What this calculator does
A shower uses less water than a bath only up to a point, and that point is closer than most people assume. At a 9 litre per minute flow rate against a 150 litre bath, the break-even is about 16 and a half minutes.
Flow rate is what decides it. A water-efficient head at 6 litres a minute gives you 25 minutes before matching the bath; an older unrestricted head at 15 litres a minute uses more than the bath after just ten. The duration matters, but the fitting matters more.
The formula
The shower volume is the flow rate multiplied by the duration, compared directly against the bath fill volume. The difference is reported both in litres and as a percentage of the bath. Neither figure accounts for the energy used to heat the water, which usually tracks the volume closely but depends on the incoming temperature.
| Term | Meaning |
|---|---|
| Flow rate | Litres per minute delivered by the shower head, the dominant variable. |
| Break-even duration | The shower length at which the two use the same water: bath volume divided by flow rate. |
| Low-flow head | Typically 6 to 9 L/min, against 12 to 15 for an older unrestricted one. |
| Bath fill volume | How much water you actually run, which is usually well below the tub capacity. |
The inputs explained
| Field | What to enter |
|---|---|
| Bath fill volume (L) | How much water you actually put in the bath, not the tub capacity. 100 to 160 L is a realistic range for a normal fill. |
| Shower flow rate (L/min) | Shower flow rate in litres per minute. Measure it by timing how long a bucket of known volume takes to fill. |
| Shower duration (min) | Shower duration, including the time the water runs while you are not under it. |
When to use it
Deciding between a bath and a shower
The answer depends entirely on your flow rate and your usual shower length, and the break-even point is the thing to know.
Judging whether a low-flow head is worth it
Comparing the same shower duration at two flow rates shows the saving directly.
Cutting household water use
Showers are one of the largest domestic uses, and the two levers are the fitting and the duration.
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.
At what point does a shower use more than a bath?
A range of shower durations against a fixed bath.
| Shower duration | Shower uses | Difference | Difference as a share of the bath |
|---|---|---|---|
| 4 min | 36 L | 114 L | 76.0% |
| 8 min | 72 L | 78 L | 52.0% |
| 12 min | 108 L | 42 L | 28.0% |
| 16 min | 144 L | 6 L | 4.0% |
| 20 min | 180 L | 30 L | 20.0% |
Questions
Does a bath use more water than a shower?
Usually, but not always. At a typical 9 L/min flow rate, a 150 L bath equals a shower of about 16 and a half minutes. Shorter than that and the shower wins; longer and the bath does. Power showers can exceed a bath in under ten minutes.
How do I measure my shower flow rate?
Time how long the shower takes to fill a container of known volume. A 10 litre bucket filling in 60 seconds is 10 L/min. Do it at your normal temperature and pressure setting, since both affect the result.
What is a typical shower flow rate?
Modern water-efficient heads run 6 to 9 L/min. Older unrestricted heads are commonly 12 to 15, and power showers can exceed 20. The difference between the extremes is larger than the difference between a short shower and a long one.
Does this account for heating energy?
No, it compares water volume only. Energy broadly tracks volume, since most of the cost of hot water is heating it, but the exact figure depends on the incoming cold temperature and how hot you run it. A shower drawing colder mains water in winter costs more to heat per litre.
For the cost of a shower in money terms, see the shower cost calculator. For pool salt levels, see the pool salt calculator.