What is a good first hour rating for a family of four?

A household of four with a typical mix of morning showers and dishwashing commonly needs a first hour rating in the 55 to 70 gallon range, though the exact number depends on how many showers run back to back and whether a dishwasher or clothes washer runs during that same hour. The water heater size chart by household size on this site works out a specific peak hour demand figure to compare against a label.

Does a bigger tank always mean a higher first hour rating?

No. A 50 gallon gas tank with a strong burner can post a higher first hour rating than an 80 gallon electric tank with a weak element, because FHR depends on recovery speed as much as stored volume. Comparing gallon capacity alone, without checking the FHR on the EnergyGuide label, can lead to buying a larger tank that still runs out of hot water sooner.

Why is only 70 percent of a tank considered usable for first hour rating?

As hot water is drawn from the top of a tank, incoming cold water enters and gradually mixes with the remaining hot water rather than displacing it cleanly, so delivered temperature drifts down before the tank is physically empty. A 70 percent usable-storage figure is a common planning convention for where that mixing makes further draws too cool to count as usable, though the DOE test measures a specific tank's actual behavior rather than assuming this fraction.

Where do I find the actual first hour rating for a specific water heater?

Every residential storage water heater sold in the United States carries a yellow EnergyGuide label listing its DOE-measured first hour rating in gallons, usually near the Uniform Energy Factor figure. That printed number reflects an actual test of that model, and it should be checked against a household's peak hour demand before buying rather than relying on gallon capacity alone.

Does raising the thermostat increase first hour rating?

Only if it increases the effective temperature rise the tank is working against for incoming cold water, since the recovery-rate portion of FHR is calculated per degree of rise, not per gallon at any fixed temperature. In practice, most FHR comparisons hold the target delivered temperature constant, so thermostat changes mainly affect standby loss and scald risk rather than the published FHR figure.

Why do two 50 gallon gas tanks have different first hour ratings?

Burner input and recovery efficiency vary between models even at the same 50 gallon capacity; a 36,000 BTU burner and a 40,000 BTU burner on otherwise similar 50 gallon tanks produce noticeably different recovery rates, and tank baffling affects how much of that 50 gallons counts as usable before delivered temperature drops. The printed FHR on each model's label reflects those real differences.

Sizing

First Hour Rating Chart by Tank Size

Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated 2026-09-17.

Quick answer

A 50 gallon electric tank with a 4,500 watt element has an estimated first hour rating of about 55 gallons at a 90 F temperature rise, calculated as 70 percent usable storage (35 gallons) plus one hour of recovery (about 20 gallons per hour). A 50 gallon gas tank with a 36,000 BTU burner reaches roughly 71 gallons at the same rise, because a gas burner recovers faster than an electric element. The number stamped on the EnergyGuide label is measured under the DOE test procedure and governs over any estimate on this page.

First hour rating, or FHR, is the amount of hot water a storage tank delivers in one hour starting from a full tank, and it is the single number that best predicts whether a tank will run out of hot water during a busy morning. Two tanks of the same gallon capacity can carry very different FHR figures depending on their heating element or burner, which is why gallon capacity alone is a poor way to shop.

This chart works the arithmetic behind FHR for common tank sizes so you can see where the number comes from, then compare it against the DOE-tested figure printed on a specific model's EnergyGuide label. The first hour rating calculator runs this same math for a tank and rise you enter directly.

What does first hour rating actually measure?

First hour rating measures how many gallons of hot water a tank can deliver in one hour, starting completely full, under the DOE's standardized test procedure. It combines two separate things: the gallons already sitting hot in the tank, and the gallons the burner or element can heat and add during that same hour while the tank is being drawn down. A tank with a weak burner and a tank with a strong burner, at the identical gallon capacity, can post very different FHR numbers because the second half of that equation differs so much.

Where does the 70 percent usable figure come from?

Not every gallon in a storage tank is usable before the outgoing water cools past a comfortable temperature, because cold incoming water mixes with the remaining hot water near the bottom of the tank rather than the tank draining like a bucket. A commonly used planning convention treats about 70 percent of nameplate capacity as usable before that mixing drops delivered temperature too far. This page uses that 70 percent figure to estimate FHR by hand; it is a convention, not a measured value, and the DOE test result on an actual label accounts for a specific tank's real mixing behavior rather than an assumed fraction.

What is the estimated first hour rating for an electric tank?

A 4,500 watt element is the most common residential electric configuration. Recovery in gallons per hour comes from GPH = (kW x 3,412) / (8.33 x temperature rise), then FHR = 70 percent usable storage plus that one hour of recovery.

Estimated first hour rating, 4,500 watt electric element, by tank size and temperature rise
Tank size	60 F rise	70 F rise	80 F rise	90 F rise	100 F rise
30 gal	52 gal	47 gal	44 gal	41 gal	39 gal
40 gal	59 gal	54 gal	51 gal	48 gal	46 gal
50 gal	66 gal	61 gal	58 gal	55 gal	53 gal
66 gal	77 gal	73 gal	69 gal	67 gal	65 gal
75 gal	83 gal	79 gal	76 gal	73 gal	71 gal
80 gal	87 gal	82 gal	79 gal	76 gal	74 gal

Convention Source: Calculated from GPH = (kW x 3,412) / (8.33 x rise) for a 4,500 watt element, added to a 70 percent usable-storage convention. The DOE-measured figure on a specific model's label governs.. A 3,800 watt or 5,500 watt element shifts every figure down or up from this baseline; the recovery rate chart on this site works those wattages separately.

What is the estimated first hour rating for a gas tank, at 36,000 BTU?

Estimated first hour rating, 36,000 BTU gas burner, by tank size and temperature rise
Tank size	60 F rise	70 F rise	80 F rise	90 F rise	100 F rise
30 gal	75 gal	67 gal	62 gal	57 gal	53 gal
40 gal	82 gal	74 gal	69 gal	64 gal	60 gal
50 gal	89 gal	81 gal	76 gal	71 gal	67 gal
66 gal	100 gal	93 gal	87 gal	82 gal	79 gal
75 gal	107 gal	99 gal	93 gal	89 gal	85 gal
80 gal	110 gal	102 gal	97 gal	92 gal	88 gal

Convention Source: Calculated from GPH = (BTU/hr x 75 percent assumed recovery efficiency) / (8.33 x rise), added to a 70 percent usable-storage convention. A specific model's published recovery efficiency and DOE-measured FHR govern.. 75 percent recovery efficiency is an assumption this page uses for the worked figures; published recovery efficiency varies by burner design.

What is the estimated first hour rating for a gas tank, at 40,000 BTU?

Estimated first hour rating, 40,000 BTU gas burner, by tank size and temperature rise
Tank size	60 F rise	70 F rise	80 F rise	90 F rise	100 F rise
30 gal	81 gal	72 gal	66 gal	61 gal	57 gal
40 gal	88 gal	79 gal	73 gal	68 gal	64 gal
50 gal	95 gal	86 gal	80 gal	75 gal	71 gal
66 gal	106 gal	98 gal	91 gal	86 gal	82 gal
75 gal	113 gal	104 gal	98 gal	93 gal	89 gal
80 gal	116 gal	107 gal	101 gal	96 gal	92 gal

Convention Source: Calculated from GPH = (BTU/hr x 75 percent assumed recovery efficiency) / (8.33 x rise), added to a 70 percent usable-storage convention. A specific model's published recovery efficiency and DOE-measured FHR govern.. A 40,000 BTU burner is common on larger residential tanks; a specific nameplate rating can fall anywhere in the 30,000 to 76,000 BTU range covered elsewhere on this site.

Why does a gas tank post a higher FHR than an electric tank of the same capacity?

A 36,000 BTU gas burner delivers roughly 27,000 BTU per hour of net recovery heat at a typical 75 percent recovery efficiency, versus about 15,354 BTU per hour from a 4,500 watt electric element converted at 3,412 BTU per kWh. That is nearly double the recovery heat from the gas burner, which is why gas tanks of identical gallon capacity consistently post higher FHR numbers than electric tanks in every row of the tables above.

Why does the label figure sometimes differ from this chart's estimate?

This chart's numbers rest on two conventions: 70 percent usable storage and, for gas, an assumed 75 percent recovery efficiency. A real tank's usable fraction depends on its internal baffling and dip tube design, and its recovery efficiency depends on burner and heat exchanger design, so an actual DOE first hour rating test can land a few gallons above or below this chart's estimate for the same nameplate capacity and input. The label figure is measured, not assumed, and it is the number to shop against.

Use this chart to sanity-check a label, not to replace one. If a specific model's printed FHR sits far outside the range this chart estimates for its capacity and input, that is worth a second look at the listing rather than an assumption the label is wrong.

Representative tanks at these capacities

These illustrate the electric and gas FHR figures worked out below; check the specific EnergyGuide label before buying.

Electric, 50 gal

Rheem

Rheem PROE50 M2 50 Gallon

$945.99

A 50 gallon electric tank with a 4,500 watt element, the exact configuration used in the worked example above.

Best for: A straight 50 gallon electric replacement

Check price on Amazon

Gas, 40 gal

GE RealMAX Choice 40 Gallon Natural Gas

$1,051.49

A 40 gallon gas tank, illustrating how a gas burner raises first hour rating well above an electric element of the same capacity.

Best for: A like-for-like swap into an existing masonry or B-vent flue

Check price on Amazon

Low-clearance, 40 gal

GE 40 Gallon Lowboy

$522.00

A 40 gallon low-profile tank for installs where ceiling height, not first hour rating, is the limiting factor.

Best for: Height restricted closets and crawlspaces

Check price on Amazon

Frequently asked questions

What is a good first hour rating for a family of four?: A household of four with a typical mix of morning showers and dishwashing commonly needs a first hour rating in the 55 to 70 gallon range, though the exact number depends on how many showers run back to back and whether a dishwasher or clothes washer runs during that same hour. The water heater size chart by household size on this site works out a specific peak hour demand figure to compare against a label.
Does a bigger tank always mean a higher first hour rating?: No. A 50 gallon gas tank with a strong burner can post a higher first hour rating than an 80 gallon electric tank with a weak element, because FHR depends on recovery speed as much as stored volume. Comparing gallon capacity alone, without checking the FHR on the EnergyGuide label, can lead to buying a larger tank that still runs out of hot water sooner.
Why is only 70 percent of a tank considered usable for first hour rating?: As hot water is drawn from the top of a tank, incoming cold water enters and gradually mixes with the remaining hot water rather than displacing it cleanly, so delivered temperature drifts down before the tank is physically empty. A 70 percent usable-storage figure is a common planning convention for where that mixing makes further draws too cool to count as usable, though the DOE test measures a specific tank's actual behavior rather than assuming this fraction.
Where do I find the actual first hour rating for a specific water heater?: Every residential storage water heater sold in the United States carries a yellow EnergyGuide label listing its DOE-measured first hour rating in gallons, usually near the Uniform Energy Factor figure. That printed number reflects an actual test of that model, and it should be checked against a household's peak hour demand before buying rather than relying on gallon capacity alone.
Does raising the thermostat increase first hour rating?: Only if it increases the effective temperature rise the tank is working against for incoming cold water, since the recovery-rate portion of FHR is calculated per degree of rise, not per gallon at any fixed temperature. In practice, most FHR comparisons hold the target delivered temperature constant, so thermostat changes mainly affect standby loss and scald risk rather than the published FHR figure.
Why do two 50 gallon gas tanks have different first hour ratings?: Burner input and recovery efficiency vary between models even at the same 50 gallon capacity; a 36,000 BTU burner and a 40,000 BTU burner on otherwise similar 50 gallon tanks produce noticeably different recovery rates, and tank baffling affects how much of that 50 gallons counts as usable before delivered temperature drops. The printed FHR on each model's label reflects those real differences.

Check the label before the tank size. Two tanks of identical gallon capacity can carry FHR figures ten or more gallons apart, and the printed, DOE-tested number on the EnergyGuide label is what actually predicts a busy morning, not the number on the tank's side panel.