Water Pressure Calculator

1. Enter supply pressure. This is the pressure at the starting point of your pipe run — typically at the meter, PRV, or main shutoff. Use a pressure gauge reading if available.

2. Enter elevation change. Measure the vertical distance from the supply point to the fixture. Uphill (positive) loses pressure; downhill (negative) gains it.

3. Select pipe details. Choose the pipe diameter, material, and enter the total developed length of the pipe run from supply to fixture.

4. Enter design flow rate. This is the GPM that will be flowing through the pipe during peak demand. Use our Fixture Unit Calculator to convert fixture units to GPM.

5. Count fittings. Each fitting adds equivalent pipe length. The calculator converts fitting counts to equivalent feet and adds them to the total friction calculation.

This calculator subtracts all pressure losses from the supply pressure to determine what remains at the fixture.

Friction loss increases exponentially with flow rate and decreases with larger pipe diameter. A pipe that works fine at 5 GPM might have unacceptable pressure drop at 15 GPM.

Verifying upper-floor pressure. Before quoting a bathroom remodel on the third floor, calculate if adequate pressure will reach the new fixtures with the existing supply piping.

Diagnosing low-pressure complaints. Walk through the calculation with real measurements to pinpoint whether the problem is elevation, undersized pipe, excessive fittings, or supply pressure.

Designing booster pump systems. If the calculation shows available pressure below 8 PSI, you need a booster pump. The deficit tells you the minimum pump boost required.