Water Heater Size Calculator
Find the right size of geyser, hot water cylinder or tank heater from your busiest hour, or the kW and BTU/h a tankless heater needs. Advanced adds heat-up time, running cost, solar and mixing.
Hot water uses in your busiest hour, often the morning rush. The amounts are typical; change them to match your home.
Many dishwashers fill with cold water and heat it themselves: put 0 if yours does.
Most washing machines are cold-fill and heat their own water: put 0 unless yours takes hot water.
Showers, baths and sinks mix hot with cold, so only part comes from the tank. Many guides advise keeping stored water at 60 °C to stop bacteria growing, with a mixing valve to stop scalding.
Cold water mixes in as you draw off, so you get about 70% of a tank hot. Recovery is the hot water the heater makes in an hour: a 3 kW element gives about 54 L at a 45 °C rise, gas heaters often more. US labels list it as recovery in gallons an hour.
Electric elements turn nearly all their power into heat (90–100%); gas tanks are often 60–80%. Standby loss is the heat a full tank loses through its walls and pipes, added on top of the energy a day; better insulation lowers it.
Taps and showers that may run at the same time. Flows are typical; check the label or time a bucket.
Gas tankless heaters are often 80–96% efficient; electric ones about 99%. Type a heater's output to see how much flow it can heat. Big electric tankless heaters draw a lot of current: check your supply and wiring with an electrician.
Mains water is colder in winter and in cold places, so size for winter. Rooftop tanks in hot places can be 30 °C or more.
Gas is often billed per kWh too (or per m³ or therm; your bill shows the kWh). The solar figure is only a rough rule for flat-plate collectors in sunny places: cloudy places need more area, evacuated tubes less.
How your water heater size is worked out
The hot water your busiest hour needs, or the heat a tankless heater must give for every tap running at once.
- 1
Busy-hour water
3 × 8 min × 9 L/min+ 10 L baths, sinks= 226 L at the tap
- 2
Hot part only
(40 − 15) ÷ (60 − 15)= 55.6% hot= 125.6 L hot
- 3
Tank from first hour
(125.6 − 50) ÷ 70%recovery 50 L/h= 107.9 L
- 4
Pick a size
next common size upfirst hour 134 L= 120 L
Read more: the formulas, a worked example, typical figures and common sizes
A storage heater (geyser, hot water cylinder or tank) must cover your busiest hour: the hot water it holds, less the part that cools as cold water mixes in, plus what it heats during that hour. A tankless heater stores nothing, so it must heat the full flow of every tap running at once, straight away. Both come from the same fact: heating 1 litre of water by 1 °C takes 4.186 kJ.
The formula
hot share = (tap temperature − cold) ÷ (tank temperature − cold)
busiest hour = (showers × minutes × flow + baths × bath size + sinks × litres) × hot share + dishwasher and washer hot water
first-hour rating = tank × usable share + recovery in an hour
tank needed = (busiest hour − recovery) ÷ usable share, then the next common size up
recovery of a heater (L an hour) = kW × 3600 × efficiency ÷ (4.186 × rise)
heat-up time (hours) = litres × 4.186 × rise ÷ (3600 × kW × efficiency)
tankless power (kW) = flow in L/min × 4.186 × rise ÷ 60
gas input = power ÷ efficiency · BTU/h = kW × 3,412.14 · amps = watts ÷ volts
energy a day (kWh) = people × litres each × 4.186 × rise ÷ 3600 ÷ efficiency × (1 + standby %)
cost = kWh × your price · solar area = hot water a day ÷ litres per m²
water at the tap from a full tank = tank × usable share ÷ hot share
Worked example
Storage tank. In the busiest hour, 3 showers of 8 minutes at 9 litres a minute use 3 × 8 × 9 = 216 litres, and 2 sink uses of 5 litres add 10: 226 litres at the tap. With cold water at 15 °C, the tank at 60 °C and the tap at 40 °C, the hot share is (40 − 15) ÷ (60 − 15) = 25 ÷ 45 = 55.6%, so 226 × 25 ÷ 45 = 125.6 litres come from the heater. A tank gives 70% of its water hot and recovers 50 litres in the hour, so it needs (125.6 − 50) ÷ 0.7 = 107.9 litres (28.5 US gallons). The next common size is 120 litres, with a first-hour rating of 120 × 0.7 + 50 = 134 litres; in US gallons it is a 30-gallon tank. A 3 kW element at 95% heats it from 15 °C to 60 °C in 120 × 4.186 × 45 ÷ (3600 × 3 × 0.95) = 2.2 hours, about 2 h 12 min. Four people using 50 litres each a day need 200 × 4.186 × 45 ÷ 3600 = 10.47 kWh, or 10.47 ÷ 0.95 × 1.15 = 12.67 kWh a day with 15% standby loss. A full tank gives 120 × 0.7 ÷ (25 ÷ 45) = 151.2 litres at 40 °C, about 16.8 minutes of shower.
Tankless. One shower at 9 litres a minute and a kitchen tap at 6 make 15 litres a minute. From 15 °C to 45 °C is a 30 °C rise, so the heater must give 15 × 4.186 × 30 ÷ 60 = 31.4 kW (107,124 BTU/h). A gas heater at 90% needs an input of 31.395 ÷ 0.9 = 34.9 kW, about 119,027 BTU/h. An electric one would draw 31,395 ÷ 230 = 136.5 amps at 230 V.
Typical figures used
| Use | Typical figure |
|---|---|
| Shower | 8 min at 9 L/min (2.4 US gpm) |
| Bath, filled | 150 L (40 US gal) |
| Sink or basin use | 5 L |
| Dishwasher on hot fill | 15 L a load |
| Washing machine on hot fill | 20 L a load |
| Usable share of a tank | 70% |
| Hot water per person a day | 50 L |
| Shower flow (tankless) | 9 L/min (2.4 US gpm) |
| Kitchen tap flow (tankless) | 6 L/min (1.6 US gpm) |
| Basin tap flow (tankless) | 4 L/min (1.1 US gpm) |
| Bath tap flow (tankless) | 15 L/min (4 US gpm) |
| Dishwasher flow (tankless) | 5 L/min (1.3 US gpm) |
| Washing machine flow (tankless) | 7 L/min (1.8 US gpm) |
These are rough typical figures, not measurements: low-flow shower heads can use 6 L/min or less, power showers 12 or more. To measure a flow, time how long a 10-litre bucket takes to fill: 10 ÷ seconds × 60 = litres a minute.
Common sizes
Geysers and cylinders are usually sold in litres: 10, 15, 25, 35, 50, 80, 100, 120, 150, 200, 250, 300, 400, 500 L. US tank heaters are sold in gallons: 20, 30, 40, 50, 55, 65, 75, 80, 100, 120 US gal. Small 10–35 L geysers suit one bathroom or a kitchen; larger homes use 150 L and up, or two heaters. Makers' sizes vary, so check the label.
Tips
- Size for winter: colder mains water means each litre needs more heat, and tankless heaters give less flow.
- A bigger tank costs more to keep hot all day. If your busy hour is short, a faster element (more kW) can do the job of a bigger tank.
- Low-flow shower heads cut the busiest-hour demand and the size of tankless heater you need.
- Large electric tankless heaters can need 100 A or more; gas ones need the right gas pipe size and flue. Ask a licensed installer.
This gives a starting size, not a design. Local rules on stored water temperature, valves and installation vary by country. No electricity or gas price is filled in: type your own from your bill.
Common questions
What size water heater do I need for a family of 4?
Add up the hot water used in your busiest hour. Three 8-minute showers at 9 litres a minute plus two 5-litre sink uses is 226 litres at the tap. With the tank at 60 °C, cold water at 15 °C and showers at 40 °C, only (40 − 15) ÷ (60 − 15) = 55.6% of that comes from the heater: 125.6 litres. A tank gives about 70% of its water hot, plus what it heats during the hour (say 50 litres), so you need (125.6 − 50) ÷ 0.7 = 107.9 litres. The next common size is 120 litres, or 30 US gallons.
How do I size a tankless (instant) water heater?
Add the flow of every tap and shower that may run at once, then multiply by 4.186 and the temperature rise, and divide by 60. One shower at 9 litres a minute and a kitchen tap at 6 is 15 litres a minute. Heating it from 15 °C to 45 °C is a 30 °C rise, so 15 × 4.186 × 30 ÷ 60 = 31.4 kW. A gas heater at 90% efficiency needs an input of 31.4 ÷ 0.9 = 34.9 kW, about 119,000 BTU/h. In US units, BTU/h is about 500 × gpm × the rise in °F.
What is a first-hour rating?
The first-hour rating is how much hot water a tank heater can deliver in one busy hour, starting full of hot water. It is roughly the usable part of the tank (often about 70%) plus what the heater warms up during that hour. A 120-litre tank that recovers 50 litres an hour gives about 120 × 0.7 + 50 = 134 litres. In the US it is printed on the yellow EnergyGuide label; pick a tank whose rating covers your busiest hour.
How long does a geyser take to heat up?
Time in hours = litres × 4.186 × temperature rise ÷ (3600 × kW × efficiency). A 120-litre tank heated from 15 °C to 60 °C by a 3 kW element at 95% takes 120 × 4.186 × 45 ÷ (3600 × 3 × 0.95) = 2.2 hours, about 2 h 12 min. A small 15-litre geyser with a 2 kW element takes about 25 minutes for the same rise.
How much hot water does a shower use?
A shower mixes hot with cold, so the heater only gives part of it. The hot share is (shower temperature − cold) ÷ (tank temperature − cold). At 40 °C from a 60 °C tank with 15 °C cold water, that is 25 ÷ 45 = 55.6%. An 8-minute shower at 9 litres a minute uses 72 litres at the shower head, of which 40 litres come from the tank.
How much electricity does a water heater use a day?
Heating each litre by 1 °C takes 4.186 kJ. Four people using 50 litres of hot water each, heated from 15 °C to 60 °C, need 200 × 4.186 × 45 ÷ 3600 = 10.47 kWh. At 95% efficiency that is 11.02 kWh, and with 15% added for heat lost from the tank, about 12.67 kWh a day. Multiply by your own price per kWh for the cost.
Related calculators
Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.