Flow unit conversions show up when a water-supply table, hydrant report, or pump curve mixes gallons per minute (gpm) with cubic feet per second (cfs). Using U.S. liquid gallons, 1 cfs = 448.831 gpm (exactly 7.48051945 gal/ft³ × 60 s/min), so gpm ÷ 448.831 = cfs and cfs × 448.831 = gpm. This page explains that sketch and gives a free browser calculator that converts either way. It is for learning and quick checks—not a substitute for the criteria adopted by your authority having jurisdiction, manufacturer data, a sealed hydraulic calculation, or review by a licensed design professional.
How the conversion is used
- Confirm which unit your source report uses (gpm is common on U.S. fire-flow and sprinkler sheets; cfs appears on some civil / open-channel and older supply notes).
- Convert with the freshwater U.S. gallon factor above: Qcfs = Qgpm / 448.831, or Qgpm = Qcfs × 448.831.
- Keep the same flow meaning (rated pump flow, test flow, or demand) when you switch units—do not mix residual pressure from one test with a converted flow from another without checking the report.
- After units match, continue with friction loss, hydrant, or pump sketches in consistent units.
Free calculator
Educational approximation only. Uses 1 cfs = 448.831 gpm (U.S. liquid gallons). Nothing you enter leaves your browser. For hydrant supply sketches see the hydrant flow test calculator; for pipe friction see the friction loss calculator or the fuller Hazen-Williams calculator. Multi-segment paths live in the Sprinkler Pipe Friction Loss workbook.
Worked example
A hydrant report lists 1,000 gpm available at residual pressure: 1,000 ÷ 448.831 ≈ 2.23 cfs. Conversely, 2.0 cfs × 448.831 ≈ 897.7 gpm. Always confirm whether the source used U.S. liquid gallons (this page) versus another gallon definition, and check the result against the adopted edition, manufacturer data, and AHJ—not this page alone.
Next steps: sketch supply with the free hydrant flow test calculator or residual pressure calculator, convert pressure and head with PSI to feet of head, size segment losses with the friction loss calculator, and build multi-segment paths in the Sprinkler Pipe Friction Loss workbook ($29). Pump and tank tables live in the Fire Pump & Tank Sizing workbook ($49).