What is a first-hour rating?

The first-hour rating (FHR) is how many gallons of hot water a storage heater can deliver in a busy hour starting full. It combines stored volume and recovery rate, so a smaller tank with a powerful burner can match a larger tank with a weak one. Match the FHR to your busiest-hour demand, not just the tank gallons.

How is tankless capacity determined?

A tankless heater is rated by how many gallons per minute it can raise by a given temperature rise. Required rise is the target temperature, usually 120 F, minus the incoming groundwater temperature. You then need a unit that delivers your simultaneous-fixture GPM at that specific rise.

Why does groundwater temperature matter so much?

Tankless output falls as the temperature rise grows. A unit that gives 7 GPM raising water 45 F in a warm region might only give 4 GPM raising it 75 F in a cold one. Always read the unit's GPM-versus-rise curve, not its single headline GPM figure.

How is the BTU/h figure calculated?

It uses Q = GPM × 60 × 8.33 × ΔT, where 8.33 is the weight of a gallon of water in pounds and ΔT is the temperature rise in F. This is the heat input the water absorbs; divide by the unit's efficiency to get the gas input rating you actually need to buy.

Storage or tankless for my home?

Storage suits homes with short, intense peaks and lower budgets; tankless suits homes wanting endless hot water and lower standby loss, but needs a large gas line or electrical service and struggles in very cold-groundwater regions. This tool sizes either path so you can compare.

Water Heater Sizing Calculator

Pick a water heater that runs out during the morning rush and you will get complaints; oversize it and you waste money and energy. This calculator sizes both storage and tankless heaters the right way — storage by first-hour rating from your peak-hour demand, tankless by flow rate at the real temperature rise.

How it works

The two heater types are sized on completely different principles:

Storage:   peak-hour demand = sum of hot-water events × gallons each
           choose a heater whose First-Hour Rating ≥ that demand

Tankless:  required GPM      = sum of simultaneous fixture hot flows
           temperature rise  = target (120 F) − groundwater temp
           energy needed     = GPM × 60 × 8.33 × ΔT  (BTU/h)

Storage cares about the busiest single hour; tankless cares about the worst-case simultaneous flow and how cold your incoming water is.

Example and notes

Two back-to-back showers (10 gal each) plus a dishwasher (6 gal) in one hour give 26 gallons of peak demand, so you want a heater with an FHR of at least 26 — a 40-gallon storage tank usually qualifies. On the tankless side, a shower plus a kitchen sink at a 70 F rise needs roughly 3.5 GPM and about 122,000 BTU/h of heat absorbed, so look for a unit rated for that GPM at that exact rise. Remember recovery and rise curves, not headline numbers, decide whether you ever run cold.