Water Flow Rate Calculator (PSI → GPM)

Convert water pressure into gallons per minute using pipe inside diameter and discharge type — estimate fixture capacity, hose output, or outlet flow before you spec valves or test on site.

System Details

Pressure (PSI)

Pipe Size

Discharge Type

Live Results

Flow Rate

109.86 GPM

Velocity

66.01 ft/s

Excessive velocity — water hammer and pipe damage risk. Increase pipe size or reduce pressure.

GPH

6,591.41 GPH

Pipe ID

0.824 in

Discharge Coeff.

0.70

Based on Bernoulli discharge with a coefficient (Cd). Actual flow varies with pipe length, fittings, and system demand — verify against field measurements.

How to Use This Water Flow Rate Calculator

Enter the water pressure in PSI. Input the available gauge pressure at the discharge point — static pressure at the hose bib, fixture supply, or test port. Typical residential supply ranges from 40–80 PSI; use the pressure measured at the outlet, not at the street main.
Select pipe size. Choose the nominal pipe size from the dropdown. Each option maps to a standard inside diameter (ID) for copper or similar rigid pipe — 1/2" through 1-1/2". Larger IDs allow more flow at the same pressure.
Select discharge type. Pick the connection or outlet type to apply the correct discharge coefficient (Cd). A smooth open pipe uses Cd = 0.98; a standard spigot or valve connection uses 0.70; a spray nozzle or restrictor uses 0.60.
Review flow rate and classification. Results update instantly. Check GPM against fixture requirements, GPH for tank fill times, and velocity to confirm the pipe can handle the discharge without excessive erosion or noise.

Formulas & Example

Flow rate uses a Bernoulli discharge relationship with inside diameter in inches, pressure in PSI, and a discharge coefficient for the outlet type.

GPM = 29.84 × ID² × √PSI × Cd
GPH = GPM × 60
Velocity (ft/s) = (0.408 × GPM) ÷ ID²

Discharge coefficients (Cd):
  Straight open pipe / smooth bore:     0.98
  Standard spigot / valve connection:   0.70
  Spray nozzle / restrictor:            0.60

Worked Example

1 PSI through 1/2" pipe (ID = 0.622 in) with a standard spigot (Cd = 0.70):

GPM = 29.84 × 0.622² × √1 × 0.70 = 8.08 GPM
GPH = 8.08 × 60 = 484.80 GPH
Velocity = (0.408 × 8.08) ÷ 0.622² = 8.53 ft/s

Result: 8.08 GPM at 8.53 ft/s — High velocity range

At higher pressures the discharge formula yields larger flows — e.g. 60 PSI on 3/4" with a standard spigot reaches roughly 110 GPM theoretical discharge. Switching to a spray nozzle (Cd = 0.60) reduces flow proportionally; upsizing pipe increases both GPM and cross-sectional area at the same pressure.

Frequently Asked Questions

How does PSI convert to GPM?▾

This calculator uses a Bernoulli-based discharge formula: GPM = 29.84 × ID² × √PSI × Cd, where ID is the pipe inside diameter in inches, PSI is gauge pressure, and Cd is the discharge coefficient for the outlet type. It estimates volumetric flow through an orifice or open discharge — not full-pipe friction loss along a long run.

What is a discharge coefficient (Cd)?▾

The discharge coefficient accounts for energy losses at the outlet. A smooth, straight open pipe has Cd ≈ 0.98 because flow exits with minimal restriction. A standard hose bib or valve adds turbulence and contraction, reducing effective flow (Cd ≈ 0.70). Spray nozzles and restrictors further limit discharge (Cd ≈ 0.60). Always match Cd to the actual outlet geometry for realistic estimates.

What is a normal residential water flow rate?▾

Most single fixtures operate between 1.5–2.5 GPM (low-flow faucets and showerheads) up to 5–8 GPM for a garden hose on a 3/4" line. This calculator classifies discharge by velocity: below 2 ft/s may not meet demand, 2–8 ft/s is the typical safe range, 8–15 ft/s raises noise and erosion concerns at fittings, and above 15 ft/s signals water hammer and pipe damage risk — upsize the pipe or reduce pressure.

Why does pipe inside diameter matter more than nominal size?▾

Flow scales with the square of inside diameter in the discharge equation — doubling ID quadruples potential flow at the same pressure. Nominal labels like 3/4" refer to outside diameter standards; actual IDs vary by material. This tool uses common copper-equivalent IDs (e.g., 3/4" = 0.824 in) for consistent field estimates.

Can I use this for irrigation or fire sprinkler sizing?▾

This tool gives a quick discharge estimate from pressure and pipe size at a single outlet. Irrigation and fire protection design require full hydraulic calculations including pipe length, fittings, elevation change, and simultaneous demand. Use this calculator for spot checks — fixture flow tests, hose bib capacity, or verifying whether available PSI supports a target GPM — then confirm with code-compliant hydraulic software or manufacturer data for system design.

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