Water Heater Sizing Calculator — First Hour Rating (FHR)

Size a storage water heater by ASHRAE First Hour Rating. Enter tank volume, BTU input, and recovery efficiency, then compare the result to peak-hour hot water demand for showers, dishwasher, and laundry.

By TradeCalc, Plumber Calculators — Code-Referenced — ASHRAE Handbook HVAC Applications (Service Water Heating), DOE 10 CFR 430, ASPE PDH Vol. 2, UPC Chapter 5 (Water Heaters)

Related Calculators

How to Size a Water Heater (ASHRAE First-Hour Rating)

What First Hour Rating Means

The First Hour Rating (FHR) is the gallons of hot water a storage water heater can deliver in the first hour of use, starting with a full tank at setpoint, without the outlet temperature dropping below 110°F. It is the DOE / ASHRAE standard capacity metric printed on every water heater's yellow EnergyGuide label. A 50-gallon tank does not deliver 50 usable gallons — only about 70% of the tank is hot enough before dilution drops the outlet below 110°F. FHR combines that usable storage with how fast the burner can reheat incoming cold water (the recovery rate).

The ASHRAE First-Hour Rating Formula

FHR = (tankGallons × 0.70) + recoveryRate

recoveryRate (gph) = (BTU_input × recoveryEfficiency) / (8.33 × temperatureRise)

tankGallons = nominal tank volume (gal)
0.70 = DOE usable-storage factor (only 70% is hot enough before outlet drops)
BTU_input = burner firing rate (BTU/hr)
recoveryEfficiency = 0.78 gas atmospheric, 0.95 condensing/power-vent, 0.98 electric
8.33 = lb of water per gallon (specific heat 1 BTU/lb·°F)
temperatureRise = setpoint − inlet (°F); typical 100°F (40°F inlet to 140°F setpoint)

Peak-Hour Hot Water Demand

ASHRAE Handbook (Service Water Heating) gives per-use hot water consumption: a 10-minute shower at 2.5 gpm ≈ 25 gal, a dishwasher cycle ≈ 10 gal, a hot clothes-washer cycle ≈ 15 gal, a lavatory use ≈ 2 gal. Total these for the peak hour (typically morning or evening) and size so FHR ≥ demand. A family of 4 typically peaks at 65–90 gal; a family of 6 at 100–130 gal.

Worked Example

Scenario: A family of 4 is sizing a replacement gas water heater. Tank is 50 gal, burner is 40,000 BTU/hr, recovery efficiency 0.78 (atmospheric gas), temperature rise 100°F (40°F inlet → 140°F setpoint). Peak hour: 2 showers, 1 dishwasher cycle, 1 hot laundry cycle.

Recovery rate = (40,000 × 0.78) / (8.33 × 100) = 31,200 / 833 = 37.5 gph
Usable storage = 50 × 0.70 = 35 gal
FHR = 35 + 37.5 = 72.5 gal
Peak demand = (2 × 25) + 10 + 15 = 75 gal
FHR 72.5 < demand 75 → 2.5 gal short ✗ (inadequate)

This unit is just barely undersized. Options: upsize to a 50-gal/65,000 BTU high-recovery model (FHR ≈ 96 gal), or switch to a 0.95-efficiency condensing 50-gal/60,000 BTU unit (FHR ≈ 103 gal), install 1.5 gpm low-flow showerheads to cut demand to 55 gal, or go tankless.

Practical Tips

Tank size alone is misleading. A 40-gal 65,000 BTU unit often has a higher FHR than a 50-gal 35,000 BTU unit, because recovery dominates the first hour. Always check the yellow EnergyGuide FHR number, not just gallons.
Gas recovery beats electric. A typical 50-gal gas unit at 40,000 BTU recovers ~37 gph; a 50-gal electric at 4,500 W recovers only ~22 gph (15,300 BTU/hr equivalent). Electric units need larger tanks for the same FHR.
Cold inlet varies by region. Inlet temperature ranges from 40°F (Maine winter) to 70°F (Florida). A 100°F rise assumes 40°F inlet; a 70°F inlet only needs 70°F rise, raising recovery 40% — southern units recover faster for the same BTU.
Tankless skips storage entirely and is sized on peak gpm instead of FHR. A 199,000 BTU tankless delivers ~5 gpm at 70°F rise — enough for 2 simultaneous showers. But tankless can't serve a high-peak-low-average home (5 showers + laundry at once) as well as a large storage tank.
Low-flow fixtures cut peak demand. Switching from 2.5 gpm to 1.5 gpm showerheads drops each shower from 25 gal to 15 gal — a 40% reduction in peak-hour demand that can let you downsize the tank.

Code References

ASHRAE Handbook HVAC Applications (Service Water Heating), DOE 10 CFR 430, ASPE PDH Vol. 2, UPC Chapter 5 (Water Heaters)

Frequently Asked Questions

What is First Hour Rating (FHR) on a water heater?

FHR is the gallons of hot water a storage heater can deliver in the first hour starting from a full hot tank, before the outlet temperature drops below 110°F. It is printed on the DOE EnergyGuide yellow label. FHR = (tank gallons × 0.70) + recovery rate (gph). The 0.70 factor reflects that only 70% of the tank is usable before dilution drops outlet temp; recovery is how fast the burner reheats cold inlet water.

How do I calculate water heater recovery rate?

Recovery rate (gallons per hour) = (BTU input × recovery efficiency) / (8.33 × temperature rise). For a 40,000 BTU gas atmospheric heater (efficiency 0.78) at 100°F rise: (40,000 × 0.78) / (8.33 × 100) = 37.4 gph. A 4,500 W electric element (15,300 BTU/hr) at 0.98 efficiency and 100°F rise recovers only ~22 gph — gas recovers nearly twice as fast.

What size water heater do I need for a family of 4?

A family of 4 typically peaks at 65–90 gal in the busiest hour (2 showers + laundry + dishes). A 50-gallon gas unit at 40,000 BTU has FHR ≈ 72 gal — adequate but tight. For comfort and future guests, most contractors recommend a 50-gallon 65,000 BTU high-recovery unit (FHR ≈ 99 gal) or a 75-gallon 40,000 BTU unit. A condensing tankless (199,000 BTU) delivers ~5 gpm continuously and is a popular replacement.

Why is my new 50-gallon water heater running out of hot water?

Three common causes: (1) low recovery — a 35,000 BTU burner recovers only 33 gph, so the second shower draws cold; (2) a bad dip tube mixing cold inlet with hot outlet, effectively reducing usable storage; (3) a hidden leak (often a slab leak or failing washing-machine valve) drawing hot water 24/7. Check the EnergyGuide FHR first — if it is under 70 gal for a 4-person home, the unit is undersized.