Water heater size calculator: tank, tankless or geyser
This water heater size calculator answers the three sizing questions people ask most: how big a storage tank (or heat pump water heater) to buy, how powerful a tankless heater must be, and how many litres a bathroom geyser needs for a family. Each method shows the formula and every step, in US gallons and °F or in litres and °C.
The tank method follows the US Department of Energy’s peak-hour worksheet; the tankless method uses the physics of heating flowing water; the geyser method works out how much stored hot water is drawn off when several people bathe one after another.
How to use the Water Heater (Geyser) Size Calculator
- Pick Tank (peak hour), Tankless or Bathroom geyser, and choose gallons and °F or litres and °C.
- For a tank, list what uses hot water in your busiest hour and how many times — the defaults are the DOE example.
- For tankless, set which outlets run at the same time, their flow rates, the incoming water temperature and the hot water temperature you want.
- For a geyser, enter how many people bathe one after another, the warm water each uses, and the cold, bath and thermostat temperatures.
- Read the size, check the steps and copy the summary to compare models.
Water Heater (Geyser) Size Calculator features
First hour rating
Adds up peak-hour use into the first hour rating to look for on a US EnergyGuide label.
Tankless output
kW, BTU/h, gas input at your efficiency and the current an electric unit would draw.
Geyser litres
Mixing maths for bucket baths or showers, with the element reheating during the session and the heat-up time from cold.
Gallons or litres
Switch units at any time; values are converted for you.
Editable everything
Uses, flow rates and temperatures are all your own numbers.
Storage tanks: size by first hour rating, not tank volume
The DOE advises sizing a storage or heat pump water heater by its first hour rating (FHR) — the gallons of hot water it can supply in an hour starting full — and choosing a model whose FHR at least matches your peak hour demand. Its worksheet (Energy Saver, March 2025) uses about 20 gallons per shower, 2 for shaving, 3 for hand dishwashing or food prep, 7 for an automatic dishwasher and 25 (top-loader) or 15 (front-loader) for a clothes washer. Three showers, a shave and some hand washing-up add up to 65 gallons (the DOE page prints its example total as 66), so look for an FHR of at least 65–66.
Tankless water heater sizing
A tankless heater is rated by the temperature rise it can give at a flow rate. Add the flow of everything that runs at once — the DOE example is a 0.75 gpm faucet and a 2.5 gpm shower, 3.25 gpm — and subtract the incoming water temperature (assume 50 °F / 10 °C if unsure) from the output you want (120 °F / 49 °C): a 70 °F rise. Heat needed = flow × 4.186 × rise ÷ 60 kW, here about 33 kW (114,000 BTU/h). An electric unit that size draws about 140 A at 240 V, which is why DOE notes that a 70 °F rise is typically possible at about 5 gpm for gas-fired models but only about 2 gpm for electric ones.
How many litres of geyser for a family?
A bath at 40 °C made from 60 °C stored water and 15 °C cold water is (40 − 15) ÷ (60 − 15) = 56 % hot water. Three 20-litre bucket baths therefore take 33 litres from the geyser, but a 2 kW element reheats about 19 litres during the half hour they take, and only about 80 % of a tank comes out hot before cold water mixes in. That points to a 25 litre geyser, which heats from cold in about 40 minutes. Showers use far more: a 5-minute shower is 40–50 litres of warm water.
| Household (rule of thumb) | Bathroom geyser |
|---|---|
| 1–2 people, bucket baths | 10–15 L |
| 3–4 people, bucket baths | 25 L |
| 4–5 people or a shower | 35 L |
| Large family, showers or a bathtub | 50 L or more |
Water Heater (Geyser) Size Calculator FAQ
What size water heater do I need?
For a tank, one with a first hour rating at least equal to your busiest hour’s use — about 65 gallons for three showers, a shave and some washing-up in an hour. For tankless, enough output for the flow you run at once at your temperature rise. The water heater size calculator works out both, plus geyser litres.
How do I size a tankless water heater?
Add the flow rates of the fixtures used at once (gpm or L/min) and find the temperature rise from the incoming water to the hot water you want. kW = L/min × 4.186 × rise ÷ 60; BTU/h = kW × 3,412.
How many litres geyser do I need for a family of 4?
For bucket baths one after another, 25 litres is common; with showers, 35–50 litres. The geyser mode works it out from your own bath volume and temperatures.
Why does a geyser star rating matter?
In India, BEE star labels for electric storage water heaters (up to 200 litres) rate standing heat loss over 24 hours — a higher star rating loses less heat while the water waits.
What temperature should a water heater be set to?
DOE suggests about 120 °F (49 °C) for most uses. Hotter water stores more usable heat but raises scalding risk, so use a thermostatic mixing valve if you store it hotter.
Is this exact?
No — it is a guide built on typical values. Real use, pipe runs, inlet temperatures and the heater’s own ratings change the answer.