About WaterHeaterCalcs
WaterHeaterCalcs is an independent project with one goal: to gather the everyday calculations homeowners, DIYers and small plumbers reach for around a domestic water heater — sizing, operating cost & energy, replacement & installation cost, repair cost, type selection, and planning & reference — in one focused, free, no-signup hub with transparent formulas, so you can size a water heater, budget a job and sanity-check a plumber’s quote.
Who is behind it
Author and curator
To be clear about credentials: I am the author and curator of this site — not a licensed plumber, a gas fitter or any trade professional, and I claim no trade credential. What I bring is relevant and real: building deterministic online calculators (open-source Python projects) and electronic-engineering training (Systems Theory, Sapienza University of Rome). Water-heater sizing is first-hour-rating and flow/temperature math (FHR = usable storage + recovery; tankless GPM = Σ fixture flow at ΔT = output − inlet), energy is closed-form thermodynamics (BTU = gallons × 8.33 × ΔT; kWh = BTU ÷ 3412 ÷ UEF; therms = BTU ÷ 100000 ÷ UEF; recovery = input × efficiency ÷ (8.33 × ΔT)) and cost is a quantity × unit-price sum (cost = parts/unit + labor + add-ons, ×(1 + contingency)) — the same rigorous arithmetic I apply in systems and electronics, so the math of these calculators is squarely in my competence, while combustion, gas-line, venting, temperature-&-pressure-relief judgement, the scald / Legionella tradeoff and code compliance are explicitly deferred to licensed plumbers / gas fitters, the manufacturer’s instructions and local code.
Our principle: transparent & durably correct
Every calculator shows its formula, a worked example and a reference table. The tools rest only on timeless thermodynamics and sizing identities (BTU = gallons × 8.33 × ΔT; recovery = input × efficiency ÷ (8.33 × ΔT); FHR = usable storage + recovery; cost = parts/unit + labor + add-ons, ×(1 + contingency)) and stable, labeled conventions (DOE hot-water use per fixture, gallons + FHR by household, UEF and lifespan by type, inlet temperature by region, cost bands). There are deliberately no live utility rates, no equipment or labor prices, no regional cost indexes, no product catalog and no plumber directory — cost tools use the prices you enter — so the results stay valid over time.
Correctness is checked against known reference values (see the methodology and the numeric self-check). The formulas and their basis are documented under Sources & formulas. All results are planning estimates and sizing / energy guides: get itemized written quotes from licensed, insured plumbers, confirm your unit’s rated First-Hour Rating, GPM and UEF on its EnergyGuide label, and follow the manufacturer’s instructions and local code. Combustion, gas-line, venting, the temperature-&-pressure relief valve, the scald / Legionella tradeoff and code compliance are out of scope and are for a licensed professional, the manufacturer’s instructions and local code. Questions? Use the contact page.