Plumbing
Plumbing Water Heater Size Calculator
Match water heater capacity to the household's peak hour, with recovery counted alongside storage.
Result
Storage heaters are sized on the busiest hour rather than daily total, and the number that matters on the label is the first hour rating rather than the tank volume. A tank only delivers about 70% of its capacity before the outlet runs cold.
Get a Free Business AuditWhat moves the number
- Number of simultaneous fixtures during the peak hour.
- Recovery rate, which differs enormously between fuel types.
- Incoming water temperature, which is seasonal and affects how much heating is needed.
Where this one goes wrong
Sizing from occupant count alone. Two people with a large soaking tub and a rain shower have a very different peak from four with standard fixtures, and occupancy alone will not predict it.
Questions
About this calculator
How do I work out the peak hour?
Ask what happens on the busiest morning: how many showers, whether they overlap, and whether laundry or a dishwasher runs at the same time. That hour is what the heater has to cover.
Does incoming water temperature matter?
Considerably. Colder incoming water in winter means more energy for the same delivered temperature, so a heater that copes in summer can fall short in February.
Gas or electric?
Gas recovers faster for the same capacity, electric is simpler to install where gas is absent. The decision usually comes down to available services rather than preference.
The formula
peak demand = showers x minutes x GPM x 0.7 + baths + appliance loads; tank = demand / 0.7
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How to use this calculator
- People in the household. Enter your own figure; the page starts at 4.
- Showers in the peak hour. Enter your own figure; the page starts at 3.
- Minutes per shower. Enter your own figure; the page starts at 8.
- Shower flow (GPM). Enter your own figure; the page starts at 2.1.
- Baths in the peak hour. Enter your own figure; the page starts at 0.
- Dishwasher or laundry loads. Enter your own figure; the page starts at 1.
The result updates as you type, so you can try a figure, see what it does to peak hour demand, 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 |
| Showers in the peak hour | 3 |
| Minutes per shower | 8 |
| Shower flow (GPM) | 2.1 |
| Baths in the peak hour | 0 |
| Dishwasher or laundry loads | 1 |
What comes out
| Result | Value |
|---|---|
| Peak hour demand main result | 42.3 |
| Suggested tank size | 65 |
| First hour rating to look for | 46.5 |
| Per shower | 11.8 |
Peak hour demand is about 42.3 gallons of hot water, so look for a first hour rating near 46.5, which is usually a 65 gallon tank.
How sensitive the answer is
Holding everything else steady and changing only people in the household, this is what happens to peak hour demand. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| People in the household | Peak hour demand |
|---|---|
| 2 | 42.3 |
| 3 | 42.3 |
| 4 example above | 42.3 |
| 6 | 42.3 |
| 8 | 42.3 |
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 30, starting at 4.
- Showers in the peak hour
- A number, between 0 and 30, starting at 3.
- Minutes per shower
- A number, between 0 and 60, starting at 8.
- Shower flow (GPM)
- A number, between 0 and 6, starting at 2.1.
- Baths in the peak hour
- A number, between 0 and 10, starting at 0.
- Dishwasher or laundry loads
- A number, between 0 and 20, starting at 1.
What the results tell you
| Result | What it is |
|---|---|
| Peak hour demand | The headline result. A decimal figure. |
| Suggested tank size | A number. |
| First hour rating to look for | A decimal figure. |
| Per shower | 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.
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