Water Heater Sizing Calculator
Two honest ways to size it: the tank way (what the busiest hour drinks, against the First-Hour Rating) and the tankless way (what runs at the same moment, at your temperature rise).
Per-use gallons and fixture flows are DOE-style averages. Final selection — and all venting and gas-line questions — belong to a licensed plumber looking at your house.
How the math works
Tank: peak-hour demand = the draws in your busiest hour at DOE-style per-use figures — shower 10 gal, tub bath 20, dishwasher 6, washer load 7 — plus your “other” gallons. That total is the First-Hour Rating (FHR) the tank must carry; the suggestion table pairs tank sizes with typical FHR figures (30 gal ≈ 45, 40 ≈ 55, 50 ≈ 70, 65 ≈ 85, 80 ≈ 100, 100 ≈ 115) and picks the first tank whose FHR covers your hour.
Tankless: add the flow rates of everything running at once, and state your temperature rise (desired outlet − incoming groundwater; 55°F is a fair Long Beach groundwater default). A unit’s rating collapses as rise grows — the page converts your need into the equivalent rating at the industry’s 35°F reference rise so spec sheets compare honestly. Per-use gallons and flows are averages; a licensed plumber makes the final call, and venting and gas-line questions are theirs, not this page’s.
A worked example
Peak hour with 3 showers, 1 bath, a dishwasher and a washer load, plus 5 other gallons: 68 gallons — a required FHR of 68, covered by a 50-gallon class tank (typical FHR ≈ 70). Tankless at the same house: two showers (2.0 GPM each) and a faucet (1.0) = 5.0 GPM at a 65°F rise — shop for a unit rated at least 9.29 GPM at the 35°F reference rise.
Questions from the field
What size water heater does a family of four need? Size the busiest hour, not the headcount. A 68-gallon peak hour lands on a 50-gallon tank by First-Hour Rating. Bigger families with staggered showers often need less tank than smaller ones who all shower at 7 AM.
What is First-Hour Rating? How many gallons of hot water the tank can deliver in its busiest first hour — stored water plus what it reheats during that hour. It is printed on the yellow EnergyGuide label, and it is the number to shop by, not raw gallons.
How do I size a tankless unit? Add up the fixtures that run simultaneously and your temperature rise. 5.0 GPM at a 65°F rise needs a unit rated about 9.3 GPM at the standard 35°F rise — ratings are quoted at the easy rise, demand happens at yours.
Does this cover gas line or venting? No. Tankless conversions especially can need gas and vent work; that is a licensed plumber’s determination, and this page stays out of it.
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