Show the formula and working
This free hot water usage calculator works out how many litres of hot water a household uses each day, how much energy it takes to heat and what that costs a year, for homes in the UK, Ireland and Europe with a boiler, hot water cylinder, immersion heater or heat pump. It splits the total between showers, baths and other use, and converts it into litres of 60 C stored water to help size a cylinder.
Quick answer: A hot water usage calculator adds up showers, baths and other use, then multiplies the litres by 4.18 kJ and the temperature rise. Two 8 minute showers at 9 L/min, two 80 litre baths a week and 30 L of other use make 197 L a day at 40 C, about 6.86 kWh of heat.
What Is a Hot Water Usage Calculator?
A hot water usage calculator is a tool that estimates a home’s daily hot water volume and the energy and money needed to heat it from cold mains temperature.
The formula in plain words: showers per day times minutes per shower times the flow rate, plus baths per week times litres per bath divided by 7, plus other use, gives litres per day at the use temperature. Each litre of water weighs about 1 kg, and its specific heat capacity is about 4.18 kJ per kg per degree. So the heat in kWh is litres times 4.18 times the temperature rise, divided by 3,600. Divide by the heating efficiency for the fuel used, and multiply by your price per kWh for the cost.
How Do You Use the Hot Water Usage Calculator?
- Enter Showers per day, Minutes per shower and the Shower flow rate.
- Enter Baths per week and Litres per bath.
- Add Other hot water per day for basins, washing up and cleaning.
- Check the Cold mains temperature, Temperature at the tap or shower and Stored water temperature.
- Enter the Water heating efficiency and the Price per kWh of fuel from your bill.
- Read the result and open “Show the formula and working” to see the litres, kWh, cost and stored water equivalent.
How Much Hot Water Does a Shower Use?
A shower uses its flow rate times its length: 8 minutes at 9 L/min is 72 litres, which takes about 2.5 kWh to heat from 10 C to 40 C. Showers often make up most of a household’s hot water, so a shorter shower or a lower flow head has the biggest effect. Time your shower filling a bucket to find its real flow rate. Electric showers heat their own water, so for those use the electric shower cost calculator instead.
| Use | Litres at 40 C | Heat needed (kWh) | Same as 60 C stored water | Cost at 90% and an example 7p per kWh |
|---|---|---|---|---|
| 5 minute shower, 6 L/min | 30 | 1.04 | 18 L | 8.1p |
| 8 minute shower, 9 L/min | 72 | 2.51 | 43 L | 19.5p |
| 8 minute shower, 12 L/min | 96 | 3.34 | 58 L | 26.0p |
| 10 minute shower, 12 L/min | 120 | 4.18 | 72 L | 32.5p |
| Bath, 80 L | 80 | 2.79 | 48 L | 21.7p |
| Bath, 120 L | 120 | 4.18 | 72 L | 32.5p |
All rows assume 10 C cold water. The cost column uses an example rate only, so enter your own in the tool.
How Much Energy Does It Take to Heat Water?
Heating 1 litre of water by 1 C takes about 0.00116 kWh, so heating 100 litres from 10 C to 40 C takes about 3.5 kWh before losses. Boilers, cylinders and pipes lose some heat, which is why the tool divides by an efficiency figure. A heat pump can deliver more heat than the electricity it uses, so for one enter an efficiency above 100%.
What Size Hot Water Cylinder Do You Need?
Start from the “Same amount as 60 C stored water” figure, because water at the tap is mixed with cold. In the default example, 197 litres a day at 40 C equals about 118 litres of 60 C stored water. A cylinder should also cover the busiest hour, such as back to back showers in the morning, and the recovery time of the heat source. Hot water stored in a cylinder should be heated to 60 C, as the HSE guidance on Legionella explains. For immersion heated cylinders, the immersion heater cost calculator shows the cost of each heat up.
Worked Example: A Two Person Home With a Gas Boiler
Example: two 8 minute showers a day at 9 L/min, two 80 litre baths a week and 30 litres a day of other hot water, heated from 10 C to 40 C by a 90% efficient gas boiler at an example 7p per kWh. These are the tool’s default inputs.
- Showers: 2 x 8 x 9 = 144 L a day.
- Baths and other: 2 x 80 / 7 = 22.9 L, plus 30 L, so 196.9 L a day in total.
- Heat: 196.9 x 4.18 x 30 / 3,600 = 6.86 kWh a day.
- Fuel: 6.86 / 0.9 x 365 = 2,781 kWh a year, costing about 194.67 at 7p per kWh.
- Stored water equivalent: 196.9 x 30 / 50 = 118 L of 60 C water a day.
Hot Water Usage Calculator FAQs
How do I calculate hot water usage?
Add shower minutes times flow rate, bath litres and other daily use. Two 8 minute showers at 9 L/min are 144 litres, and with baths and basins the example household uses about 197 litres a day at 40 C.
How many kWh does it take to heat 100 litres of water?
About 3.48 kWh to raise 100 litres from 10 C to 40 C, and about 5.81 kWh to raise it from 10 C to 60 C, before any heating losses. Divide by your system’s efficiency for the fuel used.
Is a bath or a shower cheaper?
It depends on the shower’s flow and length. An 80 litre bath uses more hot water than an 8 minute shower at 9 L/min, which uses 72 litres. A long, powerful shower at 12 L/min for 10 minutes uses 120 litres, more than many baths.
What temperature should hot water be stored at?
Stored hot water should be heated to 60 C to control Legionella bacteria, according to HSE guidance. Water at the tap is usually mixed to around 40 C for showers, and thermostatic mixing valves help prevent scalding.
Why does cold mains temperature matter?
The colder the incoming water, the more energy it takes to heat. Mains water is colder in winter than in summer, so the same showers cost more in January. Enter a lower cold temperature for a winter estimate.
What efficiency should I use for a heat pump?
Use the heat pump’s efficiency for hot water, which is usually lower than for space heating because it heats water to a higher temperature. A figure above 100%, such as 250%, means each kWh of electricity gives 2.5 kWh of heat. Check your installer’s figures.
Checked October 2026. Sources: Wikipedia, Specific heat capacity, HSE, Legionella and Legionnaires’ disease.