Water Heater Size Calculator
Size a water heater from your peak-hour fixture use: tank first-hour rating and recovery, or tankless GPM at your design temperature rise, with a pass or fail verdict.
Peak-hour fixture use
Fill in the busiest hour of your day, not the whole day. Columns: fixture, uses in that hour, hot gallons per use, flow in GPM, and whether it can run at the same time as the others.
Tank sizing
The peak hour needs 116.0 gallons but the first-hour rating is only 94.1. You are 21.9 gallons short — a bigger tank, a hotter setting with a mixing valve, or spreading showers out will all close the gap.
Tankless flow at other temperature rises
| Rise (°F) | Inlet at 120 °F outlet | Flow (GPM) | Covers simultaneous demand? |
|---|---|---|---|
| 35 | 85 °F | 1.83 | No |
| 45 | 75 °F | 1.42 | No |
| 55 | 65 °F | 1.16 | No |
| 65 | 55 °F | 0.98 | No |
| 77 | 43 °F | 0.83 | No |
A tankless heater is rated by temperature rise, not by a single flow number. The same unit that gives you a comfortable shower in August can struggle in February when the mains come in twenty degrees colder — which is why the sizing rise should be your winter inlet temperature.
Peak-hour demand = Σ(uses × hot gallons per use). Recovery = BTU/hr × efficiency ÷ (8.33 × temperature rise), so a 40,000 BTU gas heater at 80% recovery makes about 42 gallons an hour at a 90 °F rise. First-hour rating = usable storage + recovery, which is why a 50-gallon tank is rated near 70 gallons. Tankless flow = BTU/hr × efficiency ÷ (500 × rise), where 500 is 8.33 lb per gallon × 60 minutes. Water hotter than 120 °F scalds quickly — if you store hot for capacity, fit a thermostatic mixing valve. Estimates only; sizing and gas or electrical work should be signed off by a qualified installer.
What is the Water Heater Size Calculator?
The ByteTools Water Heater Size Calculator builds your peak hour from an editable fixture table. Enter how many times each fixture is used in your busiest hour and how many hot gallons it takes, and it totals the demand — then separately totals the flow of the fixtures that can run at the same time, which is the number a tankless unit has to satisfy.
- Editable fixture table with a simultaneous-use flag for tankless sizing
- Tank recovery, usable storage and first-hour rating from burner input and efficiency
- Tankless flow in GPM at your design rise, plus a table across five rises
- Gas BTU/hr or electric watts, with the conversion handled for you
- Plain pass or fail verdict against your peak hour or your simultaneous flow
- CSV export of the demand sheet; all calculations run locally in your browser
How to use the Water Heater Size Calculator
- 1
Pick tank or tankless, gas or electric, and enter the cold inlet and hot outlet temperatures you design for.
- 2
Fill in the fixture table for your busiest hour: uses, hot gallons per use, flow in GPM, and whether each can run at the same time as the others.
- 3
Enter the burner input in BTU/hr or the element rating in watts, plus the recovery efficiency.
- 4
For a tank, add the capacity and the usable storage percentage to get recovery and a first-hour rating.
- 5
Read the pass or fail verdict, then check the table of tankless flow at other temperature rises for winter conditions.
About the Water Heater Size Calculator
The ByteTools Water Heater Size Calculator builds your peak hour from an editable fixture table. Enter how many times each fixture is used in your busiest hour and how many hot gallons it takes, and it totals the demand — then separately totals the flow of the fixtures that can run at the same time, which is the number a tankless unit has to satisfy.
For a storage tank it works out recovery as BTU/hr × efficiency ÷ (8.33 × temperature rise) and adds the usable storage fraction to get a first-hour rating. A 40,000 BTU gas heater on a 50-gallon tank at a 65 °F rise comes out close to the first-hour rating printed on the label. For a tankless unit it solves flow as BTU/hr × efficiency ÷ (500 × rise).
A table shows tankless flow at five different temperature rises, because the same unit that is comfortable in August struggles in February when the mains come in twenty degrees colder. Everything runs in your browser. Estimates only — sizing plus gas or electrical work should be signed off by a qualified installer.
Frequently asked questions
What size water heater do I need for a family of four?
Work from the peak hour, not the household size. Two back-to-back showers, a load of laundry and the dishwasher can easily be 60 to 70 gallons of hot water in one hour, which is what a 50-gallon gas tank with a first-hour rating near 70 is designed to cover.
What is a first hour rating on a water heater?
It is how many gallons of hot water the heater can deliver in one hour starting from a full, hot tank — usable storage plus what the burner can recover in that hour. It is the number to compare against your peak-hour demand, and it is why a 50-gallon tank is rated near 70.
How do I calculate water heater recovery rate?
Recovery in gallons per hour is BTU/hr × efficiency ÷ (8.33 × temperature rise). A 40,000 BTU gas heater at 80% recovery efficiency and a 90 °F rise makes about 43 gallons an hour; a 4,500 W electric element at 98% manages about 20.
What size tankless water heater do I need?
Size it on simultaneous flow at your winter temperature rise. A 199,000 BTU unit at 95% efficiency delivers about 5.4 GPM at a 70 °F rise, which covers two showers and a sink. At a 45 °F rise the same unit gives over 8 GPM, so the winter figure is the one that matters.
Why does my tankless water heater struggle in winter?
Because its output is fixed in BTU, so the colder the incoming water the less flow it can heat to the same outlet temperature. Mains water that is 70 °F in summer can be 40 °F in winter, nearly halving the usable GPM. Always size on the winter inlet.
Should I set my water heater to 120 or 140 degrees?
120 °F is the usual recommendation because water above that scalds quickly, especially for children and older adults. Some people store hotter for extra effective capacity and fit a thermostatic mixing valve to deliver 120 °F at the taps — that is a plumber's job.
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