Work out the storage size you need from your busiest hour of hot-water use — matched to first-hour rating, not just the nominal gallons on the label — and see how fast a tank recovers by fuel and temperature rise. Works for tanks and hot water cylinders in gallons or litres. A planning estimate, not a manufacturer specification.
How do you want to estimate demand?
Usage level
Light ≈ 12, medium ≈ 18, heavy ≈ 24 gallons of hot water per person in the peak hour — a public rule of thumb, an estimate.
Hot-water draws in your busiest hour
How many of each happen in that one peak hour. Per-use amounts are typical published figures, editable.
UseHow many
Fuel
Typical residential gas burners run roughly 30,000–50,000 BTU/hr.
A common single-element draw is around 4.5 kW.
gal
Used with recovery to estimate first-hour rating (about 70% of this volume plus the hourly recovery).
Units
Temperature rise & efficiency
°F
°F
Recovery efficiency — gas burners around 0.80–0.95, electric resistance near 0.98–1.0.
A planning estimate using public rules of thumb and the standard recovery formula. Confirm the final size and the fuel supply with a qualified installer and your local code before buying or fitting.
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Tank or tankless — and what will it cost to run?
Thinking about going tankless instead? The tankless water heater sizing calculator sizes an on-demand unit by peak flow and temperature rise rather than gallons. To compare what a tank costs to run on different fuels, the heating fuel cost calculator weighs gas, electric and more side by side.
Size to the peak hour, not the headcount
Most “what size water heater do I need” advice starts with bedrooms or people, but a storage heater does not care how big your house is — it cares about the single busiest hour, when showers, laundry and the dishwasher pile up at once. Size to that hour and everyone gets a warm shower; size to the daily average and someone gets a cold one. The figure that matters is your peak-hour demand in gallons (or litres), and the spec you match it to is the heater’s first-hour rating.
First-hour rating beats nominal gallons
A 50-gallon tank does not deliver 50 gallons of hot water in a busy hour. As you draw hot water off the top, cold water flows in at the bottom and mixes, so only about 70% of the stored volume comes out genuinely hot before the temperature sags. The first-hour rating (FHR) captures this: it is roughly 70% of the tank volume plus whatever the burner or element can reheat within that hour. That is why two tanks of identical gallons can perform very differently — a fast-recovering gas model adds far more during the hour than a slow electric one.
first-hour rating ≈ 0.70 × tank gallons + recovery (gal/hr) size so FHR ≥ peak-hour demand
Recovery rate, and why gas is faster than electric
Recovery rate is how many gallons the heater can reheat per hour. It comes straight from the energy in the water: raising a gallon one degree Fahrenheit takes about 8.33 BTU, so the gallons reheated per hour are the input energy times efficiency divided by 8.33 times the temperature rise.
A gas burner pours in far more BTU per hour than an electric element’s equivalent, so gas recovers more water per hour at the same temperature rise — which is exactly why a smaller fast gas tank can out-deliver a larger slow electric one. And because a colder inlet means a bigger rise, every gallon costs more energy, so recovery slows in winter for either fuel.
Worked example
Take a 40,000 BTU/hr gas burner at 0.95 efficiency, raising water 80 °F (a 40 °F inlet to 120 °F):
recovery = (40,000 × 0.95) ÷ (8.33 × 80) ≈ 57 gal/hr FHR on a 50-gallon tank ≈ 0.70 × 50 + 57 ≈ 92 gal
So that 50-gallon tank behaves like a 92-gallon delivery in the first hour — comfortably covering a medium-use family of four (around 72 gallons of peak demand), with headroom to spare. Switch the same job to a 4.5 kW element (about 15,400 BTU/hr) and recovery drops to roughly 22 gal/hr, dragging the FHR down to about 57 gallons — the same tank, far less delivered, because electric reheats so much more slowly.
A rough people-to-size guide (estimate, not a manufacturer chart)
These are computed from the peak-hour rule of thumb — light, medium and heavy use at roughly 12, 18 and 24 gallons per person — not lifted from any manufacturer’s sizing sheet. Treat them as a sanity check, then match a real unit’s first-hour rating to the demand.
Household
Light (12/person)
Medium (18)
Heavy (24)
1–2 people
~24 gal
~36 gal
~48 gal
3 people
~36 gal
~54 gal
~72 gal
4 people
~48 gal
~72 gal
~96 gal
5 people
~60 gal
~90 gal
~120 gal
6 people
~72 gal
~108 gal
~144 gal
The figures are first-hour-rating targets in gallons, not tank capacities — a 50-gallon gas tank can meet a 72-gallon target because its recovery tops up the stored volume during the hour. For a hot water cylinder, switch the tool to litres and read everything through the same lens.
Bigger is not automatically better
An oversized tank costs more up front and loses more heat sitting idle, because a larger volume is kept hot around the clock whether you use it or not. Size to your real peak hour with modest headroom rather than rounding far up. If your hot water is spread through the day instead of concentrated in one rush, a smaller tank with brisk recovery often serves better than a large, slow one — and however you land, confirm the choice and the gas or electrical supply with a qualified installer and local code.
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Frequently asked questions
What size water heater do I need?
Size a storage water heater to your peak hour of hot-water use, not to the number of people or the size of the house. Estimate the busiest hour in gallons — a rough rule of thumb is about 12 gallons per person for light use, 18 for medium and 24 for heavy — then choose a unit whose first-hour rating meets or exceeds that figure. A family of four with medium use needs roughly 72 gallons of first-hour rating, which a 50-gallon gas tank can often supply because its fast recovery adds to the stored volume.
What is first-hour rating and why does it matter more than tank gallons?
First-hour rating (FHR) is the number of gallons of hot water a heater can deliver in one busy hour, starting full. It is roughly 70% of the tank’s stored volume — because incoming cold water dilutes what is left — plus whatever the burner or element can reheat in that hour. A 50-gallon tank does not give 50 gallons of hot water in an hour; a fast-recovering gas model might deliver 80 or more, while a slow electric one delivers less. Matching FHR to your peak-hour demand is what prevents a cold shower, which is why two tanks of the same gallon size can perform very differently.
How is recovery rate calculated, and why is gas faster than electric?
Recovery rate is gallons reheated per hour: input energy times efficiency, divided by the energy to raise a gallon through your temperature rise. The water itself sets the denominator — about 8.33 BTU per gallon per degree Fahrenheit. Gas burners pour in far more BTU per hour than an electric element’s equivalent, so gas recovers more water per hour for the same temperature rise. A larger temperature rise (colder incoming water) slows recovery for either fuel, because each gallon takes more energy.
Should I size by daily total or by the peak hour?
By the peak hour. The daily total is spread across the day, with hours for the tank to recover between uses, so it rarely strains the heater. The single 60-minute window when showers, laundry and the dishwasher overlap is what empties a tank, so that is the figure to size against. A unit that comfortably covers the daily total can still run cold if its first-hour rating falls short of that one busy hour.
What size hot water cylinder do I need in litres?
The same peak-hour logic applies to a hot water cylinder; only the units change. Switch this tool to litres and it converts demand, first-hour rating and recovery into litres per hour and recommends a cylinder capacity in litres. As a rough guide a couple manages on a smaller cylinder while a larger family needs more stored volume and faster reheat, but it is the busiest hour of simultaneous use — not the household headcount alone — that should drive the size.
Is a bigger tank always better?
No. An oversized tank costs more to buy and loses more heat standing idle, since a larger volume of water is kept hot around the clock, so you pay in standby losses for capacity you rarely use. Size to your real peak hour with a little headroom rather than rounding far up. If your hot water is spread through the day rather than concentrated, a smaller tank with a fast recovery rate often beats a large slow one. Confirm the final choice and the fuel supply with a qualified installer and your local code.