Septic & well unit conversions
These are the fixed unit conversions the calculators use — not prices, not conventions that drift. A cubic foot holds 7.48 US gallons; a cubic yard is 27 cubic feet; pressure and head relate as 1 psi = 2.31 ft; and hardness converts as ppm ÷ 17.1 = gpg. Apply them in the tank capacity, total dynamic head and softener sizing tools.
| Unit | Equals | Used for |
|---|---|---|
| 1 cubic foot | 7.48 US gallons | Tank capacity = L × W × liquid depth × 7.48 |
| 1 cubic yard | 27 cubic feet | Gravel volume in a drain-field trench |
| 1 US gallon | 231 cubic inches | Fixed volume conversion |
| Hardness | ppm ÷ 17.1 = gpg | Softener sizing (as CaCO₃) |
| Pressure ↔ head | 1 psi = 2.31 ft | Total dynamic head from pressure |
| 12 inches | 1 foot | Depth conversions |
Fixed unit conversions, not prices or drifting conventions — the arithmetic backbone of the calculators.