First Hour Rating, Explained
Updated July 2026
FHR is the tested hot gallons a tank delivers in 60 minutes. A 40-gallon gas tank rates about 70; a 65-gallon electric about 66. Here is how to read and use it.
First hour rating answers the only question that matters when a tank is being emptied: how many gallons of hot water come out in 60 minutes, starting full, while the burner or element fights to keep up. It is a tested number (full tank, 3 gallon-per-minute draws until the outlet temperature sags 25°F, repeat for one clock hour) and it is printed on the EnergyGuide label of every storage water heater sold in the US, usually top-left, as "Capacity (first hour rating)."
It is not the number printed biggest on the box. The box sells gallons, and gallons mislead in both directions. A 40-gallon gas tank delivers about 70 gallons in its first hour, because a 40,000 BTU burner adds roughly 43 gallons of recovery while the hour runs. A 65-gallon electric delivers about 66, barely more than its own volume, because a 4,500-watt element only puts back 20. The small gas tank beats the big electric one at the job the family actually cares about.
How to use it
Tally your busiest hour of hot water on the DOE worksheet (our calculator does it live): 20 gallons a shower, 20 a bath, 14 a dishwasher load, 32 a hot-wash laundry load. Then buy a tank whose FHR meets or beats that tally. That is the whole method. DOE's worked example lands at 70 gallons for three showers plus sink use, which maps to a 50-gallon gas or 80-gallon electric.
Two reading tips from the field. First, FHR varies between models of the same size: a 40-gallon gas tank with a 36,000 BTU burner and one with a 40,000 BTU burner can sit five gallons apart, so compare labels, not sizes. Second, ignore the temptation to "round up for safety" past your real hour; storage you never draw is heat leaking through a jacket 24 hours a day.
Where our table numbers come from
Spec-sheet FHRs are not published in one place, so the tables on this site approximate FHR as 70% of tank volume (the share you can draw before the outlet cools) plus one hour of recovery at the standard 90°F test rise. Check that against any label you are holding; it should land within a few gallons, and it errs slightly low on electric, which is the safe direction to be wrong in.