Water Heater Sizing Calculator

Gallons is the wrong first question

Most people shopping for a water heater start with “what size tank,” but the number that actually predicts whether you’ll get a cold shower on a Sunday morning is First-Hour Rating (FHR) — how many gallons of hot water the unit can actually deliver in its single busiest hour, accounting for both the tank’s stored capacity and how fast it reheats. A 40-gallon tank with a fast recovery rate can out-deliver a 50-gallon tank with a slow one. This calculator gives you both a sensible tank-gallon starting point by household size and a target FHR to shop against on the yellow EnergyGuide label.

The household-size table, and why it’s only a starting point

Storage tank sizing by household size is a decades-old rule of thumb built around a predictable pattern: most households concentrate hot water use into a single morning peak hour, mostly showers. One to two people typically need around 40 gallons; three to four, 50 gallons; five or more, 66 to 80 gallons.

That table breaks down for atypical usage patterns — a household that runs a soaking tub, a large multi-head shower, or does back-to-back loads of hot laundry every morning needs more capacity than headcount alone predicts. If your actual routine doesn’t look like “everyone showers between 6 and 8 a.m. and that’s it,” size up from the table’s suggestion rather than down.

Storage tanks versus tankless: two different sizing questions

A storage tank answers “how much hot water do I have banked before I run out.” A tankless (on-demand) unit never runs out in that sense — it heats water as it flows through — but it has a hard ceiling on how much flow it can heat at once, measured in gallons per minute (GPM). That’s a completely different constraint, and it’s why the two technologies get sized with different math.

For tankless, this calculator adds up the flow rate of every fixture you might realistically run simultaneously — a shower runs about 2.5 GPM, a dishwasher about 1.5, a washing machine about 2.0 — and adds 15% headroom. The fixture checklist defaults to two showers running together, since that’s the classic morning-rush failure point in real households, but check whatever actually happens in your home: a dishwasher running overnight doesn’t compete with morning showers, but one running during breakfast does.

The mistake that ruins tankless installs

Tankless GPM ratings on the box are almost always measured at a generous incoming water temperature — often 50°F or warmer, which describes groundwater in the Southeast reasonably well and describes groundwater in Minnesota in February not at all. The colder the water coming in, the more temperature rise the unit has to produce, and the lower its usable flow rate becomes at your actual setpoint. A unit rated at 9.5 GPM in ideal conditions might deliver closer to 6 GPM in a cold northern winter. When shopping, look for the manufacturer’s flow-rate table across a range of incoming temperatures, not just the single headline number on the front of the spec sheet, and size against the coldest month your groundwater sees.

Recovery rate, standby loss and the case for right-sizing

Storage tanks lose a small amount of heat continuously just sitting there full of hot water — standby loss — so a tank sized well beyond your actual peak demand is paying an ongoing, small efficiency tax for capacity you never use. It also takes longer to fully reheat after a big draw, like filling a large tub, which can leave the household short on hot water for the rest of that day even though the tank is nominally “big enough.”

The practical goal is matching the unit — tank or tankless — to your household’s actual worst realistic hour, not the theoretical maximum. Use this calculator with your honest routine, not an idealized one, and re-run it if the household size changes; a tank sized for a family of two is a common and avoidable source of complaints after kids move back in or a housemate moves in. Pair it with the HVAC BTU calculator if you’re planning several mechanical replacements at once, and the energy cost calculator to estimate what the unit will cost to run.