Water Heater Size Calculator
Find the right water heater size for your home: the tank your busiest hour of hot water needs, or the tankless flow your winter tap water calls for.
A tank isn't sized by how many people live in the house but by your worst hour: how many showers run back to back in the morning rush, and whether the dishwasher or the laundry is going too. A tankless unit is sized by a different number, how many showers run at the same moment, and by how cold your tap water gets in winter.
What you're buying
The fuel you have at the heater now is usually the cheapest to keep; switching means new gas line or new wiring.
Tank or tankless?
Fuel
Your busiest hour
Think of the morning rush on a school day. The first number sizes a tank, the second sizes a tankless unit, and both answers show below.
Showers in your busiest hour
Count every shower in the worst hour of the day, usually the morning rush. A family of four is often three. Each one counts about 20 gallons of hot water.
Showers running at the same time
The most that ever run together. This is what sizes a tankless unit; most homes are one or two.
Also running in that hour
Where you live
Tap water is colder in the North, so the heater has more work to do in January.
Gas tank to buy
40 gallons
Covers a busiest hour of about 65 gallons of hot water
Tanks are checked against typical first-hour ratings: gas 40, 50 and 75 gallons at 67, 81 and 125; electric 30, 40 and 50 at 45, 53 and 61; heat pump 50, 65 and 80 at 66, 75 and 87. Each shower counts 20 gallons. Tankless flow assumes 2-gallon-a-minute showerheads mixed to 105°F and the heater set at 120°F. 2026 figures.
💡About this calculator▼
Most people size a water heater by counting heads: two people, 40 gallons; four people, 50. That's close, but it's the wrong question. What runs you out of hot water is one hour, usually the morning rush, when the showers go back to back and somebody starts the dishwasher.
This sizes the heater to that hour. Tell it how many showers run in your busiest hour, how many run at the same moment, whether the dishwasher or laundry is going too, what fuel you have and where you live. For a tank you get the size to buy and the first-hour rating to look for on the yellow label. For a tankless unit you get the flow it has to make with your winter tap water, and whether one unit can do it.
Both answers show either way. Pick tank or tankless and the other type's size appears as a row, so if you're on the fence you can see what each would take for the same house.
For a tank, the number that matters is the first-hour rating. It's printed in the top left of the yellow EnergyGuide label as "Capacity (first hour rating)," and it's how many gallons of hot water the heater can deliver in an hour starting full. A gas tank reheats faster than an electric one, so a 40-gallon gas tank can out-deliver a 50-gallon electric.
Your busiest hour sets how big it has to be. The Department of Energy's worksheet counts about 20 gallons of hot water per shower, 7 for a dishwasher and 25 for a load in a top-loading washer. This adds 5 gallons for a shave and a sink of dishes, the extras in the DOE's own example. Three showers in the morning rush is about 65 gallons.
Then it picks the smallest tank that covers it. Typical first-hour ratings run about 67, 81 and 125 gallons for 40-, 50- and 75-gallon gas tanks; 45, 53 and 61 for 30-, 40- and 50-gallon electric tanks; and 66, 75 and 87 for 50-, 65- and 80-gallon heat pump tanks. Standard electric tanks stop at 50 gallons, so an electric home that needs more steps up to a heat pump tank. Past the biggest single tank, you need two, or a smaller rush hour.
For a tankless unit, it's flow and cold water. A tankless heater has no stored hot water, so what matters is how many showers run at the same moment and how many degrees it has to heat the water. Each 2-gallon-a-minute shower mixed to 105°F takes about 1.6 gallons a minute of hot water when the tap water is 35°F, less when it's warmer. The heater lifts the water from your winter tap temperature to 120°F, and that lift is the "rise" on its spec sheet.
Worked example. Three showers in the morning rush, two at the same time, nothing else running, middle of the country with 47°F winter water: 60 + 5 = 65 gallons, so a 40-gallon gas tank rated 67 covers it. The tankless answer is 2 × 2.0 × 58 ÷ 73 = 3.2 gallons a minute at a 73-degree rise; the biggest home gas units make about 5.2 at that rise, so one covers it. An electric tankless unit would need about 34 kW, the largest size sold.
📐How it's calculated▼
Busiest-hour hot water = showers in that hour × 20 + 5 + 7 (dishwasher) + 25 (top-load laundry).
Tank = the smallest stock size whose first-hour rating ≥ that hour. Gas: 40 (67), 50 (81), 75 (125). Electric: 30 (45), 40 (53), 50 (61), then heat pump 65 (75) and 80 (87). Heat pump: 50 (66), 65 (75), 80 (87).
Tankless flow = showers at once × 2.0 × (105 − tap temperature) ÷ (120 − tap temperature), in gallons a minute, at a rise of 120 − tap temperature. Winter tap water: about 35°F in the North, 47°F in the middle, 57°F in the South, 67°F in Florida, Louisiana and Hawaii.
Largest home gas unit's reach = 199,000 × 0.95 ÷ (500 × rise). Electric size = flow × 500 × rise ÷ 3,412, in kW, rounded up to 12, 15, 20, 24, 29 or 36.
Default (3 showers, 2 at once, middle, gas): 65 gallons → 40-gallon gas tank; 3.2 GPM at a 73-degree rise → one gas unit (reach 5.2).
Northern family, 3 showers at once: 3 × 2.0 × 70 ÷ 85 = 4.9 GPM at an 85-degree rise; the biggest gas unit makes 4.4 there, so two units, or plan the third shower later.
📎Sources:Pacific Northwest National Laboratory (DOE) — Building America Solution Center, DHW First Hour Rating tool,ENERGY STAR — Certified Heat Pump Water Heaters (first-hour ratings by tank size),Rinnai — Solution Selector sizing assumptions (groundwater temperature bands by state, shower flow and temperatures),Stiebel Eltron — Tempra electric tankless technical specifications (kW, breakers, service size, flow at each rise)
🔍Finding your inputs▼
Tank or tankless: Which one you're shopping for. The panel sizes that one and shows the other in a row.
Fuel: Gas, electric or heat pump for a tank; gas or electric for tankless. The fuel you have at the heater now is usually the cheapest to keep, because switching means a new gas line or new wiring.
Showers in your busiest hour: Every shower in the worst hour of the day, usually the morning rush. A family of four is often three. This sizes a tank.
Showers running at the same time: The most that ever run together. This sizes a tankless unit. Most homes are one or two; a house with three bathrooms and teenagers may be three.
Also running in that hour: The dishwasher adds about 7 gallons, a load of laundry in a top-loader about 25. If you can run them later, pick "nothing else," because the result will be smaller.
Where you live: Winter tap water runs about 35°F across the northern states, 47°F through the middle of the country, 57°F across the South and 67°F in Florida, Louisiana and Hawaii. It barely matters for a tank and matters a lot for tankless.
⚠️Special situations▼
What size water heater do I need for a family of 4?
Usually a 40- or 50-gallon gas tank or a 50-gallon heat pump tank. What decides it is the morning rush, not the head count: three showers back to back is about 65 gallons, which a 40-gallon gas tank rated 67 covers; add a dishwasher running at the same time and it's 72, which takes a 50. On standard electric, three showers is past what a 50-gallon rates (61), so the next size up is a 65-gallon heat pump tank. For tankless, a family of four running two showers at once needs about 3.2 gallons a minute in the middle of the country, which one large gas unit covers.
Why does a 40-gallon gas tank beat a 50-gallon electric?
Because a gas burner reheats water much faster than electric elements, and the first-hour rating counts both what's stored and what the heater makes during that hour. Typical ratings are about 67 gallons for a 40-gallon gas tank and 61 for a 50-gallon electric. That's why the yellow label's first-hour rating, not the tank's gallons, is the number to compare.
The tankless box says 9 gallons a minute. Why does this say one unit won't cover three showers?
The biggest number on a box is the flow at a small temperature rise, warm southern water; Rinnai's own table for its largest home model runs from 8.8 gallons a minute with 77°F water down to 4.4 with 35°F water. In the North in January the heater has to lift 35°F water to 120°F, an 85-degree rise, and the largest home gas units make about 4.4 gallons a minute there. Three showers at once need about 4.9. Always compare the flow at your rise; the calculator shows it in the first row.
I'm all-electric. Should I go tankless?
Check the numbers first. Two showers at once in the middle of the country takes about 34 kW, which is the largest electric tankless size, three 50-amp breakers and a 300-amp electrical service, so check the main breaker before you shop. A heat pump tank covers the same household on one ordinary circuit and uses about a quarter of the electricity of a standard electric tank. Electric tankless makes the most sense for one or two people in a warm climate, or for a single bathroom far from the main heater.
❓Common questions▼
How do I figure out what size water heater I need?
Find your busiest hour of hot water and match it to the first-hour rating on the heater's yellow EnergyGuide label. Count about 20 gallons per shower in that hour, 7 for the dishwasher and 25 for a load in a top-loading washer, plus a few gallons for sinks. Three showers is about 65 gallons, so you want a first-hour rating of at least 65, which is a 40-gallon gas tank or a 50-gallon heat pump tank. For tankless, add up the showers running at the same moment and size for your winter tap water.
What is a first-hour rating?
It's the number of gallons of hot water a tank heater can deliver in one hour starting full, and it's printed in the top left corner of the yellow EnergyGuide label as "Capacity (first hour rating)." It combines the stored water with what the burner or elements heat during that hour, which is why a 40-gallon gas tank (about 67) can out-deliver a 50-gallon electric (about 61). Match it to your busiest hour, not to your household size.
What size tankless water heater do I need?
Multiply the showers you run at the same time by the hot water each one draws, which is about 1.6 gallons a minute with 35°F winter water and 1.4 with 67°F water for a standard 2-gallon-a-minute head. Two showers at once needs about 3.2 gallons a minute in most of the country; three in the North needs about 4.9. Then check the unit's flow at your temperature rise, the lift from your tap water to 120°F. The largest home gas units make about 4.4 gallons a minute at an 85-degree rise and about 7.1 at a 53-degree rise.
Is a bigger water heater better?
Not past what your busiest hour needs. A bigger tank keeps more water hot around the clock and loses more heat doing it, costs more, and takes more room. Size to the first-hour rating that covers your rush with a little margin. If you run out now, the cheapest fix is often moving the dishwasher or laundry out of the shower hour; three showers plus a load of laundry is about 90 gallons, a 75-gallon gas tank, and without the laundry it's 65, a 40.
Are electric water heaters over 50 gallons still sold?
Standard electric tanks top out around 50 gallons for most homes; the larger electric sizes sold today are heat pump models, which rate about 75 gallons in the first hour at 65 gallons and about 87 at 80. Under the Department of Energy's newer rule, effective May 6, 2029, electric tanks over 35 gallons will need heat pump technology. Heat pump tanks want a space that stays between 40°F and 90°F with about 1,000 cubic feet of air around them.
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