The Water Units That Cause Real Errors, in One Converter

The Problem

Water and wastewater work runs on a fistful of units that don't play nicely together: flow in cfs, GPM, and MGD; volume in gallons, cubic feet, and acre-feet; rainfall as a depth that has to become a volume over an area. A generic unit converter doesn't know that a gallon of water weighs 8.34 pounds, and the mistakes here aren't rounding errors — they're factor-of-694 pump-sizing catastrophes.

The Approach

A domain converter across flow, volume, intensity, area, and rainfall-over-area, with live equivalents tables, plus a dedicated loading calculator built around the one equation plant operators use more than any other [2]:

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 rainfall category extends a depth to a volume over an acreage, closing the loop with the drainage tools.

Converter showing a flow value with live equivalents across cfs, GPM, MGD, and acre-feet per day, plus the 8.34 loading calculator
Figure 1: One flow entry, every equivalent at once — and the 8.34 loading calculator alongside.
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.”
  • 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
The gotchas that cause the real errors
  • MGD vs. GPM is the classic pump-sizing catastrophe — a factor of 694. If a station “needs 2” of something, make everyone say the unit out loud.
  • Acre-feet vs. acre-inches: detention volumes get quoted in both, and a factor of 12 hides in the paperwork.
  • cfs is instantaneous; MGD usually means daily average. Converting between them is arithmetic; equating a peak to an average is a design error.

Single file, vanilla JS, localStorage persistence. All gallons are US gallons; feet here are the ordinary international foot, which is fine for volumes — the survey-foot distinction matters for coordinates, not tanks (see the Coordinate Converter for where it does) [1].

The tool: Hydrology & Water/Wastewater Unit Converter.

References

  1. [1] NIST Handbook 44, Appendix C — general tables of units of measurement.
  2. [2] Any water/wastewater operator certification manual — the 8.34 formula and its uses.