Flow Rate Calculator

Volumetric flow rate (often written Q) is the volume of fluid passing a point per unit time. In US plumbing and HVAC, the everyday unit is gallons per minute (GPM); in SI it is cubic meters per second (m^3/s), with liters per minute (L/min) and cubic feet per hour (cfh) common for water and gas. This calculator converts between all four so you can switch from a fixture-unit chart in GPM to a meter spec in m^3/s without doing the unit math by hand.

Q = A * v is the design formula: pick a pipe size (which sets the cross-sectional area A) and a target velocity v, and you get the flow you can move through that pipe. Q = V / t is the measurement formula: catch a known volume V in a bucket, time it with a stopwatch, and divide. Use the area-velocity mode when you are sizing a new line or checking against an 8 ft/s plumbing speed limit; use the volume-time mode when you actually have a hose bib in front of you. The mass-rate mode (Q = m_dot / rho) is the version used with Coriolis meters and weigh-tank tests, where you measure how fast mass accumulates and divide by fluid density.

Nominal pipe sizes (NPS) do not match the actual inside diameter. A nominal 2 in Schedule 40 steel pipe has an ID of 2.067 in; the same nominal 2 in in Schedule 80 is only 1.939 in. Copper Type L tube nominally 2 in is 1.985 in ID. Plug the wrong number in and the cross-sectional area is off by 5 to 15 percent, which pushes a 10 GPM design straight past its velocity limit. Always look up the inside diameter for your specific material and schedule, then enter that here.

Above about 8 ft/s (2.4 m/s) for cold water and 5 ft/s for hot, copper pipes start to suffer accelerated erosion-corrosion at fittings, plastic risks water-hammer damage, and noise becomes audible through walls. Codes like the IPC and UPC do not hard-code the number, but the 2018 Uniform Plumbing Code commentary and Copper Development Association handbook both cite 8 ft/s cold / 5 ft/s hot as the design ceiling. The calculator does not flag this for you, but if you back-solve a target GPM through a small pipe and the velocity output is above 8 ft/s, step up to the next pipe size.

All four scale linearly from m^3/s as the SI base. The factors used here are: 1 m^3/s = 15,850.32 US GPM, 1 m^3/s = 60,000 L/min, and 1 m^3/s = 127,132.8 ft^3/hr (cfh). Going the other way, 1 GPM is about 0.06309 L/s and 8.0208 ft^3/hr; 1 L/min is about 0.2642 GPM. The calculator reports all four for any input so you do not need to remember the chain. For natural-gas sizing in particular, watch the units: gas tables almost always quote cfh (cubic feet per hour at standard conditions), while pump curves and chillers usually quote GPM.