The conversions drainage reports and plant operations actually run on: flow across cfs/GPM/MGD/acre-feet, volumes, rainfall-over-area, intensity, area — plus the mg/L ↔ lb/day loading calculation with the 8.34 factor. Runs entirely offline; inputs persist in this browser via localStorage.
Equivalents
How the Numbers Are Derived
The factors worth memorizing anyway
1 cfs = 448.83 GPM = 0.6463 MGD ≈ 1.983 ac-ft/day — "a cfs is about two acre-feet a day" is the estimator's version.
1 MGD = 694.4 GPM = 1.547 cfs
1 ac-ft = 43,560 cf = 325,851 gal; 1 inch over 1 acre = 3,630 cf ≈ 27,154 gal
1 cf = 7.4805 gal; 1 m³ = 264.17 gal = 35.315 cf
lb/day = mg/L × MGD × 8.34 — because a gallon of water weighs 8.34 lb, and mg/L is parts per million by mass. The single most used equation in plant operations.
Unit gotchas that cause real errors
MGD vs. GPM confusion is the classic pump-sizing catastrophe — a factor of 694. If a lift station "needs 2" of something, make everyone say the unit out loud.
Acre-feet vs. acre-inches: detention volumes get quoted in both; a factor of 12 hides in the paperwork.
cfs is instantaneous, MGD often means daily average — converting between them is arithmetic; equating a peak to an average is a design error.
All gallons here are US gallons; all feet are the ordinary international foot (fine for volumes — the survey-foot distinction matters for coordinates, not tanks).
References
NIST Handbook 44, Appendix C — general tables of units of measurement.
Any water/wastewater operator certification manual — the 8.34 formula and its uses.