---
title: "Well Pressure Tank Calculator: Size Your Tank to Stop Pump Short-Cycling"
canonical: "https://theyieldgrid.com/well-pressure-tank-calculator/"
model_id: "tyg-802"
model_version: "1.0.0"
last_reviewed: "2026-04-27T19:42:43"
reviewed_by: "Umer Hayiat"
---

# Well Pressure Tank Calculator: Size Your Tank to Stop Pump Short-Cycling

> Canonical calculator: [https://theyieldgrid.com/well-pressure-tank-calculator/](https://theyieldgrid.com/well-pressure-tank-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Well Pressure Tank Calculator: Size Your Tank to Stop Pump Short-Cycling A pressure tank failure rarely announces itself in advance. What happens instead is a pump that runs for eight seconds, stops, then runs again three minutes later, repeatedly cycling through the night while the motor windings heat beyond their thermal rating. The tank itself is not the failure, it is the signal: a tank sized for the wrong pump flow rate, or a bladder that has lost its pre-charge, leaves no buffer between the water demand and the pressure switch. The pump fills the gap and pays the price.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Well Pump Flow Rate (GPM) | `wpptsz-gpm` | number | GPM | 0.5 to 100 | No |
| Pressure Switch Setting (PSI) | `wpptsz-switch` | select |  | — Select setting — = ``; 30/50 PSI (cut-in 30, cut-out 50) = `30/50`; 40/60 PSI (cut-in 40, cut-out 60) — most common = `40/60`; 50/70 PSI (cut-in 50, cut-out 70) = `50/70` | No |
| Target Minimum Pump Run Time (minutes) | `wpptsz-runtime` | number |  | 0.5 to 10 | No |

## Outputs

| Output ID | Default state |
|---|---|
| `wpptsz-results` | — gallons — recommended minimum tank size Enter your pump flow rate and pressure switch setting, then click Calculate. Tank Volume Breakdown — Drawdown vs. Air Charge Drawdown (usable water) Air pre-charge (non-usable) Sizing Breakdown Value Result Notes Reference Table — Common Pump & Tank Combinations (40/60 PSI) Pump GPM Target Run (min) Drawdown (gal) Min Tank Size (gal) Cycle Time Risk Flag 3 1.5 4.5 16.1 ~27 s fill Check run time 5 1.5 7.5 26.8 ~45 s fill Acceptable 10 1.0 10 35.7 ~1 min f |
| `wpptsz-results-inner` | — gallons — recommended minimum tank size Enter your pump flow rate and pressure switch setting, then click Calculate. Tank Volume Breakdown — Drawdown vs. Air Charge Drawdown (usable water) Air pre-charge (non-usable) Sizing Breakdown Value Result Notes Reference Table — Common Pump & Tank Combinations (40/60 PSI) Pump GPM Target Run (min) Drawdown (gal) Min Tank Size (gal) Cycle Time Risk Flag 3 1.5 4.5 16.1 ~27 s fill Check run time 5 1.5 7.5 26.8 ~45 s fill Acceptable 10 1.0 10 35.7 ~1 min f |
| `wpptsz-out-primary` | — |
| `wpptsz-num-total` | — |
| `wpptsz-warnings-box` |  |

## Formula and method

Visualizing Boyle’s Law: only a fraction of your total tank volume is usable water drawdown. Show the calculation steps Step 1: Calculate drawdown (usable gallons) Drawdown (gallons) = Pump Flow Rate (GPM) x Target Run Time (minutes) This is straightforward unit multiplication. A 10 GPM pump running for 2 minutes delivers 20 gallons per cycle. That 20 gallons must fit within the tank’s usable volume, which is the drawdown window. Step 2: Apply the pressure-switch multiplier A pre-charged bladder tank does not store all of its volume as water. The remaining space holds compressed air at a pre-charge pressure set just below the cut-in PSI. As water enters, the air compresses. As water exits, the air expands. The fraction of total tank volume available as water storage depends on the pressure ratio between cut-in and cut-out pressures, governed by Boyle’s Law. 30/50 PSI switch: multiplier 0.31 (31 of every 100 gallons of tank volume is usable water) 40/60 PSI switch: multiplier 0.28 50/70 PSI switch: multiplier 0.25 Higher operating pressures reduce the usable fraction because the pressure ratio between cut-in and cut-out is a smaller percentage of absolute pressure, leaving less room for water expansion. Step 3: Calculate required total tank volume Total Tank Volume (gallons) = Drawdown (gallons) / Multiplier Rounding rule: always round up to the next standard catalog size. Never round down. A tank that is 5 gallons short will produce a run time 10 to 25 seconds shorter than designed depending on pump GPM. Assumptions and Limits The multipliers (0.31, 0.28, 0.25) are industry-standard estimates for pre-charged bladder tanks with air pre-charge set 2 PSI below cut-in. Actual drawdown may differ up to 5% by manufacturer and bladder condition. This calculator assumes a functioning bladder or diaphragm tank. A waterlogged galvanized tank, one where the bladder has ruptured, has near-zero drawdown regardless of tank volume and cannot be corrected by resizing. Pre-charge pressure is assumed to be correctly set. If the pre-charge is higher than cut-in PSI minus 2, the usable drawdown shrinks and the pump will short-cycle even with an appropriately sized tank. The tool does not account for multiple simultaneous fixtures drawing water at the same time. If two showers run simultaneously, the effective demand may exceed drawdown and the pump will run longer than one cycle continuously, which is generally acceptable and preferable to short-cycling. Single-family residential systems typically run pumps rated 5 to 25 GPM. Values above 25 GPM on this calculator suggest a commercial or agricultural application where a licensed pump engineer should verify the full system design. The calculator does not model waterlogged tank diagnosis, bladder replacement intervals, or storage volume for emergency water supply during power outages.

## Verified worked examples

### Example 1: Light-Use Cabin with a Small Pump

Pump Flow Rate: 5 GPM Pressure Switch: 40/60 PSI (multiplier: 0.28) Target Run Time: 1.0 minute Calculation: Drawdown = 5 GPM x 1.0 min = 5.0 gallons. Total tank = 5.0 / 0.28 = 17.9 gallons. Result: Minimum 17.9-gallon tank. The next standard size is a 20-gallon pre-charged bladder tank. This is the smallest reasonable installation for this pump; any smaller tank will produce run times under 50 seconds per cycle.

### Example 2: High-Flow Submersible on a Working Farm

Pump Flow Rate: 15 GPM Pressure Switch: 40/60 PSI (multiplier: 0.28) Target Run Time: 2.0 minutes Calculation: Drawdown = 15 GPM x 2.0 min = 30.0 gallons. Total tank = 30.0 / 0.28 = 107.1 gallons. Result: Minimum 107.1-gallon tank. The nearest standard size is a 119-gallon tank. Installing a 44-gallon tank on this pump, a common cost-cutting move, would produce a run time of roughly 22 seconds per cycle and destroy the motor within months.

### Example 3: Older Shallow-Well System on 30/50 PSI

Pump Flow Rate: 8 GPM Pressure Switch: 30/50 PSI (multiplier: 0.31) Target Run Time: 1.5 minutes Calculation: Drawdown = 8 GPM x 1.5 min = 12.0 gallons. Total tank = 12.0 / 0.31 = 38.7 gallons. Result: Minimum 38.7-gallon tank. The next standard size is a 44-gallon tank. The 30/50 switch setting provides a slightly higher drawdown fraction than 40/60, so the required tank volume is modestly lower for equivalent protection.

## Assumptions

Visualizing Boyle’s Law: only a fraction of your total tank volume is usable water drawdown. Show the calculation steps Step 1: Calculate drawdown (usable gallons) Drawdown (gallons) = Pump Flow Rate (GPM) x Target Run Time (minutes) This is straightforward unit multiplication. A 10 GPM pump running for 2 minutes delivers 20 gallons per cycle. That 20 gallons must fit within the tank’s usable volume, which is the drawdown window. Step 2: Apply the pressure-switch multiplier A pre-charged bladder tank does not store all of its volume as water. The remaining space holds compressed air at a pre-charge pressure set just below the cut-in PSI. As water enters, the air compresses. As water exits, the air expands. The fraction of total tank volume available as water storage depends on the pressure ratio between cut-in and cut-out pressures, governed by Boyle’s Law. 30/50 PSI switch: multiplier 0.31 (31 of every 100 gallons of tank volume is usable water) 40/60 PSI switch: multiplier 0.28 50/70 PSI switch: multiplier 0.25 Higher operating pressures reduce the usable fraction because the pressure ratio between cut-in and cut-out is a smaller percentage of absolute pressure, leaving less room for water expansion. Step 3: Calculate required total tank volume Total Tank Volume (gallons) = Drawdown (gallons) / Multiplier Rounding rule: always round up to the next standard catalog size. Never round down. A tank that is 5 gallons short will produce a run time 10 to 25 seconds shorter than designed depending on pump GPM. Assumptions and Limits The multipliers (0.31, 0.28, 0.25) are industry-standard estimates for pre-charged bladder tanks with air pre-charge set 2 PSI below cut-in. Actual drawdown may differ up to 5% by manufacturer and bladder condition. This calculator assumes a functioning bladder or diaphragm tank. A waterlogged galvanized tank, one where the bladder has ruptured, has near-zero drawdown regardless of tank volume and cannot be corrected by resizing. Pre-charge pressure is assumed to be correctly set. If the pre-charge is higher than cut-in PSI minus 2, the usable drawdown shrinks and the pump will short-cycle even with an appropriately sized tank. The tool does not account for multiple simultaneous fixtures drawing water at the same time. If two showers run simultaneously, the effective demand may exceed drawdown and the pump will run longer than one cycle continuously, which is generally acceptable and preferable to short-cycling. Single-family residential systems typically run pumps rated 5 to 25 GPM. Values above 25 GPM on this calculator suggest a commercial or agricultural application where a licensed pump engineer should verify the full system design. The calculator does not model waterlogged tank diagnosis, bladder replacement intervals, or storage volume for emergency water supply during power outages. The multipliers (0.31, 0.28, 0.25) are industry-standard estimates for pre-charged bladder tanks with air pre-charge set 2 PSI below cut-in. Actual drawdown may differ up to 5% by manufacturer and bladder condition. This calculator assumes a functioning bladder or diaphragm tank. A waterlogged galvanized tank, one where the bladder has ruptured, has near-zero drawdown regardless of tank volume and cannot be corrected by resizing. Pre-charge pressure is assumed to be correctly set. If the pre-charge is higher than cut-in PSI minus 2, the usable drawdown shrinks and the pump will short-cycle even with an appropriately sized tank. The tool does not account for multiple simultaneous fixtures drawing water at the same time. If two showers run simultaneously, the effective demand may exceed drawdown and the pump will run longer than one cycle continuously, which is generally acceptable and preferable to short-cycling. Single-family residential systems typically run pumps rated 5 to 25 GPM. Values above 25 GPM on this calculator suggest a commercial or agricultural application where a licensed pump engineer should verify the full system design. The calculator does not model waterlogged tank diagnosis, bladder replacement intervals, or storage volume for emergency water supply during power outages. The drawdown sizing method is the established standard used by pump manufacturers, pressure tank manufacturers, and the Hydraulic Institute for residential well system design. The following checks are derived from the formula itself and from known failure modes documented by pump service contractors. Critical Warnings: Short-cycle bladder rupture cascade: Replacing a correctly sized tank with a smaller one to reduce cost while keeping the same high-GPM pump is the single most common cause of premature submersible pump failure. A 20-gallon tank on a 15 GPM pump at 40/60 PSI delivers only 5.6 gallons of drawdown, filling in roughly 22 seconds. The pump starts and stops dozens of times per hour, overheating the motor and slamming the pressure switch contacts. A replacement submersible pump costs between $800 and $3,000 installed. The tank upgrade to prevent this costs a fraction of that. Waterlogged tank illusion: A tank that has lost its air pre-charge through bladder rupture or a failed Schrader valve may physically hold 44 gallons but deliver near-zero usable drawdown. The symptom is a pressure gauge needle that bounces rapidly and a pump that runs in very short bursts. Sizing a new tank using this calculator will only solve the problem if the replacement tank has a functioning bladder and correctly set pre-charge. A waterlogged galvanized steel tank cannot be recharged; it must be replaced with a bladder-style unit. Minimum Standards: Minimum pump run time per cycle: 1.0 minute. Below this threshold, heat from motor starting accumulates faster than the motor can dissipate it between cycles. Best practice run time for motors rated 1 HP and above: 2.0 minutes per cycle. Pre-charge pressure must be set to cut-in PSI minus 2 (for a 40/60 switch, pre-charge at 38 PSI). This setting should be verified whenever a tank is drained, replaced, or moved. Competitor Trap: Many online tank sizing guides present a simplified rule of thumb such as “one gallon of tank per GPM” or “size the tank at 10 times the pump’s GPM.” These heuristics were derived for older galvanized tanks with unpredictable air-water ratios and do not apply to modern pre-charged bladder tanks. They also ignore the pressure switch setting entirely, which shifts the usable fraction by several percentage points depending on operating range. A 40/60 PSI system sized using the “1x GPM” rule will typically produce a tank 25 to 40 gallons too small for a 10 GPM pump on a 2-minute target run time. Pressure spikes from short-cycling can cause secondary problems beyond motor damage. Rapid pump starts create hydraulic shocks throughout the supply piping. The water hammer calculator can help estimate surge pressures in systems that experience these repeated start events, particularly where copper or CPVC piping runs long distances to fixtures. If your well has been recently serviced, chemically treated, or had any service work involving the casing, cross-reference your maintenance log. The well shock chlorination calculator provides volume and dosing figures for disinfecting well casings, which is typically required after pump replacement or any work inside the casing.

## Limitations and safety

The multipliers (0.31, 0.28, 0.25) are industry-standard estimates for pre-charged bladder tanks with air pre-charge set 2 PSI below cut-in. Actual drawdown may differ up to 5% by manufacturer and bladder condition. This calculator assumes a functioning bladder or diaphragm tank. A waterlogged galvanized tank, one where the bladder has ruptured, has near-zero drawdown regardless of tank volume and cannot be corrected by resizing. Pre-charge pressure is assumed to be correctly set. If the pre-charge is higher than cut-in PSI minus 2, the usable drawdown shrinks and the pump will short-cycle even with an appropriately sized tank. The tool does not account for multiple simultaneous fixtures drawing water at the same time. If two showers run simultaneously, the effective demand may exceed drawdown and the pump will run longer than one cycle continuously, which is generally acceptable and preferable to short-cycling. Single-family residential systems typically run pumps rated 5 to 25 GPM. Values above 25 GPM on this calculator suggest a commercial or agricultural application where a licensed pump engineer should verify the full system design. The calculator does not model waterlogged tank diagnosis, bladder replacement intervals, or storage volume for emergency water supply during power outages. The drawdown sizing method is the established standard used by pump manufacturers, pressure tank manufacturers, and the Hydraulic Institute for residential well system design. The following checks are derived from the formula itself and from known failure modes documented by pump service contractors. Critical Warnings: Short-cycle bladder rupture cascade: Replacing a correctly sized tank with a smaller one to reduce cost while keeping the same high-GPM pump is the single most common cause of premature submersible pump failure. A 20-gallon tank on a 15 GPM pump at 40/60 PSI delivers only 5.6 gallons of drawdown, filling in roughly 22 seconds. The pump starts and stops dozens of times per hour, overheating the motor and slamming the pressure switch contacts. A replacement submersible pump costs between $800 and $3,000 installed. The tank upgrade to prevent this costs a fraction of that. Waterlogged tank illusion: A tank that has lost its air pre-charge through bladder rupture or a failed Schrader valve may physically hold 44 gallons but deliver near-zero usable drawdown. The symptom is a pressure gauge needle that bounces rapidly and a pump that runs in very short bursts. Sizing a new tank using this calculator will only solve the problem if the replacement tank has a functioning bladder and correctly set pre-charge. A waterlogged galvanized steel tank cannot be recharged; it must be replaced with a bladder-style unit. Minimum Standards: Minimum pump run time per cycle: 1.0 minute. Below this threshold, heat from motor starting accumulates faster than the motor can dissipate it between cycles. Best practice run time for motors rated 1 HP and above: 2.0 minutes per cycle. Pre-charge pressure must be set to cut-in PSI minus 2 (for a 40/60 switch, pre-charge at 38 PSI). This setting should be verified whenever a tank is drained, replaced, or moved. Competitor Trap: Many online tank sizing guides present a simplified rule of thumb such as “one gallon of tank per GPM” or “size the tank at 10 times the pump’s GPM.” These heuristics were derived for older galvanized tanks with unpredictable air-water ratios and do not apply to modern pre-charged bladder tanks. They also ignore the pressure switch setting entirely, which shifts the usable fraction by several percentage points depending on operating range. A 40/60 PSI system sized using the “1x GPM” rule will typically produce a tank 25 to 40 gallons too small for a 10 GPM pump on a 2-minute target run time. Pressure spikes from short-cycling can cause secondary problems beyond motor damage. Rapid pump starts create hydraulic shocks throughout the supply piping. The water hammer calculator can help estimate surge pressures in systems that experience these repeated start events, particularly where copper or CPVC piping runs long distances to fixtures. If your well has been recently serviced, chemically treated, or had any service work involving the casing, cross-reference your maintenance log. The well shock chlorination calculator provides volume and dosing figures for disinfecting well casings, which is typically required after pump replacement or any work inside the casing.

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## Provenance

- Model ID: `tyg-802`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-04-27T19:42:43
- Runtime SHA-256: `01c185552397728ab1d977a94962d0f7d7d463a0fd760dd762f2f5261049ed46`

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