Water Flow Rate Calculator
Calculate water flow rate through pipes based on pressure, diameter, and pipe characteristics
Unit Conversions
| Parameter | Value |
|---|
Flow Characteristics
Step-by-Step Calculation
Recommended Water Velocities
Maintaining proper flow velocity is critical. Too slow and sediment settles; too fast and you get noise, erosion, and water hammer.
| Application | Min (ft/s) | Ideal (ft/s) | Max (ft/s) |
|---|---|---|---|
| Residential Supply | 2 | 4-5 | 8 |
| Commercial Supply | 2 | 4-6 | 8 |
| Fire Protection | — | — | 10-15 |
| Cooling Water | 3 | 5-7 | 10 |
| Pump Suction Lines | 1 | 2-4 | 5 |
| Gravity Drain Lines | 2 | 3-5 | 6 |
| Main Distribution | 2 | 4-6 | 8 |
Hazen-Williams Formula
The Hazen-Williams equation is the most widely used formula for calculating friction losses and flow rates in water distribution systems. It is empirical and valid for water at typical temperatures (40°F – 75°F) in pipes 2 inches and larger, though it is commonly applied to smaller pipes as well.
Velocity Form:
V = 1.318 × C × R0.63 × S0.54
Flow Form (US units):
Q = 0.4322 × C × d2.63 × S0.54
Head Loss per 100 ft:
hf = 10.67 × Q1.852 / (C1.852 × d4.87)
Where:
V = velocity (ft/s)
Q = flow rate (GPM)
C = Hazen-Williams roughness coefficient
d = pipe inner diameter (inches)
R = hydraulic radius (ft) = d / (4 × 12) for full pipes
S = hydraulic slope = hf / L (ft head loss per ft of pipe)
Hazen-Williams C-Factors
| Pipe Material | C-Factor |
|---|---|
| PVC, CPVC, PE, Fiberglass | 150 |
| Copper, Brass, New Steel | 140-150 |
| New Cast Iron, Ductile Iron (lined) | 130-140 |
| Concrete, Cement-Lined | 120-140 |
| General Welded Steel | 110-120 |
| Old Cast Iron (20+ years) | 100-110 |
| Corroded / Tuberculated | 60-100 |
Typical Fixture Flow Rates
When sizing residential or commercial plumbing, use these typical demand values as a starting point.
Residential Fixtures
- • Kitchen faucet: 2.0 – 2.5 GPM
- • Bathroom faucet: 1.0 – 1.5 GPM
- • Shower head: 2.0 – 2.5 GPM
- • Toilet (tank): 1.6 – 3.0 GPM (fill)
- • Dishwasher: 1.0 – 2.5 GPM
- • Washing machine: 3.0 – 5.0 GPM
- • Hose bib: 3.0 – 5.0 GPM
Commercial Fixtures
- • Flush valve toilet: 15 – 40 GPM (peak)
- • Urinal flush valve: 15 GPM (peak)
- • Commercial sink: 2.5 – 4.0 GPM
- • Pre-rinse spray: 1.3 – 1.6 GPM
- • Drinking fountain: 0.5 – 1.0 GPM
- • Fire sprinkler head: 15 – 30 GPM
- • Cooling tower: varies by tonnage
Pipe Sizing Guidelines
- Velocity limits: Keep water velocity below 8 ft/s for residential and 10 ft/s for commercial to prevent noise and water hammer
- Pressure loss: Total system friction losses should not exceed 50% of available supply pressure
- Minimum velocity: Maintain at least 2 ft/s to prevent sediment settling and stagnation
- Fittings: Account for fittings by adding their equivalent pipe length (e.g., a 1" 90° elbow ≈ 2.7 ft equivalent length)
- Elevation: Add 0.433 psi per foot of elevation gain (1 ft of water head = 0.433 psi)
- Sizing rule of thumb: Use the next larger standard pipe size when calculated velocity exceeds recommended limits
- Hot water recirculation: Typical flow rates are 1-3 GPM for residential loops
Related Calculators
Note: This calculator uses the Hazen-Williams equation, which is empirically derived for water flow at typical temperatures (40°F–75°F). For fluids other than water, high temperatures, or very small pipes, the Darcy-Weisbach equation may be more appropriate. Results are approximations and should be verified by a licensed engineer for critical applications. Always follow local plumbing codes and manufacturer specifications.
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