Plumbing
Water Heater Size Calculator
Size a storage water heater from peak demand and recovery rate rather than from tank capacity alone.
Result
Pipe and drain sizing is governed by plumbing code (IPC/UPC or local equivalent) and must be designed and installed by a licensed plumber. This is an educational estimate, not a design. Tank sizing depends on first-hour rating rather than capacity alone. A well-recovering 40 gallon heater can outperform a slow 50. Incoming water temperature drops sharply in winter, which is when tankless units disappoint if undersized.
Get a Free Business AuditWhat moves the number
- First hour rating, which combines stored volume with recovery and is the figure that matters.
- Peak demand period, usually a morning hour rather than a daily total.
- Fuel type, since gas recovers far faster than electric resistance.
Where this one goes wrong
Sizing on tank volume alone. A smaller tank with a fast recovery can outperform a larger slow one across a busy hour, which is exactly when the customer notices.
Before you quote
- Whether the gas line and venting suit the input required.
- Whether the electrical service supports an electric unit of the size proposed.
- Whether an expansion tank is required on a closed system.
Questions
About this calculator
What is first hour rating?
The amount of hot water the heater can deliver in a busy hour, combining what is stored with what it can reheat during that hour. It describes real performance better than tank size does.
Is a bigger tank always better?
No. A larger tank costs more to buy, takes more space and loses more standby heat. Matching first hour rating to peak demand is the better target.
Does a closed system need an expansion tank?
Where a check valve or pressure reducing valve prevents backflow, thermal expansion has nowhere to go and an expansion tank is generally required. Local code governs it.
The formula
tankless BTU/h = GPM × temperature rise × 500
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- People in the household. Enter your own figure; the page starts at 4.
- Bathrooms. Enter your own figure; the page starts at 2.
- Peak hour hot water use (gallons). Roughly 20 gallons per shower, 15 per dishwasher load.
- Incoming water temperature (°F). Enter your own figure; the page starts at 55.
- Desired hot water temperature (°F). Enter your own figure; the page starts at 120.
- Peak simultaneous flow (GPM). Enter your own figure; the page starts at 5.
The result updates as you type, so you can try a figure, see what it does to tank size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| People in the household | 4 |
| Bathrooms | 2 |
| Peak hour hot water use (gallons) | 60 |
| Incoming water temperature (°F) | 55 |
| Desired hot water temperature (°F) | 120 |
| Peak simultaneous flow (GPM) | 5 |
What comes out
| Result | Value |
|---|---|
| Tank size main result | 50 gallon tank |
| First-hour rating needed | 68 |
| Tankless capacity needed | 162,500 |
| Temperature rise required | 65.0 |
A household like this needs a first-hour rating of about 68 gallons, typically a 50 gallon tank. For tankless, 5 GPM at a 65.0°F rise needs roughly 162,500 BTU/h.
How sensitive the answer is
Holding everything else steady and changing only people in the household, this is what happens to tank size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| People in the household | Tank size |
|---|---|
| 2 | 50 gallon tank |
| 3 | 50 gallon tank |
| 4 example above | 50 gallon tank |
| 6 | 65 gallon tank |
| 8 | 100 gallon tank |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- People in the household
- A number, between 0 and 50, starting at 4.
- Bathrooms
- A number, between 0 and 30, starting at 2.
- Peak hour hot water use (gallons)
- Roughly 20 gallons per shower, 15 per dishwasher load.
- Incoming water temperature (°F)
- A number, between 0 and 100, starting at 55.
- Desired hot water temperature (°F)
- A number, between 0 and 180, starting at 120.
- Peak simultaneous flow (GPM)
- A number, between 0 and 50, starting at 5.
What the results tell you
| Result | What it is |
|---|---|
| Tank size | The headline result. A category or planning selection, rather than a measured quantity. |
| First-hour rating needed | A number. |
| Tankless capacity needed | A number. |
| Temperature rise required | A decimal figure. |
Before you rely on this number
Pipe sizing, flow and pressure figures depend on the actual fittings, the run, the material and local code. Code requirements vary by jurisdiction and override any general calculation, so check what applies where the work is happening.
Keep going
Related calculators
Water Flow Calculator
Work out volumetric flow from pipe size and velocity, or check one against the other.
Open calculatorPlumbing Cost Calculator
Cost a plumbing job from burdened labour, materials and the access the work actually requires.
Open calculatorHVAC BTU Calculator
Get a planning figure for the BTU a space needs, as a sanity check against a proper load calculation rather than a substitute for one.
Open calculatorEstimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.