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RemoveAndReplace.com · Water Heater Sizing
Buy the right size water heater the first time
Answer a few questions about your household and hot water habits — we’ll recommend a tank size or tankless GPM rating.
Use this water heater sizing calculator to find the right tank size or tankless GPM (gallons-per-minute) rating for your home. It works from four inputs — how many people live in the house, how many bathrooms you have, your fuel type, and how often hot water use overlaps during a typical busy hour — and returns both a recommended tank size and a tankless GPM rating so you can compare either path before you buy.
Getting this wrong has real costs: an undersized water heater runs out of hot water during busy mornings, while an oversized one wastes energy keeping water hot that never gets used. The goal is to size for your peak hour of demand, not your average day.
How this calculator works
The calculator builds a recommendation in five steps:
- Start from household size:
30 galfor 1–2 people,40 galfor 3–4,50 galfor 5–6,65 galfor 7+. - Add
15 galif the home has 3 or more bathrooms. - Add
5 galfor moderate usage overlap, or20 galfor high overlap (back-to-back showers, laundry, and dishes at once). - Add
10 galif the fuel type is electric, since electric elements recover more slowly than gas or propane. - Round up to the nearest standard tank size: 30, 40, 50, 65, 80, or 100 gallons.
For a tankless recommendation, the calculator estimates peak simultaneous flow: a base of 1.5 GPM, plus 0.9 GPM per bathroom, plus 0.6 GPM for households over 4 people, plus 1.2–2.5 GPM depending on usage overlap — then rounds up to the nearest common tankless rating (6, 8, 9, or 10 GPM).
Worked example
A household of 4 people, 2 bathrooms, gas fuel, with moderate usage overlap:
| Input | Value |
|---|---|
| Base tank size (4 people) | 40 gal |
| Bathroom adjustment (2 baths) | +0 gal |
| Usage adjustment (moderate) | +5 gal |
| Fuel adjustment (gas) | +0 gal |
| Rounded to standard size | 50 gal tank, or 8 GPM tankless |
Sizing tips that matter
Tank size is about peak, not average
You don’t need enough hot water for a whole day at once — you need enough for your worst hour, like two showers back to back plus a load of dishes.
Tankless is rated by flow, not volume
A tankless unit’s GPM (gallons per minute) rating needs to cover every fixture you might run at the same time, not just one shower.
Undersizing shows up fast
A too-small tank or tankless unit means cold showers during busy mornings. Oversizing wastes energy keeping water hot you don’t use.
Common questions
Tanks cost less upfront and handle simultaneous high-volume use well. Tankless units cost more upfront, take up less space, and never run out of hot water, but need enough GPM capacity for your peak demand. Both are shown here so you can compare.
It’s how many gallons of hot water a tank can deliver in one hour, starting full. It matters more than raw tank size when comparing models — look for it on the yellow EnergyGuide label.
Not the gallons or GPM needed, but it changes recovery speed. Gas and propane heaters recover faster than electric, so an electric tank of the same size may need to be a size larger to keep up.
No — the range shown is equipment only. Installation (venting, gas lines, electrical, permits) varies widely and typically adds several hundred to a few thousand dollars.
Most families of 4 with 1–2 bathrooms need a 40–50 gallon tank, or a 6–8 GPM tankless unit. The exact number depends on your bathroom count, fuel type, and how often hot water use overlaps in your home.
A 2-bathroom home typically needs 6–8 GPM of tankless capacity, depending on household size and how often showers, laundry, and dishwashing overlap.
As a rough guide: 30 gallons for 1–2 people, 40 gallons for 3–4, 50 gallons for 5–6, and 65+ gallons for 7 or more. Bathroom count, fuel type, and usage habits shift this up or down.
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