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.

Reference sources

The standards and conventions behind these calculators are published by IAPMO, International Code Council and Water Quality Association. We link them at section level rather than deep-linking a document: codes and guidance are revised over time, so the current edition on the publisher's own site is the authoritative wording.