Calculate water flow rate from pipe diameter and flow velocity.
How to Use the Water Flow Calculator
Enter the "Pipe Diameter (mm)" — use the internal (bore) diameter of the pipe.
Enter the "Flow Velocity (m/s)" of the water moving through the pipe.
Click "Calculate" to see the resulting flow rate in litres per minute and US gallons per minute.
Worked Example: 25 mm Pipe at 1.2 m/s
Convert the diameter to metres: 25 mm = 0.025 m, so the radius is 0.0125 m.
Area = π × 0.0125² ≈ 0.000491 m².
Flow = 0.000491 × 1.2 ≈ 0.000589 m³ per second.
Multiply by 60,000 to get litres per minute: about 35.34 L/min, which the calculator also shows as 9.34 US GPM.
Other useful forms of the same answer: 35.34 L/min × 60 ≈ 2,121 litres per hour, or about 2.12 m³/h, the unit pump catalogues often use.
How Long Will It Take to Fill a Tank?
Divide the tank capacity in litres by the flow in L/min to get the filling time in minutes. At 35.34 L/min, a 1,000 litre overhead tank fills in about 28 minutes. Common combinations:
Flow (L/min)
500 L tank
1,000 L tank
10
50 min
100 min
20
25 min
50 min
30
16.7 min
33.3 min
40
12.5 min
25 min
60
8.3 min
16.7 min
Actual filling time is usually longer, because pump output drops as the delivery height increases and municipal supply pressure varies through the day.
When You Don't Know the Velocity
This calculator needs a velocity, which is rarely known on site. Often it is easier to measure the flow directly and work backwards. Time how long an outlet takes to fill a bucket of known size: if a 10 litre bucket fills in 40 seconds, the flow is 10 ÷ 40 × 60 = 15 L/min.
To find the velocity, divide the flow in m³/s by the pipe area. 15 L/min through a 15 mm bore is 0.00025 m³/s ÷ 0.0001767 m² ≈ 1.41 m/s. Velocities well above 2 to 3 m/s in household pipes tend to cause noise and water hammer, which suggests a larger pipe.
If you know the pressure and pipe length rather than the velocity, use the Pipe Flow Rate Calculator, which estimates flow from friction losses instead.
Frequently Asked Questions
It uses the continuity equation: Flow Rate = Cross-Sectional Area × Velocity, where the area is calculated from the pipe's internal diameter (Area = π × r²).
Cold water supply pipes are usually designed for 1.0-2.0 m/s, while hot water pipes are often kept lower (around 0.9-1.2 m/s) to reduce noise and heat loss.
It should be the internal (bore) diameter of the pipe, since that's the actual area through which water flows.
Many pump, filter and fixture specifications, especially imported ones, quote flow in US GPM. One US gallon is about 3.785 litres, so the tool multiplies L/min by 0.264172 for easy comparison.
At the same velocity, the flow becomes four times larger because the cross-sectional area depends on the diameter squared. In practice velocity also changes with pipe size, so use this as a comparison rather than a prediction.