What size water heater do I need?
Sized by peak hour demand rather than by tank volume alone. Enter your details and the answer appears with every step of the arithmetic shown, so you can check it against any quote you have been given.
This is an estimate from the standard industry rule of thumb, not an engineering calculation. For anything you are about to spend money on, have the figure confirmed by a load calculation.
How water heater capacity is worked out
The number that matters is first hour rating — how much hot water the heater can deliver in the busiest hour, which is the tank volume plus what it can reheat during that hour. A 40 gallon tank with a strong burner can out-deliver a 50 gallon tank with a weak one.
The calculation is shown step by step under the result rather than hidden behind the answer. That is deliberate: most sizing guides online are lead capture forms that give you a number and a phone call, and a number you cannot check is a number you cannot argue with when a contractor disagrees. Every adjustment is listed, so you can see what changed the answer and by how much.
Nothing you type is sent anywhere. The arithmetic runs in your browser, there is no form to fill in, and no one calls you afterwards. That is the whole reason this page exists — you should be able to what size water heater do i need without trading your phone number for it.
What going too big or too small actually costs
An oversized tank keeps more water hot than the household ever uses, and standby loss runs continuously whether anyone showers or not. On a gas tank that is a modest but permanent addition to the bill.
Running out of hot water mid-shower is the symptom, and it appears exactly when the household schedule bunches up — everyone showering within the same hour before work. Peak hour demand, not daily total, is what to size against.
Both directions cost money, which is why it is worth ten minutes to what size water heater do i need before you take a quote at face value. The asymmetry is worth knowing though: for most equipment on this site, being slightly under is a comfort problem you notice on a handful of extreme days a year, while being significantly over is a permanent efficiency and equipment-life problem you pay for every month.
By household size
| Household | Tank size | First hour rating | Tankless flow |
|---|---|---|---|
| 1 person | 30 gallons | 35 to 45 gal | 3 to 4 GPM |
| 2 people | 40 gallons | 45 to 55 gal | 4 to 5 GPM |
| 3 people | 40 to 50 gallons | 55 to 65 gal | 5 to 6 GPM |
| 4 people | 50 gallons | 65 to 75 gal | 6 to 7 GPM |
| 5 people | 60 to 66 gallons | 75 to 85 gal | 7 to 8 GPM |
| 6 or more people | 75 to 80 gallons | 85 to 95 gal | 8 GPM or two units |
Household size is a proxy for peak hour demand, and it is a rough one. Four people who shower at four different times need far less capacity than four people who all leave at eight. If your household bunches up, size against the tier above yours.
Where a rule of thumb stops being enough
Every number on this page comes from the standard industry rule of thumb, and a rule of thumb is not an engineering calculation. The real method is a Manual J load calculation, which models your wall and attic insulation, your window area and orientation, how leaky the building envelope is, and how much your ducts lose. It routinely lands a size away from any square-footage estimate — in either direction, which is the part people forget.
So treat this as a sanity check rather than a specification. Ten minutes to size it properly tells you whether a quote is in the right neighbourhood, which is genuinely useful — it is enough to catch a contractor who has quoted you two sizes up because that is what was on the truck. It is not enough to order equipment from.
Tankless changes the question entirely. Instead of gallons you size by flow rate in gallons per minute and by temperature rise, and cold incoming water in winter can halve the flow a unit can support. A tankless heater that is generous in Florida is undersized in Minnesota.