Some civil, open-channel, and older water-supply notes list flow in cubic feet per second (cfs) while U.S. fire-flow and sprinkler sheets usually use gallons per minute (gpm). Using U.S. liquid gallons, 1 cfs = 448.831 gpm (exactly 7.48051945 gal/ft³ × 60 s/min), so cfs × 448.831 = gpm and gpm ÷ 448.831 = cfs. This page is the inverse companion to the free GPM to CFS calculator: same teaching constant, same browser tool, either direction. 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 (cfs on some civil / open-channel notes; gpm on most U.S. fire-flow and sprinkler sheets).
- Convert with the freshwater U.S. gallon factor: Qgpm = Qcfs × 448.831, or Qcfs = Qgpm / 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, tank duration, 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 the same conversion starting from gpm see GPM to CFS; for hydrant supply sketches see the hydrant flow test calculator; for tank run-time see the water supply duration calculator. Multi-segment paths live in the Sprinkler Pipe Friction Loss workbook.
Worked example
A civil note lists 2.0 cfs available: 2.0 × 448.831 ≈ 897.7 gpm. Conversely, 1,000 gpm ÷ 448.831 ≈ 2.23 cfs. 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: convert the other way with the free GPM to CFS calculator, sketch supply with the hydrant flow test calculator or residual pressure calculator, estimate tank run-time with the water supply duration calculator, 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).