What this calculator does
A shower costs water plus the energy to heat it, and the heating is much the larger part. An eight minute shower at 8 litres a minute costs about 16 cents of water and 62 cents of energy, so heating is roughly four fifths of the total.
That split is why flow rate and duration matter more than the water price. Every litre saved saves both the water and the energy to heat it, which is why a low-flow head is one of the more effective domestic energy measures rather than just a water measure.
The formula
The water volume is the flow rate multiplied by the duration. Water cost is that volume at the price per thousand litres. Heating energy uses the specific heat capacity of water, 4.186 kJ per kg per degree, multiplied by the volume and the temperature rise, divided by 3,600 to give kWh and by the heater efficiency. The two costs are added.
| Term | Meaning |
|---|---|
| Flow rate | Litres per minute from the shower head. Standard heads run 9 to 12; low-flow heads 6 to 8. |
| Temperature rise | How far the water must be heated, from mains temperature to shower temperature. |
| Specific heat capacity | 4.186 kJ per kilogram per degree for water, the constant that sets the energy required. |
| Heater efficiency | The share of input energy reaching the water. Around 0.9 for electric resistance, lower for gas. |
The inputs explained
| Field | What to enter |
|---|---|
| Shower length (minutes) | Shower duration in minutes. |
| Flow rate (L/min) | Flow rate in litres per minute. Measure it by timing how long a bucket of known volume takes to fill. |
| Water price ($/1000 L) | Water price per thousand litres. |
| Electricity price ($/kWh) | Electricity or gas price per kWh. |
| Temperature rise needed (°C) | Temperature rise in degrees. Mains water is typically 10 to 15°C and a shower around 38 to 40°C, so 25 is a reasonable figure. |
| Water heater efficiency | Heater efficiency as a decimal. 0.9 for electric, around 0.8 for gas. |
When to use it
Working out what showers cost a household
Multiplying by the number of showers a year turns an invisible cost into a line that can be compared against other household spending.
Assessing a low-flow shower head
Comparing the cost at 12 litres a minute against 8 gives the annual saving directly, which is usually enough to pay for the head within months.
Comparing shower against bath
A bath is a fixed volume, typically 100 to 150 litres, so comparing it against the calculated shower volume settles which is cheaper in a particular household.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does shower length cost?
A range of shower durations at a fixed flow rate.
| Shower length | Cost per shower | Water used | Heating cost | Cost of 365 showers |
|---|---|---|---|---|
| 4 min | $0.39 | 32.0 L | $0.31 | $142.38 |
| 8 min | $0.78 | 64.0 L | $0.62 | $284.75 |
| 12 min | $1.17 | 96.0 L | $0.93 | $427.13 |
| 20 min | $1.95 | 160.0 L | $1.55 | $711.89 |
Questions
Why is heating so much more than the water?
Because raising water temperature takes a great deal of energy. Heating 64 litres by 25 degrees needs about 2 kWh, which at typical electricity prices costs several times the water itself. This is why hot water is usually the second largest energy use in a home after space heating.
How do I measure my flow rate?
Time how long the shower takes to fill a container of known volume. A 10 litre bucket filling in 50 seconds is 12 litres a minute. Doing this is worthwhile, since flow rates vary far more between installations than people expect.
Is a shower cheaper than a bath?
It depends entirely on the shower. A bath is typically 100 to 150 litres. At 8 litres a minute a shower passes 100 litres at around twelve and a half minutes, so a short shower is much cheaper and a long one is not.
What temperature rise should I use?
The difference between mains temperature and shower temperature. Mains water runs roughly 10 to 15°C depending on season and location, and a comfortable shower is 38 to 40°C, so 25 degrees is a reasonable year-round figure. Winter mains temperatures push it higher.
For appliance running costs generally, see the electricity bill calculator. For heating water in an EV context, see the EV charging cost calculator.