Well Water & Off-Grid Hydraulics

Well Pump Pressure Tank & Pressure Switch Sizing Calculator

Calculate required usable water drawdown, Boyle's Law acceptance factor, pre-charge air PSI, and commercial bladder tank capacity to protect your well pump motor from premature short-cycling.

1. Pump & Pressure Specifications Live Auto-Calculation

Submersible or jet pump output (typically 5 to 25 GPM for residential wells)
GPM
Select standard Square D Pumptrol preset or enter custom pressures below
Pump turns ON (e.g., 40)
PSI
Pump turns OFF (e.g., 60)
PSI
1.0 min (<1.5 HP) or 2.0 min (2+ HP)
MIN
Standard bladder offset is 2 PSI
PSI

2. Tank Sizing & Drawdown Output Waiting for Inputs

Enter pump flow rate (GPM) and pressure switch PSI specs to calculate tank size.
Recommended Commercial Pressure Tank Size
0 Gallons
Exact minimum theoretical volume: 0.0 gal
Required Drawdown
0.0 gal
Actual tank yields: 0.0 gal
Tank Air Pre-Charge
0 PSI
Set with 0 PSI water pressure
Boyle's Acceptance Factor
0.0%
Usable water % of total tank
Max Cycles Per Hour
0 cycles/hr
Est. actual run time: 0.0 min
Thermodynamic Sizing Note (Boyle's Law): Pre-charged diaphragm tanks hold compressed air above a flexible membrane. At sea level ($P_{\text{atm}} = 14.7\text{ PSIA}$), increasing system pressure from 30/50 to 40/60 PSI slightly compresses the acceptance ratio, requiring a larger physical tank to deliver the same gallon drawdown.

Recommended Well Tank & Switch Hardware Dynamically Matched to Your Specs

Hardware search links automatically update to match your calculated gallon capacity and pressure switch range.

1. Pre-Charged Well Tank
Pre-Charged Diaphragm Well Pressure Tank

Heavy-duty vertical butyl diaphragm well tank (Amtrol Well-X-Trol / WaterWorker) rated for potable well systems.

Check Tank Prices on Amazon →
2. Pump Pressure Switch
Square D Pumptrol Well Pressure Switch

Heavy-duty diaphragm-actuated water pump pressure switch with optional low-pressure cut-off protection.

Shop Pressure Switches →
3. Brass Tank Tee & Gauge Kit
Lead-Free Brass Well Tank Tee Installation Kit

Complete manifold with brass union tank tee, 0-100 PSI liquid-filled gauge, pressure relief valve, and boiler drain.

View Brass Tee Kits →

Well Pressure Tank Sizing Reference Matrix (30/50 vs. 40/60 PSI)

Use this engineering reference chart to compare required usable drawdown and standard residential diaphragm tank capacities across common well pump flow rates (GPM) and pressure switch differentials ($P_{\text{atm}} = 14.7\text{ PSIA}$, pre-charge set $2\text{ PSI}$ below cut-in).

Pump Flow Rate Switch Setting (PSI) Air Pre-Charge Drawdown Factor Min. Tank (1-Min Run) Heavy-Duty Tank (2-Min Run)
5 GPM 30 / 50 PSI 28 PSI 30.7% 20 Gallon (6.1 gal drawdown) 32 Gallon (9.8 gal drawdown)
5 GPM 40 / 60 PSI 38 PSI 26.5% 20 Gallon (5.3 gal drawdown) 44 Gallon (11.7 gal drawdown)
7 GPM 30 / 50 PSI 28 PSI 30.7% 32 Gallon (9.8 gal drawdown) 44 Gallon (13.5 gal drawdown)
10 GPM 30 / 50 PSI 28 PSI 30.7% 32 Gallon (9.8 gal) or 44 Gal 62 Gallon (19.0 gal) or 86 Gal
10 GPM 40 / 60 PSI 38 PSI 26.5% 44 Gallon (11.7 gal drawdown) 86 Gallon (22.8 gal drawdown)
15 GPM 40 / 60 PSI 38 PSI 26.5% 62 Gallon (16.4 gal drawdown) 119 Gallon (31.5 gal drawdown)
20 GPM 40 / 60 PSI 38 PSI 26.5% 86 Gallon (22.8 gal drawdown) 2x 86 Gal Manifold (45.6 gal)

Technical FAQ: Well Pressure Tank & Switch Engineering

1. What size pressure tank do I need for a 10 GPM well pump at 40/60 PSI?

For a 10 GPM submersible or jet pump operating on a 40/60 PSI pressure switch, a 1-minute minimum motor run time requires 10 gallons of usable water drawdown. Because a bladder tank operating between 40 and 60 PSI (with a 38 PSI pre-charge) has a Boyle's Law acceptance factor of 26.5%, the minimum physical tank volume is $10 \div 0.265 = 37.7\text{ gallons}$. Stepping up to the nearest commercial tank size requires a 44-gallon tank (which yields 11.7 gallons of actual drawdown). If you want a 2-minute run time (20 gallons of drawdown) for maximum motor life, install an 86-gallon pressure tank.

2. How do you calculate pressure tank drawdown using Boyle's Law?

Pre-charged well tanks trap a captive air charge above a heavy butyl diaphragm. Using Boyle's Law ($P_1 V_1 = P_2 V_2$) in absolute pressure ($\text{PSIA} = \text{PSIG} + 14.7\text{ psi atmospheric}$), the usable drawdown acceptance factor ($F_d$) is calculated as:
$$F_d = \frac{P_{\text{precharge}} + 14.7}{P_{\text{cut-in}} + 14.7} - \frac{P_{\text{precharge}} + 14.7}{P_{\text{cut-off}} + 14.7}$$ Multiplying the total physical tank volume by $F_d$ tells you the exact volume of water pushed out of the tank between the cut-off and cut-in pressures.

3. Why is tank air pre-charge set 2 PSI below the pressure switch cut-in setting?

Setting the empty tank air pre-charge 2 PSI below cut-in pressure (e.g., 28 PSI for a 30/50 switch or 38 PSI for a 40/60 switch) ensures a small reserve buffer of water remains inside the diaphragm when the pressure switch clicks on at cut-in. Without this 2 PSI offset, the bladder would completely bottom out against the tank outlet before the submersible pump builds head pressure, causing a momentary drop to zero water pressure ("stutter") at your household faucets.