What is a first hour rating?

First hour rating is the number of gallons of hot water a storage heater delivers in one hour, starting with the tank fully heated. It combines the usable part of the stored water with everything the burner or element reheats during that hour. It is the single most useful number on a water heater label, and it is the one to compare between models rather than the gallons.

Why is the first hour rating higher than the tank size?

Because the heater keeps working while you draw. A 50 gallon tank does not simply hand over 50 gallons and stop: the burner or element runs the whole time, reheating incoming cold water as it arrives. A 50 gallon gas tank with a fast burner can show a first hour rating above 80 gallons, which is why two tanks of the same size can behave very differently.

Why is only 70 percent of the tank usable?

Cold water entering through the dip tube mixes with the hot water above it, so the outlet temperature falls through a long draw well before the tank is empty. Planning figures of 60 to 75 percent are standard. This is a convention rather than a measured value, and it is the main reason an estimated first hour rating and the label figure can differ by several gallons.

Does a gas tank recover faster than an electric one?

Usually by a wide margin. A 36,000 BTU gas burner at 80 percent efficiency recovers roughly 38 gallons per hour through a 90 F rise, while a 4,500 watt electric element recovers about 20. That gap is why a 40 gallon gas tank often out-delivers a 50 gallon electric one, and why switching from gas to electric without going up a size disappoints people.

Sizing calculator

First Hour Rating Calculator

Quick answer

First hour rating is the usable storage plus one hour of recovery. A 50 gallon electric tank with a 4,500 watt element recovers about 20 gallons per hour through a 90 F rise, so 35 gallons of usable storage plus 20 gives an estimated first hour rating near 55 gallons.

Result

Estimated first hour rating 56 gal Convention

Usable storage 35 gal Convention

Recovery in one hour 20.5 gal Published standard

Minutes to reheat the whole tank 146 min Published standard

This is an estimate of a measured value. The DOE first hour rating on the yellow EnergyGuide label is the real figure and it governs wherever it is available.

The formula

FHR = (tank gallons x usable fraction) + recovery in one hour. Gas recovery GPH = BTU x efficiency / (8.33 x rise). Electric recovery GPH = kW x 3412 / (8.33 x rise).

The published DOE first hour rating is a MEASURED test result, not this estimate, and where a manufacturer publishes one that figure governs. This calculator exists for the case where you are looking at a tank whose label you cannot find. The 70 percent usable fraction is a convention: real tanks deliver between about 60 and 75 percent at the stored temperature before the outlet cools.

The figures above are computed with these exact inputs: Tank size 50 gal, Heat source electric, Element rating 4.5 kW, Burner input 36000 BTU/h, Temperature rise 90 F, Usable fraction 70 %.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
First hour rating estimated as usable storage plus one hour of recovery	Convention	First hour rating estimated as usable storage plus one hour of recovery. The published DOE FHR is measured, not estimated
Usable Fraction: 0.7	Convention	Planning figure for the fraction of a tank delivered at the stored temperature before the outlet cools. The DOE first hour rating is a measured test value and governs where one is published
Water Weight: 8.33	Published standard	Published density of water at 60 F, 8.33 lb per gallon
Gas Efficiency: 0.8	Convention	Assumed thermal efficiency for an atmospheric gas burner. Use the efficiency on your own unit data plate where it is published

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Intermediate

Rheem

Rheem PROE50 M2 50 Gallon

$945.99

The professional line 50 gallon electric, which is the single most common residential water heater size in the country.

Best for: A straight 50 gallon electric replacement

Check price on Amazon

Beginner

GE 40 Gallon Lowboy

$522.00

A lowboy is short and wide, which is the answer when the tank has to fit under a stair or in a crawlspace and the tall version physically will not go in.

Best for: Height restricted closets and crawlspaces

Check price on Amazon

Beginner

GE 30 Gallon Electric

$576.00

Thirty gallons suits one or two people. Going smaller than the household needs is the cheapest mistake to make and the most annoying to live with.

Best for: One or two people, or an apartment

Check price on Amazon

Beginner

GE Smart 40 Gallon Short

$621.00

A ten year warranty usually means a thicker anode rod rather than a better tank, and the anode is a part you can replace yourself for about thirty dollars.

Best for: A short-height 40 gallon slot

Check price on Amazon

Intermediate

GE Smart 40 Gallon Short, 12 Year

$675.00

The twelve year version of the same tank. Compare the price difference against the cost of one anode rod before paying for it.

Best for: Hard water areas where the anode does real work

Check price on Amazon

Expert

Bradford White

Bradford White RE340T6 40 Gallon

$1,003.13

Bradford White sells through plumbing supply rather than retail, so this is the tank a plumber is most likely to already stock parts for.

Best for: Anyone who wants the plumber to have parts on the truck

Check price on Amazon

Frequently asked questions

What is a first hour rating?: First hour rating is the number of gallons of hot water a storage heater delivers in one hour, starting with the tank fully heated. It combines the usable part of the stored water with everything the burner or element reheats during that hour. It is the single most useful number on a water heater label, and it is the one to compare between models rather than the gallons.
Why is the first hour rating higher than the tank size?: Because the heater keeps working while you draw. A 50 gallon tank does not simply hand over 50 gallons and stop: the burner or element runs the whole time, reheating incoming cold water as it arrives. A 50 gallon gas tank with a fast burner can show a first hour rating above 80 gallons, which is why two tanks of the same size can behave very differently.
Why is only 70 percent of the tank usable?: Cold water entering through the dip tube mixes with the hot water above it, so the outlet temperature falls through a long draw well before the tank is empty. Planning figures of 60 to 75 percent are standard. This is a convention rather than a measured value, and it is the main reason an estimated first hour rating and the label figure can differ by several gallons.
Does a gas tank recover faster than an electric one?: Usually by a wide margin. A 36,000 BTU gas burner at 80 percent efficiency recovers roughly 38 gallons per hour through a 90 F rise, while a 4,500 watt electric element recovers about 20. That gap is why a 40 gallon gas tank often out-delivers a 50 gallon electric one, and why switching from gas to electric without going up a size disappoints people.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.