Methodology

This page explains how the SepticCalcs calculators are derived and verified — and why they need no ongoing maintenance to stay correct.

1. Timeless math, stable conventions

Every tool computes from a closed-form formula: tank capacity = L × W × liquid depth × 7.48; design flow = occupants × gpd/person; absorption area = design flow ÷ soil loading rate; trench length = area ÷ effective width, laterals = ceil(length ÷ max lateral); pumping frequency = usable volume ÷ (occupants × accumulation); well yield = gallons ÷ fill-seconds × 60; pressure-tank drawdown = run-time × GPM, tank = drawdown ÷ acceptance factor; softener grains/day = hardness gpg × daily gallons, regen interval = capacity ÷ grains/day; project cost = quantity × your unit price; cost per unit = your total ÷ quantity. The only baked-in numbers are stable conventions — 7.48 gallons per cubic foot; 27 cubic feet per cubic yard; ppm ÷ 17.1 = gpg; psi × 2.31 = feet of head; tank-size-by-bedroom, perc → soil-loading and pressure-tank acceptance planning bands. These do not drift, so the statements stay true over time.

2. No prices, no feeds

There is deliberately no material, drilling or service price, no labor-rate table, no regional cost index, no live loan rate and no product catalog. Every cost tool works on the prices you enter from your own quotes and bills ($/ft of drilling, $/gallon of pumping, $/sq ft, $/lb of salt, per-unit price, labor $/hour). That is why the site is correct regardless of what materials, labor, drilling or service prices do.

3. Numeric self-check

Every formula is asserted against a worked example with known numbers (for instance: an 8 × 5 × 4 ft tank holds ≈ 1,197 gallons; a 300 gpd load at 0.5 gpd/sq ft needs 600 sq ft of drain field; a 600 sq ft field at 3 ft width is 200 LF of trench in 2 laterals; a 10 GPM pump with a 1.5-minute run and a 30/50 switch wants a 50-gallon pressure tank; 20 gpg × 300 gallons/day is 6,000 grains/day). A release gate runs all of these and fails on any mismatch, so "verification" here is mathematical correctness plus accurate conventions — not a time-based check.

4. Estimate, not a bid, design or water test

The tank-size, perc → soil-loading, trench, pressure-tank and per-person values are labeled planning bands — a starting point, not an engineering spec. Every cost result is a planning estimate: get itemized written quotes from licensed contractors, and treat septic design, drain-field sizing and well siting as licensed-designer / professional-well-driller, local-health-department, permit-and-inspection work. Treatment tools are about sizing and cost, not safety — test your well water with a certified lab. The well-vs-city ROI is illustrative math on your figures, not financial advice. We make no water-safety or potability claims.