Steep Construction Embankment with Concentrated Runoff

Slope Run: 20 ft Slope Rise: 8 ft Slope Width: 30 ft Target Blanket Depth: 2 in Type of Flow: Concentrated Flow Result: Slope = 40% (2.5:1 ratio). Volume = (20 x 30 x 0.167) / 27 = 3.70 cubic yards. Risk zone: Washout Warning. This scenario triggers the tool’s washout alert. At 40% with concentrated flow, gravity and shear stress exceed the friction capacity of any loose organic material. The calculated 3.70 cubic yards represents what you would need to order, but it cannot be applied as a loose blanket. The correct sequence is: install a pinned erosion control blanket (ECB) or turf reinforcement mat (TRM) first, then apply compost underneath the fabric. Concentrated flow paths must also be redirected or armored with check dams before any surface treatment is effective.

## Assumptions

This diagram highlights the critical 3:1 slope ratio where gravity overcomes the friction of loose compost. Show the calculation steps Step 1: Slope Percentage Slope (%) = (Rise / Run) x 100 This is the foundational measurement. All safety thresholds, product recommendations, and material limits in this tool are indexed to slope percentage. Round to one decimal place for display; the safety threshold logic uses the unrounded value internally. Step 2: Slope Ratio Ratio = (Run / Rise) : 1 The ratio format is the construction and grading industry standard. A 3:1 slope has 3 horizontal feet for every 1 vertical foot. Ratios are rounded to one decimal place. On flat or near-flat ground (rise near zero), the ratio approaches infinity and is displayed as “Flat.” Step 3: Slope Angle in Degrees Angle = arctan(Rise / Run), converted from radians to degrees This is a supplemental output for reference. Slope angle in degrees is less commonly used in field erosion control than slope percentage, but it is useful when cross-referencing geotech reports or seed germination guides that specify angle. Step 4: Cubic Yards of Material CY = (Run x Width x (Depth / 12)) / 27 Depth is divided by 12 to convert inches to feet, then the three dimensions are multiplied to get cubic feet. Dividing by 27 converts cubic feet to cubic yards. This uses the plan-view area (Run x Width), which is the horizontal footprint, not the actual slope surface area. For slopes below 30%, the difference is minor (less than 5%). For steeper slopes, surface area can be meaningfully larger; advanced users can substitute slope length for run in this formula if surface-area accuracy is required. Step 5: Washout Safety Check If Slope is greater than 33% AND flow type is Concentrated Flow, the washout alert fires. If slope is greater than 33% with sheet flow only, the high-risk warning fires without the full washout flag. The 33% threshold (3:1 ratio) is derived from the friction limit of bulk compost on compacted soil: above this grade, gravitational force along the slope face reliably exceeds the material’s resistance to sliding or being displaced by moving water. Assumptions and Limits All calculations use horizontal run, not slope surface length. Entering slope surface length instead of horizontal run will understate slope steepness. Compost density is assumed at approximately 800 to 1,000 lbs per cubic yard (standard bulk screened compost). Finer or wetter composts can be heavier; very dry or coarse material can be lighter. The volume calculation assumes uniform depth across the entire area. Irregular terrain, berms, or concave slopes will produce variable actual depth and may require more material than calculated. The 3:1 (33%) washout threshold applies to loose-applied compost blankets on compacted or graded slopes. Freshly tilled or disturbed soil has lower cohesion and the effective safe threshold may be lower. This calculator does not account for soil erodibility class (K-factor), design storm intensity, or return period. Sites subject to regulatory stormwater permits require engineered erosion and sediment control plans. Vegetation establishment is not modeled. A compost blanket is a temporary measure. Permanent erosion control requires deep-rooted plant cover; the tool’s material estimate covers the application phase only. For slopes where the difference between horizontal run and slope surface length is significant (grades above 50%), substitute slope length for run in the formula for a surface-accurate volume estimate. All calculations use horizontal run, not slope surface length. Entering slope surface length instead of horizontal run will understate slope steepness. Compost density is assumed at approximately 800 to 1,000 lbs per cubic yard (standard bulk screened compost). Finer or wetter composts can be heavier; very dry or coarse material can be lighter. The volume calculation assumes uniform depth across the entire area. Irregular terrain, berms, or concave slopes will produce variable actual depth and may require more material than calculated. The 3:1 (33%) washout threshold applies to loose-applied compost blankets on compacted or graded slopes. Freshly tilled or disturbed soil has lower cohesion and the effective safe threshold may be lower. This calculator does not account for soil erodibility class (K-factor), design storm intensity, or return period. Sites subject to regulatory stormwater permits require engineered erosion and sediment control plans. Vegetation establishment is not modeled. A compost blanket is a temporary measure. Permanent erosion control requires deep-rooted plant cover; the tool’s material estimate covers the application phase only. For slopes where the difference between horizontal run and slope surface length is significant (grades above 50%), substitute slope length for run in the formula for a surface-accurate volume estimate. Visualizing the difference between material failure and successful erosion control on steep residential grades. Critical Warnings Bark mulch floats. Standard shredded bark mulch has a density lower than water when saturated. On any slope above 4:1 (25%), even moderate rainfall produces enough shear stress to carry bark mulch downhill as a floating mass. This is not a theoretical failure mode; it is a predictable physical outcome. Bark mulch is not an appropriate erosion control material on any graded slope. Compost blankets have a hard upper slope limit. Even screened compost, which is heavier and more cohesive than bark, loses its frictional stability above 3:1 (33%). At a 2:1 (50%) slope, gravity along the slope face generates a force component that exceeds the material’s weight-to-friction ratio. Applying more material does not solve the problem; it creates a larger mass primed to fail. Concentrated flow is a multiplier, not just a modifier. A slope that would pass a sheet-flow check at 28% can exhibit complete compost displacement under a concentrated flow path. Any upslope impervious surface (roof, driveway, compacted path) that drains onto the treated area converts what appears to be sheet flow into concentrated flow at ground level. Depth creep does not improve erosion control performance. Applying 3 or 4 inches of compost instead of the standard 1 to 2 inches does not increase slope stability. The additional depth adds dead weight, which can cause the entire blanket layer to detach and move as a single unit, a failure mode worse than gradual surface erosion. Minimum Standards for Safe Application Loose compost application is limited to slopes at or below 3:1 (33%). On any steeper grade, a pinned erosion control blanket or turf reinforcement mat is required before compost is applied. On slopes between 4:1 (25%) and 3:1 (33%), biodegradable jute or coir netting should be staked over the compost blanket at a minimum of 2 stakes per linear foot. 12-inch wooden landscape stakes driven flush to the netting surface are the standard for most residential and light commercial applications. Concentrated flow paths must be redirected or armored with check dams, rock rip-rap, or drainage swales before any organic blanket is applied to the affected slope face. Treating the symptom (erosion on the slope) without addressing the cause (concentrated water energy) guarantees repeat failure. Competitor Trap: Most erosion control guides online present a single cubic-yard calculation and a generic note to “check slope conditions.” They skip the flow-type variable entirely, treating sheet flow and concentrated flow as interchangeable. The practical consequence is that a landscaper following a generic guide applies a standard compost blanket on a 28% slope with upslope pavement, selects “sheet flow” because the water appears to spread visually, and loses all material in the first storm. The flow type selection is not a checkbox; it is the variable that determines whether an otherwise acceptable slope grade becomes a washout scenario. Use the retaining wall and slope stability context from The Yield Grid’s retaining wall calculator to assess whether your site’s slope requires structural reinforcement beyond erosion control alone. For sites where concentrated runoff originates from upslope drainage, the rain garden sizing calculator can help route and infiltrate that flow before it reaches the treated slope. Loose compost application is limited to slopes at or below 3:1 (33%). On any steeper grade, a pinned erosion control blanket or turf reinforcement mat is required before compost is applied. On slopes between 4:1 (25%) and 3:1 (33%), biodegradable jute or coir netting should be staked over the compost blanket at a minimum of 2 stakes per linear foot. 12-inch wooden landscape stakes driven flush to the netting surface are the standard for most residential and light commercial applications. Concentrated flow paths must be redirected or armored with check dams, rock rip-rap, or drainage swales before any organic blanket is applied to the affected slope face. Treating the symptom (erosion on the slope) without addressing the cause (concentrated water energy) guarantees repeat failure. Competitor Trap: Most erosion control guides online present a single cubic-yard calculation and a generic note to “check slope conditions.” They skip the flow-type variable entirely, treating sheet flow and concentrated flow as interchangeable. The practical consequence is that a landscaper following a generic guide applies a standard compost blanket on a 28% slope with upslope pavement, selects “sheet flow” because the water appears to spread visually, and loses all material in the first storm. The flow type selection is not a checkbox; it is the variable that determines whether an otherwise acceptable slope grade becomes a washout scenario. Use the retaining wall and slope stability context from The Yield Grid’s retaining wall calculator to assess whether your site’s slope requires structural reinforcement beyond erosion control alone. For sites where concentrated runoff originates from upslope drainage, the rain garden sizing calculator can help route and infiltrate that flow before it reaches the treated slope.

## Limitations and safety

All calculations use horizontal run, not slope surface length. Entering slope surface length instead of horizontal run will understate slope steepness. Compost density is assumed at approximately 800 to 1,000 lbs per cubic yard (standard bulk screened compost). Finer or wetter composts can be heavier; very dry or coarse material can be lighter. The volume calculation assumes uniform depth across the entire area. Irregular terrain, berms, or concave slopes will produce variable actual depth and may require more material than calculated. The 3:1 (33%) washout threshold applies to loose-applied compost blankets on compacted or graded slopes. Freshly tilled or disturbed soil has lower cohesion and the effective safe threshold may be lower. This calculator does not account for soil erodibility class (K-factor), design storm intensity, or return period. Sites subject to regulatory stormwater permits require engineered erosion and sediment control plans. Vegetation establishment is not modeled. A compost blanket is a temporary measure. Permanent erosion control requires deep-rooted plant cover; the tool’s material estimate covers the application phase only. For slopes where the difference between horizontal run and slope surface length is significant (grades above 50%), substitute slope length for run in the formula for a surface-accurate volume estimate. Visualizing the difference between material failure and successful erosion control on steep residential grades. Critical Warnings Bark mulch floats. Standard shredded bark mulch has a density lower than water when saturated. On any slope above 4:1 (25%), even moderate rainfall produces enough shear stress to carry bark mulch downhill as a floating mass. This is not a theoretical failure mode; it is a predictable physical outcome. Bark mulch is not an appropriate erosion control material on any graded slope. Compost blankets have a hard upper slope limit. Even screened compost, which is heavier and more cohesive than bark, loses its frictional stability above 3:1 (33%). At a 2:1 (50%) slope, gravity along the slope face generates a force component that exceeds the material’s weight-to-friction ratio. Applying more material does not solve the problem; it creates a larger mass primed to fail. Concentrated flow is a multiplier, not just a modifier. A slope that would pass a sheet-flow check at 28% can exhibit complete compost displacement under a concentrated flow path. Any upslope impervious surface (roof, driveway, compacted path) that drains onto the treated area converts what appears to be sheet flow into concentrated flow at ground level. Depth creep does not improve erosion control performance. Applying 3 or 4 inches of compost instead of the standard 1 to 2 inches does not increase slope stability. The additional depth adds dead weight, which can cause the entire blanket layer to detach and move as a single unit, a failure mode worse than gradual surface erosion. Minimum Standards for Safe Application Loose compost application is limited to slopes at or below 3:1 (33%). On any steeper grade, a pinned erosion control blanket or turf reinforcement mat is required before compost is applied. On slopes between 4:1 (25%) and 3:1 (33%), biodegradable jute or coir netting should be staked over the compost blanket at a minimum of 2 stakes per linear foot. 12-inch wooden landscape stakes driven flush to the netting surface are the standard for most residential and light commercial applications. Concentrated flow paths must be redirected or armored with check dams, rock rip-rap, or drainage swales before any organic blanket is applied to the affected slope face. Treating the symptom (erosion on the slope) without addressing the cause (concentrated water energy) guarantees repeat failure. Competitor Trap: Most erosion control guides online present a single cubic-yard calculation and a generic note to “check slope conditions.” They skip the flow-type variable entirely, treating sheet flow and concentrated flow as interchangeable. The practical consequence is that a landscaper following a generic guide applies a standard compost blanket on a 28% slope with upslope pavement, selects “sheet flow” because the water appears to spread visually, and loses all material in the first storm. The flow type selection is not a checkbox; it is the variable that determines whether an otherwise acceptable slope grade becomes a washout scenario. Use the retaining wall and slope stability context from The Yield Grid’s retaining wall calculator to assess whether your site’s slope requires structural reinforcement beyond erosion control alone. For sites where concentrated runoff originates from upslope drainage, the rain garden sizing calculator can help route and infiltrate that flow before it reaches the treated slope.

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## Provenance

- Model ID: `tyg-2769`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-24T10:37:31
- Runtime SHA-256: `9a2964d3c3541092e3d606a7292d3855884e33cc1f58612470bc0751c124a65f`

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