Flow Rate Calculator — GPM, L/min, m³/h

Two people asking for a flow rate usually mean two different measurements. One has a bucket and a stopwatch. The other has a pipe size on a drawing and a velocity they were told to design to. Both arrive at the same number by different routes, so this tool keeps them as separate methods and reports the answer in ten units at once.

Whichever route you take, adding the inside diameter turns the volume answer into a velocity — and velocity is the figure that actually tells you whether a line is sized sensibly. Pick a fluid and you get mass flow as well, which is what heat and pump duty calculations want rather than litres.

Worked Example: the 10-Litre Bucket Test

The tool opens on the lead sample, Volume ÷ time with 10 L collected in 45 s from a 15 mm bore carrying water:

• Flow rate: 13.333 L/min, which is 3.522 US gal/min, 2.933 UK gal/min, 0.8 m³/h and 0.47086 CFM
• Velocity: 1.26 m/s through a bore area of 176.7 mm², and the advisory comes back green
• Mass flow: 0.22182 kg/s, or 798.56 kg/h at water’s 998.2 kg/m³

Press Load Sample again and it moves on to a 100 mm main sized by velocity, a 2 in line measured in US gallons, a diesel line in feet per second, and an air duct.

How It Calculates Velocity from Bore

Volumetric flow is Q = V / t on the timed-fill route and Q = A × v on the other, where the cross-section is A = π(d/2)² from the inside diameter. Run it backwards and velocity is v = Q / A, which is why a bore is optional on the first method but required on the second. Leave it out there and you get “Bore × velocity needs an inside diameter. Measure across the inside of the pipe, not the outside.” Mass flow is density times volume, so it is the only output that changes when you switch fluids: the same 13.333 L/min is 0.22182 kg/s as water and 0.00026756 kg/s as air, a factor of 829.

Because area goes with the square of the diameter, bore matters far more than it looks. Keep that 13.333 L/min and narrow the pipe from 15 mm to 10 mm and velocity climbs from 1.26 to 2.83 m/s — past the 2.4 m/s top of the working band and into the range where copper starts to sing.

Precision, Densities and the Velocity Bands

Results carry four significant figures, so 0.22222 L/s is shown in full rather than rounded to 0.22. Densities are single values at roughly 20 °C and one atmosphere — fine for water, rough for a gas, whose density moves with both pressure and temperature, so treat the air and natural gas mass flows as order-of-magnitude. The feature most people miss is that the velocity advisory swaps its thresholds with the Fluid picker: a liquid is judged against 0.5–2.4 m/s with 3 m/s as the ceiling, while a gas is judged against 3–10 m/s with a 15 m/s ceiling, because ductwork tolerates speeds that would wreck a service pipe. Nominal pipe sizes are not bores either: 15 mm copper measures about 13.6 mm inside, and using 15 mm overstates the area by roughly 22%.