Undersize it and the last shower runs cold; oversize it and you paid for capacity nobody uses. Size a tank from its First Hour Rating (peak-hour demand) or a tankless from flow rate and temperature rise. Enter your fixtures below. Runs in your browser; nothing is uploaded.
Two 50 gallon tanks can perform completely differently. What matters is the First Hour Rating: how much hot water the heater can actually deliver in the first busy hour, which is the tank's stored volume plus how fast it reheats. A 50 gallon gas heater recovers quickly and might deliver 80 to 90 gallons in that first hour; a 50 gallon electric recovers slowly and might deliver only 60 to 70. Size to the peak-hour demand this calculator adds up, then match it to a heater's First Hour Rating, not its sticker gallons.
A tankless heater never runs out, so gallons do not apply. It can only heat the water passing through it at a given moment, so the two numbers that size it are simultaneous flow (add up the GPM of every fixture that might run at once) and temperature rise (your target temperature minus the incoming groundwater temperature). The catch every installer knows: a unit's GPM rating falls as the required rise climbs. A tankless rated for 7 GPM in Florida might only manage 4 GPM in Minnesota, where the incoming water is 30 degrees colder. That is why this calculator asks for your climate, and why you size a tankless at your rise, from the manufacturer's chart, not off the box.
Sizing is half the job. The other half is quoting and billing the replacement so the customer says yes and pays fast. Print a clean quote from the free plumbing invoice generator, or, if you are doing more than a couple of these a week, let FieldForge build the invoice by voice from your price book the moment the new heater is firing, and text the customer a payment link before you have loaded the old tank in the truck.
Add up the hot water used in your busiest hour and pick a heater whose First Hour Rating meets it. Gas recovers faster than electric, so a smaller gas tank can beat a larger electric one.
Add the GPM of all fixtures that could run at once, subtract your incoming water temperature from your target to get the rise, and choose a unit rated for that flow at that rise.
It depends on your peak hour, not headcount. A morning rush of two showers plus a sink commonly lands around a 60 to 70 gallon First Hour Rating. Size from your actual demand above.