PSI to GPM Calculator

Estimate water flow in GPM from tank pressure, exit pressure, and pipe size.

Supports US and UK gallon flow units with metric and imperial pipe sizes.

Updated August 9, 2026

CreatorFrank Zhao

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Introduction / overview

The PSI to GPM Calculator estimates how fast water leaves a pressurized tank or pipe, based on three things: the pressure inside the tank, the pressure where the water exits, and the size of the pipe. Type a pressure and a pipe diameter and it fills in the flow rate in gallons per minute — or work backward and ask what pressure or pipe size a target flow needs.

PSI and GPM measure different things: PSI (pounds per square inch) is a unit of pressure, while GPM (gallons per minute) is a unit of flow rate. They only connect through the pipe itself — the opening area and the pressure difference that pushes the water through it. That is why a high pressure reading alone doesn't tell you the flow: the same pressure moves far more water through a wide pipe than a narrow one.

The unit math is exact: the pipe area and the US gallon conversions are precisely defined, and the calculator keeps full working precision. What is approximate is the physics — this is a frictionless, water-only model, so real installations flow a little less.

Who is this for?

Plumbers and irrigation installers estimating what a line can deliver from a pressure reading.
Well and pump owners checking whether a tank pressure produces enough flow.
DIYers and engineers sizing a drain, overflow, or gravity-fed line.
Anyone working backward from a required flow to the pressure or pipe size needed to reach it.

How to use / quick start

1Enter the pressure inside the tank in psi — the reading from a gauge on the tank or supply line.
2Check the pressure at the exit. It defaults to 14.7 psi (standard atmospheric pressure), which is correct when the pipe opens to the air. Change it only if the pipe discharges into another pressurized space.
3Enter the pipe diameter (or its cross-sectional area — pick either one; the tool fills in the other). Pick the units your pipe is labelled in, such as inches or millimetres.
4Read the flow rate in the Flow rate field. It recalculates instantly as you edit, in US gal/min by default.
5Work in any direction. Type into the pressure, diameter, area, or flow field and the calculator derives whichever one you left blank — so sizing a line for a target flow is just as easy as reading a flow from a known pressure.

Quick example: 72 psi through a 2.5 in pipe

Enter 72 for the pressure inside the tank, leave the exit pressure at 14.7 psi, and set the pipe diameter to 2.5 in.

The tool first finds the pipe's cross-sectional area, then combines it with the pressure difference to compute the flow:

A = \pi \left( \frac{2.5}{2} \right)^2

=

4.9087\ \mathrm{in^2}

Q = 448.83 \times \frac{4.9087}{144} \times \sqrt{ \frac{2 \times 32.174 \times 144 \times (72 - 14.7)}{62.4} }

=

1411.31\ \mathrm{US\ gal/min}

So a 72 psi tank discharging to the open air through a 2.5 in pipe delivers about 1,411 US gallons per minute. Switch the flow unit to US gal/hr and the same flow reads about 84,679 gal/hr; choose UK gal/min and it reads about 1,175, because a UK gallon is larger.

How to interpret the results

Highlighted value = calculated automatically from the other fields, and it stays in sync as you edit.
Plain value = the number you typed. You can switch which field is the input at any moment.
Blank or error in Flow rate? The tank pressure must be greater than the exit pressure. If they are equal or reversed, the tool cannot compute a flow and leaves the result blank.

Real-world examples / use cases

1) What pressure do I need for a target flow?

Background: A gravity-fed line must deliver about 1,000 gal/min through a 2.5 in pipe that discharges to the open air.

Input: Enter the flow as 1,000 US gal/min, the diameter as 2.5 in, leave the exit at 14.7 psi, and leave the tank pressure blank.

P_{\mathrm{tank}} = P_{\mathrm{exit}} + \frac{Q^2}{K \cdot A^2 \cdot C^2}

=

14.7 + 28.77

=

43.47\ \mathrm{psi}

What it means: A tank pressure of about 43.5 psi (with the pipe open to the air) will push roughly 1,000 gal/min through a 2.5 in line — handy for sizing a pump before you buy it.

2) Why pipe size changes the flow so much

Background: You keep the same 72 psi tank and 14.7 psi exit, but compare pipe sizes.

Input: Run the same tank pressure with a 1.25 in, 2.5 in, and 5 in pipe.

Q \propto A \propto D^2

\Rightarrow

\text{1.25 in} \to 353

\text{2.5 in} \to 1411

\text{5 in} \to 5645\ \mathrm{gal/min}

What it means: Doubling the diameter quadruples the flow, because the pipe's cross-sectional area scales with the square of the diameter. A small change in pipe size has a large effect — check your inner diameter carefully.

Calculation method / formula explanation

Two steps turn the inputs into a flow. First the pipe's open area is found from its diameter; then that area is combined with the pressure difference using Bernoulli's (Torricelli's) relation for water leaving a tank.

1) Cross-sectional area of a round pipe

The open area of a circular pipe is the area of a circle with diameter $D$ :

A = \pi \left( \frac{D}{2} \right)^{2}

2) Flow rate from the pressure difference

The velocity of the water leaving the pipe comes from the pressure drop, and the flow is that velocity times the pipe's area. Written with every unit-conversion factor shown:

Q = 448.83 \cdot A \cdot \sqrt{ \frac{2 \times 32.174 \times 144 \times \Delta P}{62.4} }

where the pressure drop is simply:

\Delta P = P_{\mathrm{tank}} - P_{\mathrm{exit}}

All of the pressure, diameter, area, and flow fields are reversible: type into any of them and the tool solves for the one you left blank.

Where the constants come from

144 converts psi to pounds-force per square foot (since $1\ \mathrm{ft} = 12\ \mathrm{in}$ ). 32.174 is standard gravity, converting pound-force to mass-times-acceleration. 62.4 is the density of water in lb/ft³ — water is about $62.4\ \mathrm{lb/ft^3}$ at typical temperatures, and its exact value varies slightly with temperature, per the physical properties of water. 448.83 converts cubic feet per second to US gallons per minute — one US gallon is exactly 231 cubic inches, and one cubic foot holds about $7.48\ \mathrm{US\ gal}$ per the US gallon definition. A psi is one pound-force per square inch, and standard atmospheric pressure is about 14.7 psi at sea level, per the definition of psi.

Assumptions behind the estimate

The model treats water as incompressible, ignores friction in the pipe, and assumes the tank's surface moves slowly enough to count as still — the usual assumptions for Bernoulli's equation at a constant height. Real pipes with elbows, valves, and length flow somewhat less than this ideal estimate.

Tips & best practices

Keep both pressures on the same scale
The calculator uses the difference between the tank and exit pressures. If your gauge reads pressure above atmosphere (psig) and the pipe opens to the air, add about 14.7 psi to the tank value so it matches the absolute scale of the default 14.7 psi exit — otherwise the driving pressure is under-counted. The tool requires both pressures to be greater than zero, so a “0” exit for open air isn't accepted.
Diameter is the most sensitive input
Because the area scales with the square of the diameter, a small measurement error has a large effect on the result. Measure the inner diameter, and enter the diameter or the area — not both.
US and UK gallons are different sizes
A UK (imperial) gallon is about 20% larger than a US gallon, so the same physical flow reads as a smaller number in UK gal/min. Pick the unit your pump, pipe, or supplier documents.
Expect real pipes to flow a little less
This is a frictionless estimate. Fittings, valves, bends, and pipe length all reduce real flow, so treat the number as an upper bound and verify with a flow test for anything critical.

Related concepts / background info

PSI, gauge, and absolute pressure

PSI (pounds per square inch) is the pressure a force of one pound exerts on one square inch. A gauge usually reads pressure above the surrounding atmosphere (psig), while absolute pressure (psia) includes the atmosphere — about 14.7 psi at sea level. The calculator compares tank and exit on the same scale, so keep the default 14.7 psi exit and add 14.7 to a psig tank reading.

Flow rate and the US vs UK gallon

GPM (gallons per minute) is how much water passes a point each minute. A US gallon is exactly 231 cubic inches (3.785 L), while a UK gallon is 4.546 L — about 20% larger. The tool supports both in per-second, per-minute, per-hour, and per-day units, so you can read the result in the unit your project uses.

Next steps with your result

If your pressure gauge reads in a different unit such as bar or kPa, convert the reading to psi first with the Pressure Conversion Calculator. To turn a flow volume into a water weight — for example, to find how many pounds per hour a 1,411 gal/min flow represents — use the Gallons to Pounds Conversion.

Frequently asked questions (FAQs)

Is GPM the same as PSI?

No. PSI measures pressure and GPM measures flow rate — they are different physical quantities. This calculator connects them for water in a pipe: given the pressure difference and the pipe size, it estimates the flow, or given a target flow and pipe size, it finds the pressure needed.

Why is my flow rate blank or showing an error?

The flow can only be computed when the tank pressure is greater than the exit pressure and both are positive. If the tank pressure equals or falls below the exit pressure, the result stays blank. Raise the tank pressure (or lower the exit pressure) to get a flow.

Do I need to enter both the pipe diameter and its area?

No. Enter either the diameter or the cross-sectional area and the tool derives the other automatically. They stay in sync, so you can also switch between them to check your work.

Can I work backward from a target flow?

Yes. Type a flow into the Flow rate field and leave the field you want to solve for blank — the tool returns the tank pressure, exit pressure, pipe diameter, or pipe area needed to produce that flow. This is the easiest way to size a line for a specific delivery.

Limitations / disclaimers

This is a frictionless, water-only model (Bernoulli / Torricelli at constant height). Real pipes with valves, elbows, and length flow less than the estimate.
The result is for water at a typical density (about 62.4 lb/ft³). Other fluids, or water at extreme temperatures, give different flows.
Both pressures must be positive and on the same scale, and the tank pressure must exceed the exit pressure for a flow to be defined.
Treat the result as a planning estimate. For pumps, fire flows, or system design, confirm with a flow meter or an engineer before relying on the number.

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