Water heater size calculator: what size water heater do I need?
60 gallons in the peak hour
2 showers 40 gal · 1 shave 2 gal · 1 hand and face wash 4 gal · 1 dishwasher load 14 gal · first hour rating 60 gal or more · by the AHRI worksheet
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Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
60 gallons in the peak hour
2 showers 40 gal · 1 shave 2 gal · 1 hand and face wash 4 gal · 1 dishwasher load 14 gal · first hour rating 60 gal or more
Source: AHRI (Air-Conditioning, Heating, and Refrigeration Institute), Worksheet for Estimating Peak Hour Demand/First Hour Rating: average gallons of hot water per use, read November 7, 2026. Data sheet (PDF)
Two showers, a shave, a hand and face wash and a dishwasher load in the same hour use 60 gallons of hot water by the AHRI worksheet, so the tank water heater needs a first hour rating of 60 gallons or more.
A worked example
A Saturday morning hour with three showers, a bath, a load of laundry and breakfast cooking needs 117 gallons in the peak hour. 3 showers 60 + 1 bath 20 + 1 washer load 32 + food preparation 5 = 117 gallons: a first hour rating of 117 gallons or more.
What size water heater for a family of 4?
It depends on the busiest hour. Three showers and a shampoo, 64 gallons; four showers, 80; four showers and a dishwasher load, 94. Pick a first hour rating at or above that.
What is a first hour rating?
The figure a tank water heater is rated by for its busiest hour of hot water. The AHRI worksheet matches it to your peak hour demand: its own example, three showers, a shave, a shampoo and hand dishwashing, comes to 70 gallons.
What else it is called
Also searched as what size water heater for a family of 4 or for a 3 bedroom house, a water heater size calculator or a water heater size chart. The size follows the busiest hour, not the people or the bedrooms: four showers one after another in a morning hour use 80 gallons.
Gallons per use in the peak hour
| Use | Once in the hour | Twice | Three times |
|---|---|---|---|
| Shower | 20 gal | 40 gal | 60 gal |
| Bath | 20 gal | 40 gal | 60 gal |
| Shave | 2 gal | 4 gal | 6 gal |
| Hand and face wash | 4 gal | 8 gal | 12 gal |
| Shampoo | 4 gal | 8 gal | 12 gal |
| Hand dishwashing | 4 gal | 8 gal | 12 gal |
| Dishwasher load | 14 gal | 28 gal | 42 gal |
| Food prep | 5 gal | 10 gal | 15 gal |
| Wringer washer load | 26 gal | 52 gal | 78 gal |
| Washer load | 32 gal | 64 gal | 96 gal |
AHRI, Worksheet for Estimating Peak Hour Demand/First Hour Rating: average gallons of hot water per use. Add the uses of your busiest hour; the total is the first hour rating to look for. The worksheet assumes no water conservation measures.
Where the numbers come from
- AHRI, Worksheet for Estimating Peak Hour Demand/First Hour Rating, average gallons of hot water per use: shower 20, bath 20, shaving 2, hands and face washing 4, hair shampoo 4, hand dishwashing 4, automatic dishwasher 14, food preparation 5, wringer clothes washer 26, automatic clothes washer 32. Read November 7, 2026. AHRI worksheet
- AHRI's example: 3 showers 60, 1 shave 2, 1 shampoo 4, 1 hand dishwashing 4, “Peak Hour Demand = 70.” “The above worksheet assumes no water conservation measures.” AHRI worksheet
- The wringer washer, 26 gallons, is on the worksheet but not in the calculator. Every gallon figure here is AHRI's; none comes from another method.
The worksheet assumes no water conservation measures, so low flow showerheads use less than it counts. A first hour rating is the figure for a tank water heater; a tankless one is sized by flow and temperature rise instead.
By Javier Miralles · Last updated . How this site is made.
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