Sizing

The deterministic sizing cluster — the highest-demand water-heater queries: what size water heater do I need (the signature metric — the tank whose First-Hour Rating meets your peak-hour demand), a peak-hour hot-water demand calculator, a first-hour-rating calculator (FHR ≈ 0.70 × storage + recovery), a tankless GPM sizing calculator (Σ fixture flow at ΔT = output − inlet), a recovery-rate / reheat-time calculator (straight thermodynamics), size by household, and expansion-tank sizing. Confirm your unit's rated FHR and GPM on its EnergyGuide label; round up and leave peak headroom.

Typical planning values. Your real hot-water use, incoming water temperature, recovery and efficiency vary by household, unit, fuel and region — confirm your unit’s rated First-Hour Rating, GPM and UEF on its EnergyGuide label and the manufacturer’s instructions. Round sizing up, and leave headroom for peak demand.
What Size Water Heater Do I NeedSize a water heater by the DOE first-hour-rating method — add your busiest hour of hot-water use to get the tank gallons and First-Hour Rating you need.Peak-Hour Hot-Water DemandAdd up every hot-water draw in your busiest hour — showers, baths, shaves, dishwashers — to find the peak-hour demand that sets the First-Hour Rating you need.First-Hour-Rating CalculatorFirst-Hour Rating is the hot water a tank really delivers in hour one: usable storage plus recovery. Compute the FHR from your tank size and burner input.Tankless GPM SizingSize a tankless water heater by flow, not gallons: add the GPM of the fixtures you run at once at your temperature rise, and get the required GPM rating.Recovery Rate & Reheat TimeHow fast does a water heater reheat? Compute the recovery rate in gallons per hour and the reheat time for a full tank from the burner or element input and ΔT.Size by HouseholdGet the recommended storage tank gallons, First-Hour Rating and tankless GPM band for your household size, then confirm it with the peak-hour demand method.Expansion-Tank SizingSize a thermal-expansion tank for a closed system from your tank gallons, supply pressure and relief pressure, and get the acceptance volume you need.

Every Sizing tool is calculated server-side — visible immediately — with its formula, a worked example and a reference table, and works on the routine you measure and the prices you enter from your own quotes. Background reading is in the guides.