First-hour-rating (FHR) calculator

First-Hour Rating is the number that actually matters on the EnergyGuide label — the gallons a tank can deliver in a busy first hour. It is the usable stored hot water plus whatever the burner or element reheats while you draw.

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.

1 Enter your numbers

gallons
Nominal tank size on the label.
of stored water
LABELED typical ~0.70 (not all stored water is usable before it turns cold).
gph
From the recovery-rate tool, or the label.
Your result
First-Hour Rating75 gal in first hour
Usable stored hot water35 gal
Recovery in the first hour40 gph

First-Hour Rating is what a tank actually delivers in the busy first hour — the usable stored hot water (about 70% of the tank) plus what the burner or element reheats. Recovery depends on the input, so two 50-gallon tanks can have very different FHRs. This one is about 75 gal.

Nominal tank gallons mislead. A 50-gallon tank does not hand you 50 gallons of hot water before it runs cold, and it does not stop at 50 either — the burner keeps reheating while you draw. The metric that captures both effects is the First-Hour Rating, and it is what you should compare against your peak-hour demand.

FHR has two parts: the usable slice of the stored hot water (roughly 70%, because the bottom of the tank turns lukewarm as cold water refills it), plus the recovery the heat source adds in that hour. A powerful gas burner can add more in an hour than a small tank stores, which is why a modest gas tank can out-deliver a bigger electric one.

Formula

First-Hour Rating in gallons:

FHR = usable_fraction × storage_gallons + recovery_gph

The usable fraction is a LABELED typical near 0.70; recovery (gallons per hour the source reheats) comes from the burner or element input — compute it with the recovery-rate tool or read it off the label.

Worked example

A 50-gallon tank, 0.70 usable, recovering 40 gph:

0.70 × 50 + 40 = 35 + 40 = 75 gallons

So this tank posts an FHR of 75 gallons — exactly the family-of-four peak from the demand tool. Now the lever is visible: drop recovery to a slow electric 26 gph and the FHR falls to about 61; raise it with a strong gas burner and the same 50-gallon shell clears 80. Storage sets the floor, recovery sets the ceiling.

Measure first, avoid a wrong size

Compare FHR, not gallons. When two tanks show the same nominal size, the one with the higher FHR delivers more in the rush hour. That is the number to shop on.

  • Recovery is the hidden variable. It depends on the input rating (BTU/hr for gas, kW for electric) and the temperature rise — a colder inlet lowers it. Get it from the recovery-rate tool for your own numbers.
  • The 0.70 fraction is a planning typical. Tank geometry and how far you let the outlet cool both move it; the manufacturer’s rated FHR on the EnergyGuide label is the authority.
  • Electric tanks lean on storage. With slow recovery, their FHR is mostly the usable stored water, so they need more gallons to match a gas unit’s first-hour delivery.

Reference table

LABELED First-Hour Rating bands by household (from the signature dataset) — a sanity check on your computed FHR.

HouseholdStorage (gal)First-Hour Rating (gal)
1–2 people30–4040–55
2–3 people40–5050–65
3–4 people50–6060–75
5+ people60–8075–90

Frequently asked questions

What is a First-Hour Rating?
The First-Hour Rating (FHR) is the gallons of hot water a storage tank can supply in one busy hour, starting full. It is the usable stored hot water plus the recovery the burner or element adds during that hour — the number to match against your peak-hour demand.
Why is FHR different from the tank size?
Because only about 70% of stored water is usable before it turns cold, and because the heater keeps reheating as you draw. A 50-gallon tank might have an FHR of 61 or 80 depending on how fast it recovers.
Where do I find recovery to plug in?
Use the recovery-rate tool with your burner BTU/hr (or element kW) and temperature rise, or read the recovery figure off the unit’s specification sheet. A gas burner recovers far faster than a 4,500-watt element.
Should I match FHR exactly to my peak?
Match or slightly exceed it, then round up. Leaving a size of headroom covers guests, a colder winter inlet and the gradual drop in recovery as sediment builds up.

Related calculators

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.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.