Flow Rate Calculator
Calculate flow rate from pipe parameters and convert between common flow units.
💧 Flow, Velocity and Bore Are One Equation
Flow rate, water speed and pipe bore are locked together by continuity: flow equals speed times cross-sectional area. Fix any two and the third follows. The first tab solves for flow given a speed and a size, the third solves for speed given a flow and a size, and the middle tab is a straight unit conversion with no hydraulics in it at all.
Formula Reference
Bore, not nominal size, is what carries the water. Pick the material and this calculator uses that material's own dimensions: copper Type L, PEX SDR-9, CPVC CTS SDR-11, and Schedule 40 for PVC, galvanized and steel. At ¾ inch nominal that is 0.671 in of bore in PEX against 0.824 in in Schedule 40 — and area goes as the square, so the same velocity moves half as much water again through the steel.
The conversions are exact by definition rather than measured. The US gallon is defined as 231 cubic inches, which fixes 7.48052 gallons per cubic foot; the litre conversion rests on the international inch of exactly 25.4 mm, giving 3.785411784 litres per gallon.
📐 The Conversions, and Why They Are Exact
Every constant in the converter tab derives from two definitions and nothing else, which is why they can be quoted to as many decimals as you like:
- 1 GPM = 3.785411784 LPM — from the US gallon of 231 in³ and the inch of exactly 25.4 mm
- 1 GPM = 0.133681 CFM — 231 ÷ 1728
- 1 GPM = 0.227125 m³/hr — 3.785411784 × 60 ÷ 1000
- 1 GPM = 60 GPH, and 1 LPM = 1/60 L/s
Watch the difference between US and imperial gallons if you are working from British or Canadian data: an imperial gallon is about 20% larger, and nothing on this page uses it.
🚿 What Fixtures Actually Draw
Working flow rates for common fixtures are capped by federal law rather than by convention. Showerheads manufactured after 1 January 1994 may not exceed 2.5 gallons per minute measured at 80 psi; lavatory and kitchen faucets are capped at 2.2 GPM at 60 psi; water closets at 1.6 gallons per flush and urinals at 1.0. An EPA WaterSense labelled showerhead must do better still, at no more than 2.0 GPM.
Two things follow. Those numbers are ceilings measured at a stated pressure, so a fixture on a starved line will deliver less than its rating. And they are manufacturing standards, not installed reality — an older head predating 1994 can easily draw twice a modern one, which is worth knowing before you size anything around it.
⚠️ Reading the Velocity Result
The velocity tab compares your result against 8 ft/s for copper and the plastics and 6 ft/s for steel and galvanized. The 8 figure comes from the Copper Development Association, which advises designers not to exceed 8 feet per second in cold water, 5 feet per second in hot water up to about 140°F, and 2 to 3 feet per second where water routinely runs hotter. The tool applies the cold figure to hot lines as well, so on a hot line judge the result against 5 ft/s. In practical terms 3/4-inch Type L copper reaches 8 ft/s at about 12 GPM and 5 ft/s at about 7.5 GPM.
❓ Questions People Actually Ask
How to calculate GPM flow rate?
If you know the water speed and the pipe bore, multiply the velocity in feet per second by the cross-sectional area in square feet and then by 448.831, which is the number of gallons per minute in one cubic foot per second. Area is pi times the square of the bore in inches divided by 24. If you do not know the velocity, measure instead: time how long a container of known volume takes to fill and divide, which is the only method that accounts for whatever the tap, the meter and the run are actually doing. Calculated flow assumes a bore and ignores every restriction between the source and the outlet.
How many gpm can a 3/4 pipe flow at 60 PSI?
Pressure alone does not set flow — the length of the run, the fittings, and what is at the end all decide it, so the same 3/4-inch pipe at 60 psi delivers very different flows over 20 feet and over 200. What can be stated is the sensible ceiling: 3/4-inch Type L copper has a 0.785-inch bore, so at the Copper Development Association's 8 ft/s cold-water limit it carries about 12 gallons per minute, and pushing more than that means noise and erosion-corrosion rather than useful flow. At around 10 GPM that tube loses roughly 0.098 psi per foot, so 60 psi is entirely consumed by about 600 feet of straight run before any fitting or lift is counted.
How to calculate gpm to gph?
Multiply by 60, since an hour contains 60 minutes: 5 GPM is 300 GPH, and 12 GPM is 720 GPH. Going the other way, divide by 60. There is no rounding or approximation involved, unlike conversions between US and metric units, and unlike anything to do with imperial gallons, which are about 20 percent larger than US gallons and are not used anywhere on this page.
Calculated flow and velocity assume a clean pipe of the stated bore and account for nothing else — no meter, no valves, no fittings, no scale, no pressure loss along the run. A real tap will deliver less. Measure if the answer matters.
The velocity comparison is design guidance, not a code check, and it applies to copper. This does not replace a licensed plumber or an engineered design.
Maximum fixture flow rates are set in federal law at 10 CFR 430.32, paragraphs (o) for faucets, (p) for showerheads, (q) for water closets and (r) for urinals, including the 80 psi and 60 psi pressures at which they are measured; the stricter 2.0 GPM figure is the EPA WaterSense showerhead specification. The velocity limit for copper and its Type L bores are from the Copper Development Association's Copper Tube Handbook (CDA publication A4015-26/24), which also publishes the friction loss figure quoted above. The conversion constants come from the definition of the US liquid gallon as 231 cubic inches and the international inch of exactly 25.4 mm. To turn a flow into a pipe size, use our pipe sizing calculator; to find where the pressure went, the pressure drop calculator. The questions above came from Google's People Also Ask panel for "flow rate calculator gpm" on 31 July 2026. Bores for the other materials come from their own specifications: PEX from ASTM F876 CTS-OD SDR-9 as published in manufacturers' technical data, CPVC from ASTM D2846 CTS SDR-11, and PVC, galvanized and steel from Schedule 40 — identical walls and outside diameters in ASTM D1785 and ASME B36.10M. Cast iron is the one approximation: supply-side bore varies by class and era, so it borrows the Schedule 40 figures.