Water Heater Size Calculator

What size water heater do you actually need?

Water heaters are not sized by how much hot water a household uses in a day. They are sized by the worst hour of the week — usually a weekday morning when showers, a dishwasher and a laundry load all land inside sixty minutes. Get that hour right and the heater is right; average it out across the day and you will buy something too small, or pay for a tank far bigger than the house needs.

Storage tanks and tankless units answer that hour with completely different numbers. A tank answers with gallons delivered in the first hour, the metric printed on the yellow EnergyGuide label as the first-hour rating (FHR). A tankless unit answers with flow: gallons per minute (GPM) it can raise from your incoming water temperature to a usable one. That is why this calculator switches its whole worksheet when you change the heater type.

Worked example: a family of four at 7 a.m., storage tank

Three showers back to back at roughly 10 gallons of hot water each is 30 gallons. Add a dishwasher cycle at 6, a laundry load at 7, and two quick sink or shaving uses at 2 gallons each, and the peak hour comes to 47 gallons of hot water. That number, 47, is the first-hour rating to shop for. It maps to a 50-gallon tank: a 50-gallon gas model typically carries an FHR near 80 and a 50-gallon electric near 60, so either clears 47 with headroom. A 40-gallon electric, with an FHR around 50, would only just scrape by and would leave nothing for a guest.

Note what did not happen: nobody added the family's whole daily usage of 200-plus gallons. Sixty minutes is the whole question.

The same family, tankless

Now the question changes to how many fixtures run simultaneously. Two showers at 2.0 GPM each plus a kitchen sink at 1.0 GPM is 5.0 GPM. Then comes the part most buyers skip: temperature rise. If the household is in Georgia, incoming water is around 65°F and the rise to a 105°F shower is 40°F — a small-to-mid whole-home unit handles 5 GPM there easily. If the same family lives in Minnesota or northern Britain, incoming water sits near 40°F, the rise is 65°F, and the same unit delivers far less.

Temperature rise math you can check by hand

Heating one gallon per minute by one degree Fahrenheit takes about 500 BTU per hour. So 5 GPM at a 65°F rise needs 5 × 65 × 500 = 162,500 BTU per hour of delivered heat. At 82 percent thermal efficiency the unit has to fire at roughly 198,000 BTU — which is exactly why the biggest residential gas tankless units are rated 199,000 BTU. Cold-climate homes wanting two simultaneous showers are shopping at the very top of the residential range.

Electric is stricter. One GPM per degree of rise costs about 0.1465 kW, so 5 GPM at a 65°F rise would need roughly 48 kW — more than a typical 200-amp service can spare. A 27 kW electric tankless, the common whole-home size, delivers about 2.8 GPM at that rise. That is one shower, and nothing else running.

How to read the yellow label

On a storage tank, the number that matters is the first-hour rating, printed in small type near the top of the EnergyGuide label. It is always larger than the tank volume because the burner or element keeps working while you draw. Gas recovers roughly twice as fast as electric, which is why a 40-gallon gas tank can out-deliver a 50-gallon electric one in that first hour. Compare FHR to FHR across models; comparing gallons to gallons across fuel types is misleading.

On a tankless unit, the headline GPM on the box is measured at a small rise, often 35°F. Ignore it and find the manufacturer's flow chart, then read the GPM at your own rise. Two models with identical box numbers can differ by 1.5 GPM at 65°F.

When to size up

Size a tank one step up if you have a large soaking tub, a body-spray shower, a household that grows at holidays, or a well pump feeding especially cold water. Size up if your showers routinely run 15 minutes rather than 8 — the 10-gallon figure used here assumes a low-flow head and a normal shower. Do not size up "just in case" beyond one step: every extra gallon of storage loses heat around the clock, and the running cost never stops.

For tankless, do not size up on gallons, size on simultaneous fixtures and rise. Adding a second smaller unit in parallel is often cheaper and more efficient than one oversized unit, and it gives you redundancy when one fails.

Limits of this estimate

The gallon and GPM figures here are typical residential values: 2.0 GPM showerheads, modern dishwashers, front-load washers. Old fixtures use considerably more, and a 1990s showerhead alone can hit 3.5 GPM. Recirculation loops, long uninsulated pipe runs, hard water scale and a thermostat set below 120°F all shift the answer. Use this to walk into the shop with the right number in your head, then let a licensed installer confirm gas line size, venting and electrical capacity before you commit.