Methodology

This page explains how the PipeCalcs 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: tankless load = GPM × 500 × temperature rise (electric kW = GPM × ΔT × 0.1465); recovery = BTU/hr × efficiency ÷ (8.34 × ΔT); pressure head = psi × 2.31; pipe volume = 0.0408 × diameter² × length; drain fall = length × slope; gas CFH = BTU/hr ÷ heating value; total gas load = Σ appliance BTU/hr; project cost = quantity × your unit price; cost per fixture = your total ÷ fixtures. The only baked-in numbers are stable conventions — 7.48 gallons per cubic foot; 1 psi = 2.31 ft of head; nat-gas 1000 / propane 2516 BTU per cubic foot; 1 kWh = 3412 BTU; WSFU → peak-GPM, DFU → drain/vent-size and gas-capacity planning bands; drain slope 1/4 in per foot. These do not drift, so the statements stay true over time.

2. No prices, no feeds

There is deliberately no material 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 ($/fixture, $/ft, $/hour, $/gallon). That is why the site is correct regardless of what materials, labor or service prices do.

3. Numeric self-check

Every formula is asserted against a worked example with known numbers (for instance: a 4 GPM tankless heater at a 45 °F rise needs 90,000 BTU/hr or 26.4 kW; 20 water-supply fixture units is about 14 GPM → a 3/4-in supply line; 10 drainage fixture units wants at least a 3-in branch; a 200,000 BTU/hr gas load over a 40-ft run is about a 1-in line; a 12-fixture repipe at $150/fixture plus 24 hours of labor and a 10% contingency is about $4,521). 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 code approval

The fixture-unit, pipe-size, gas-capacity, drain-slope and flow 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 plumbers. Pipe, drain, vent and gas sizing follows standard conventions, but your local plumbing code and inspector govern — confirm with a licensed plumber and pull the required permits. All gas work must be done by a licensed professional with a permit and inspection; gas leaks are dangerous. Savings and comparison tools are illustrative math on your figures, not financial advice.