Flow Rate Converter

Enter a flow rate and the unit it's measured in, then read the equivalents below. Liters per minute, gallons per minute (US and imperial), cubic meters per hour, cubic feet per second — they all stay in sync.

Useful when reading a pump spec sheet in metric while sizing a system in imperial, or comparing fire-flow numbers from one jurisdiction's hydraulics manual against another's. The conversion factors are constants drawn from the standard NIST tables.

Watch the unit gallons. US and imperial gallons differ by about 20 percent — selecting the right one matters more than the math.

Flow rate is volume per time, so the conversions are products of two ratios — the volume conversion and the time conversion. One US gallon equals exactly 3.785411784 liters (defined by the US gallon being 231 cubic inches and the inch being 2.54 cm). One imperial gallon is 4.54609 liters (defined since 1985). The two gallons differ by about 20%, which is the source of more confusion than any other unit mismatch in plumbing and pump specs. The tool labels each gallon explicitly as US or imperial; never assume.

Worked example. A pump spec sheet says it delivers 100 GPM. If that's 100 US GPM, the metric equivalent is 378.5 L/min, 22.7 m³/h, or 6.31 L/s. If the spec is in imperial GPM (more common in UK and Commonwealth datasheets), the same number is 454.6 L/min, 27.3 m³/h. A 20% sizing error on a fire-water main is the kind of thing that fails inspection. The tool puts both flavors in the dropdown so you pick the one the datasheet actually meant.

For very small flows (medical IV pumps, lab dosing, drip irrigation), the natural units are mL/min or μL/min. The tool extends down to those scales without losing precision. For very large flows (rivers, municipal water supply), the natural unit is m³/s or cfs. The conversion holds across the whole range — internally, everything normalizes to m³/s with double-precision floats, then converts out to the display unit.

Three gotchas worth flagging. First, "cfs" and "ft³/s" are the same thing; "cfm" is cubic feet per minute and is sixty times smaller. Second, MGD (million gallons per day) shows up in US municipal documents — 1 MGD is 694.4 GPM (US) or 3.785 m³/min. Third, "head" and "flow" are not the same; pump curves quote both, and a pump rated for 100 GPM at 10 ft of head will deliver less than 100 GPM at 50 ft of head. The tool converts the flow number; the head/flow relationship belongs in your pump curve.