PSI to GPM Calculator
PSI and GPM measure different things, so there's no direct conversion — this calculator uses the real physics formula to get your actual flow rate from pressure and pipe size.
Why PSI can't be directly converted to GPM
PSI (pounds per square inch) measures pressure. GPM (gallons per minute) measures flow rate. These are fundamentally different physical quantities — pressure describes a force applied over an area, while flow rate describes a volume of liquid moving past a point over time. There's no fixed multiplication factor between them the way there is between, say, inches and centimeters. Getting from one to the other requires knowing at least one more piece of information: the size of the pipe or opening the water is flowing through.
This calculator uses Bernoulli's equation, the real physics behind fluid flow, to bridge that gap properly — not an approximation or a rule of thumb.
The actual physics behind this calculator
v = √(2 × ΔP ÷ ρ), then Flow = v × Area
Bernoulli's equation for fluid flow at constant height reduces to this: exit velocity (v) equals the square root of twice the pressure difference (ΔP) divided by the fluid's density (ρ, 62.4 lb/ft³ for water). Once velocity is known, multiplying it by the pipe's cross-sectional area gives the volumetric flow rate, which this calculator then converts into gallons per minute.
Step 1: Pressure difference
Subtract the exit pressure from the source pressure, then convert from psi to lbf/ft² (× 144) and then to consistent units for the velocity formula.
Step 2: Pipe cross-section
Area = π × (diameter ÷ 2)², converting the diameter from inches to feet first to keep every unit consistent through the calculation.
Worked example: 72 PSI tank, 2.5-inch pipe
With a source pressure of 72.0 psi and atmospheric exit pressure of 14.7 psi, the pressure difference is 57.3 psi. Running that through the full formula gives an exit velocity of roughly 92.2 ft/s. The 2.5-inch pipe has a cross-sectional area of about 0.0341 ft², so multiplying velocity by area gives a flow rate of about 3.14 ft³/s — which converts to approximately 1,412 gallons per minute after applying the standard 448.83 ft³/s-to-gpm conversion factor.
Try halving the pipe diameter to 1.25 inches with the same pressure, and the flow rate drops to roughly a quarter of that figure — not half — because cross-sectional area scales with the square of the diameter, not linearly with it.
Working through other specs on the same job?
Flow rate calculations like this one often come up alongside torque specs on the same plumbing, irrigation, or mechanical project — tightening valve fittings or pump housings correctly matters just as much as getting the flow numbers right. The inch-pounds to foot-pounds converter and inch-lbs to Newton-meters converter both handle the torque side of that same kind of job, so you're not switching tools mid-project for a quick unit check.
PSI to GPM calculator — FAQ
Can I directly convert PSI to GPM like other unit conversions?
No — this is the single most important thing to understand before using this tool. PSI measures pressure and GPM measures flow rate; they're different physical quantities entirely, not two units for the same thing like inches and centimeters. There's no fixed multiplication factor between them. Calculating flow rate from pressure requires knowing at least one more variable — the pipe's diameter — and applying Bernoulli's equation for fluid flow, which is exactly what this calculator does behind the scenes.
What is Bernoulli's equation, and why does it matter here?
Bernoulli's equation describes how pressure, velocity, and height relate for a flowing fluid, based on energy conservation — as a fluid's velocity increases, its pressure decreases, and vice versa. Applied to water exiting a tank through a pipe, it lets you solve for the water's exit velocity purely from the pressure difference between the tank and the outside atmosphere, without needing to measure the velocity directly. Multiplying that velocity by the pipe's cross-sectional area then gives the flow rate.
Why does pipe diameter matter so much for the result?
Flow rate depends on cross-sectional area, and area scales with the square of the diameter (Area = π × (diameter ÷ 2)²) — so doubling a pipe's diameter roughly quadruples its cross-sectional area, and with it, the flow rate at the same pressure. This is why a relatively small increase in pipe size can produce a dramatically larger flow rate, and why pipe diameter is just as important an input here as the pressure reading itself.
What pressure should I use for the "exit" or "downstream" value?
For water exiting into open air, this is simply atmospheric pressure — 14.7 psi at sea level, which is the default this calculator uses. If the water is instead flowing into another pressurized system rather than the open air, use that system's actual pressure instead of the atmospheric default.
Does this calculator work for fluids other than water?
The underlying Bernoulli-based formula works for any incompressible fluid, but the specific numbers this calculator uses — most notably water's density of 62.4 lb/ft³ — are specific to water. Using this tool for a significantly different fluid (like oil or a chemical solution) would require substituting that fluid's actual density into the formula, which this calculator doesn't currently support directly.
Why might my real-world flow rate be lower than this calculator's result?
This calculator models an idealized, frictionless flow, which represents a theoretical maximum. Real-world pipes introduce friction losses from the pipe's interior surface, plus additional pressure loss from bends, fittings, valves, and pipe length — all of which reduce actual flow rate below this idealized figure. For rough estimates and comparing scenarios, this calculator is genuinely useful; for a precise engineering flow calculation on a specific real pipe run, those real-world losses need to be accounted for separately.
How do I convert my result from GPM to gallons per hour?
Multiply the GPM value by 60, since there are 60 minutes in an hour. This calculator's unit toggle does that conversion automatically, but the underlying math is exactly that simple once you have a GPM figure.
