Water Heater Size Calculator
Create a preliminary storage water-heater estimate from peak shower demand, other hot-water use, recovery rate, reserve and temperature rise.
Default project preview
40 gallons
Shower demand
40 gal
Recovery credit
20 gal/hr
First-hour guide
60 gal
Storage requirement
35 gal
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Showers in peak hour
Enter this project assumption in a unitless value. The current imperial default is 2. Minimum: 0.
Minutes per shower
Enter this project assumption in min. The current imperial default is 10 min.
Shower flow
Enter this project assumption in gal/min. The current imperial default is 2 gal/min.
Other peak-hour hot water
Enter this project assumption in gal. The current imperial default is 10 gal. Minimum: 0.
Heater recovery in one hour
Enter this project assumption in gal. The current imperial default is 20 gal. Minimum: 0.
Demand reserve
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Required temperature rise
Enter this project assumption in °F. The current imperial default is 70 °F.
Heating efficiency
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 1. Maximum: 100.
Energy price
Enter this project assumption in per kWh. The current imperial default is 0.18 per kWh. Minimum: 0.
How to read the outputs
Shower demand
2 showers. Default preview: 40 gal.
Recovery credit
entered heater recovery. Default preview: 20 gal/hr.
First-hour guide
tank plus entered recovery. Default preview: 60 gal.
Storage requirement
before standard-size rounding. Default preview: 35 gal.
Peak heating energy
thermal input adjusted for efficiency. Default preview: 10.5 kWh.
Peak energy cost
entered energy rate. Default preview: $1.88.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Peak demand = shower count × duration × flow + other demand, with reserve. Storage need = peak demand − one-hour recovery, rounded up to a planning tank size.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
This calculator estimates storage or first-hour demand from household fixtures and usage assumptions. It is useful for comparing scenarios, but final selection must consider recovery rate, fuel, incoming-water temperature and simultaneous demand.
Calculation steps
- 1List hot-water uses during the busiest hour.
- 2Estimate actual hot-water volume rather than total mixed water.
- 3Add simultaneous fixture demand.
- 4Compare demand with published first-hour rating or recovery performance.
Worked numerical example
If the peak hour includes two 20-gallon showers, 8 gallons of sink use and 7 gallons for a dishwasher, the gross peak demand is 55 gallons before accounting for mixed-water temperature and recovery.
Purchasing and rounding
Storage volume and first-hour rating are different. Compare the calculator result with the certified performance of the exact heater, not tank gallons alone.
Interpret the result
A larger tank can buffer peaks; a faster heater can recover sooner. The best choice depends on both demand pattern and energy source.
Page-specific assumptions
- Peak-hour use is represented by entered fixtures.
- Incoming and delivery temperatures are considered separately if needed.
- No unusual process loads.
- Distribution losses are not hidden in fixture volume.
Common mistakes
- Sizing only by number of bedrooms.
- Comparing tank volume instead of first-hour rating.
- Using total shower water as 100% hot water.
- Ignoring recovery at winter inlet temperature.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Water Heater Size Calculator questions
What is first-hour rating?+
It describes how much hot water a storage heater can deliver during a high-demand first hour, combining stored water and recovery.
Why does incoming-water temperature matter?+
Colder incoming water requires more energy and can reduce recovery performance.
Can this size a tankless heater?+
Not directly. Tankless systems are selected by simultaneous flow and required temperature rise rather than storage volume.