About HeaterMath

HeaterMath answers one question, what size water heater a given household needs, with published worksheet numbers and open arithmetic, run by the same code in the calculators and in the static tables. This page lists every constant so you can check our work, and says plainly what the site is not.

The tank method

Peak hour demand comes off the AHRI/DOE worksheet: 20 gallons of hot water per shower, 20 per bath, 2 per shave, 4 each for hand-washed dishes, a sink hair wash, or hands-and-face washing, 14 per dishwasher load, 32 per hot-wash laundry load. The worksheet assumes no conservation fixtures; its 20-gallon shower is a 2.5 GPM head running 10 minutes with about 79% of the mix drawn hot. Our low-flow alternative is the same arithmetic at 2.0 GPM and 8 minutes: 13 gallons, our derivation. The recommendation is then the smallest standard tank whose first hour rating covers the demand.

Since spec-sheet FHRs vary by model, our tables approximate FHR as 70% of tank volume plus one hour of recovery at the standard 90°F test rise. Recovery is physics: input BTU × efficiency ÷ (8.33 × rise). We use 80% recovery efficiency for atmospheric gas tanks and 98% for immersed electric elements, with standard inputs of 32,000 to 75,000 BTU for gas by size and 4,500 W (15,354 BTU) for electric. The approximation lands within a few gallons of published Rheem and AO Smith labels and errs slightly small on electric, the safe direction. Always confirm against the EnergyGuide label in front of you.

The tankless method

DOE's: add the flows that can run at once, multiply by the rise from winter inlet water to 120°F. Delivered heat needed is 500 × GPM × rise. Fixture flows: 2.5 GPM per shower (the federal maximum head), 4 for a filling tub (spouts run 4 to 7; we plan low), 1.0 bathroom sink, 1.5 kitchen sink, 1.5 dishwasher, 2.0 clothes washer. Gas unit classes assume 82% efficiency, mid-range for non-condensing units; condensing units around 95% buy you one size class of margin. Electric kW is the same heat restated: GPM × rise × 500 ÷ 3412.

Inlet water presets (42, 50, 58, 66, 72°F) are typical winter figures by region; DOE's own default is 50°F. A thermometer on your February cold tap outranks all five.

What this site is not

A quote, a code review, or an install spec. Venting, gas line sizing, pan and drain requirements, seismic strapping, and permit rules are local and physical, and a plumber standing in your mechanical room is the only calculator for them. Our numbers size the purchase; they do not install it. Nothing here estimates operating cost or advises on money.

Who runs this

HeaterMath is one of a family of small single-topic calculators. It takes no money from manufacturers; if referral links appear, they are marked sponsored and never change the math. Corrections are the most useful mail we get: if a label in your hand disagrees with a table here, the contact page is open.