Sources & formulas

Every calculator on SepticCalcs rests on established arithmetic and geometry plus stable septic and well conventions. Unlike topics with a time dependency, "verification" here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Geometry & volume

  • Tank / pump-out capacity: gallons = length × width × liquid depth (ft) × 7.48.
  • Gravel volume: cubic yards = LF × width × depth (ft) ÷ 27.
  • Units: 12 in = 1 ft; 27 cu ft = 1 cu yd; 231 cu in = 1 gallon; 1 psi = 2.31 ft of head; ppm ÷ 17.1 = grains per gallon.

Septic sizing (labeled planning bands)

  • Design flow: occupants × gpd/person (default 75 gpd, user-adjustable).
  • Absorption area: design flow ÷ soil loading rate (from the perc test).
  • Perc → loading: faster soil (fewer min/inch) → a higher gpd/sq ft loading rate — the local health department’s table governs.
  • Trench: length = absorption area ÷ effective width (~3 ft); laterals = ceil(length ÷ max lateral ~100 ft).
  • Tank minimum by bedrooms: ~750 gal (2 bed) → 1,500+ gal (5 bed) — set by your local health department.
  • Pumping frequency: years ≈ usable volume ÷ (occupants × ~60 gal/person/yr accumulation).

Well pump, pressure & yield

  • Peak demand: fixtures × ~1.5 gpm/fixture (planning convention).
  • Total dynamic head: pumping water level + pressure head (psi × 2.31) + friction loss.
  • Pressure tank: drawdown = run-time × pump GPM; tank = drawdown ÷ acceptance factor (20/40 ≈ 0.20, 30/50 ≈ 0.30, 40/60 ≈ 0.40).
  • Yield (bucket test): GPM = gallons ÷ fill-seconds × 60.

Water treatment, cost, budget & ROI

  • Softener: grains/day = hardness gpg × daily gallons; regen interval = grain capacity ÷ grains/day.
  • Every cost tool uses the prices you enter ($/ft, $/gallon, $/sq ft, $/lb, per-unit, labor $/hour) — no material, drilling or service price, labor rate, cost index or live loan rate is stored, so the site needs no maintenance.
  • Cost per unit: your total ÷ quantity (e.g. $/ft of well = total ÷ depth; $/gallon of capacity = total ÷ gallons).
  • Well-vs-city ROI: well monthly = your well total ÷ (life years × 12); savings = your city monthly − well monthly — illustrative only.

Permit rules, setbacks, local code and health-department requirements vary by place and change over time — always confirm sizing, setbacks, permits and code with your local health department, have septic design and well siting verified by a licensed designer or professional well driller, and test well water potability with a certified laboratory.