Flow Rate Calculator
Calculate water flow rate two ways: pipe velocity × area or volume ÷ time. Standard US pipe sizes, GPM, GPH, liters/min, CFS, and daily usage estimate.
🌊 Flow Rate Calculator
PIPE
FLOW RATE
DAILY ESTIMATE
💡 How to Calculate Flow Rate
Flow rate is the volume of fluid passing through a pipe per unit of time. It's the most fundamental measurement in plumbing design — every pipe size, pump selection, and fixture specification depends on it.
The calculator above supports two calculation methods: engineering formula (pipe area × velocity) with 8 standard pipe sizes, and practical measurement (volume ÷ time for bucket tests). Both output in GPM, GPH, liters/min, and CFS.
Calculate pipe volume with our pipe volume calculator. For tanks, see our tank volume calculator. Check flow speed with our water velocity calculator.
Flow Rate Formula: Q = A × v
The volumetric flow rate equals the pipe cross-sectional area multiplied by fluid velocity:
Q = A × v — where Q = flow rate, A = pipe area, v = velocity
For a round pipe: A = π × (D/2)²
Example: 1" copper pipe (1.049" ID) at 5 ft/s:
- A = π × (1.049 ÷ 24)² = π × 0.001907 = 0.005993 sq ft
- Q = 0.005993 × 5 = 0.02997 CFS
- GPM = 0.02997 × 448.831 = 13.45 GPM
Alternate Formula: Q = V / t
The bucket test is the easiest way to measure actual flow rate at a fixture:
Q = V / t — where V = volume collected, t = time
Example: Fill a 5-gallon bucket from your garden hose: takes 3 minutes → Q = 5 ÷ 3 = 1.67 GPM.
Fixture Flow Rate Standards (US)
| Fixture | Standard GPM | WaterSense GPM | Federal Max |
|---|---|---|---|
| Kitchen faucet | 2.2 | 1.5 | 2.2 GPM |
| Bathroom faucet | 1.5 | 1.0 | 2.2 GPM |
| Showerhead | 2.5 | 2.0 | 2.5 GPM |
| Toilet (per flush) | 1.6 GPF | 1.28 GPF | 1.6 GPF |
| Dishwasher | 2.0–3.0 | — | — |
| Washing machine | 3.0–5.0 | — | — |
| Garden hose (½") | 5.0–10.0 | — | — |
| Irrigation sprinkler | 2.0–4.0 | — | — |
| Bathtub filler | 4.0–6.0 | — | — |
| Hose bibb (¾") | 8.0–15.0 | — | — |
GPF = gallons per flush. WaterSense is the EPA's voluntary program for water-efficient products. Federal maximums are set by the Energy Policy Act.
Flow Rate by Pipe Size (at 5 ft/s)
| Pipe (Copper ID) | Area (sq in) | GPM at 5 ft/s | GPH |
|---|---|---|---|
| ½" (0.622") | 0.304 | 4.6 | 274 |
| ¾" (0.824") | 0.533 | 8.0 | 481 |
| 1" (1.049") | 0.864 | 13.5 | 808 |
| 1¼" (1.368") | 1.470 | 22.9 | 1,374 |
| 1½" (1.610") | 2.036 | 31.7 | 1,903 |
| 2" (2.067") | 3.356 | 52.3 | 3,138 |
| 3" (3.068") | 7.393 | 115.2 | 6,912 |
| 4" (4.026") | 12.730 | 198.3 | 11,900 |
5 ft/s is the recommended maximum for residential supply lines. Higher velocities increase noise and water hammer risk.
Unit Conversions
- 1 CFS = 448.831 GPM = 7.481 gal/sec
- 1 GPM = 3.785 liters/min = 0.0631 liters/sec
- 1 GPM = 0.00223 CFS
- 1 liter/sec = 15.85 GPM