Bath vs Shower Calculator
Compare a bath's fill volume with a shower's running-water volume. See the water difference and the shower duration at which usage becomes equal.
How the result changes
Only shower flow changes; all other inputs stay at your current values. The line illustrates the formula across the shown range. This is not a forecast of real-world outcomes.
About this calculation
A bath can use more or less water than a shower depending on how full the tub is and how long the shower runs. This calculator compares measured volumes rather than assuming either activity always wins. Compare bath vs shower when estimating personal care needs.
Formula
shower litres = flow × running minutes; equal-use minutes = bath litres ÷ shower flow
Worked example
With Bath fill volume: 120 L, Shower flow: 8 L/min, Shower running time: 10 min, shower water is 80 L.
Inputs and units
Enter litres used to fill the bath, not the advertised tub capacity to the rim. Use showerhead litres per minute and minutes of actual flow; an off-and-on shower should count only the running intervals.
- Bath fill volume (L): Estimate water actually added to the tub, not its brim-full rating.
- Shower flow (L/min): Use the actual showerhead rate at normal operating pressure.
- Shower running time (min): Count only time that water flows.
Calculation and worked example
Multiply shower flow by running minutes for shower litres. Subtract that from the bath volume for a signed difference. Divide bath litres by shower flow to find the shower length at which water use is equal In compact notation, shower litres = flow × running minutes; equal-use minutes = bath litres ÷ shower flow.
With a 120 L bath fill and an 8 L/min shower, ten running minutes use 80 L. Bath minus shower is +40 L, and a fifteen-minute shower reaches the same 120 L volume.
How to use the answer
A positive difference means the entered bath uses more water; a negative result means the shower uses more. The equal-use time is a threshold for this particular flow rate, not a universal shower recommendation.
Change the shower duration to see when the comparison reverses, or test a different showerhead flow. If the bath is shared, decide whether to compare total household water or per-person use before interpreting the difference.
Assumptions and checks
Shower flow remains constant and bath fill volume is measured accurately. Heating energy, temperature, fixed water charges, and water left in pipes are outside the volume comparison.
Do not compare a tub's brim-full manufacturer capacity with a modest actual fill. Likewise, a nominal showerhead rating may not match flow at home pressure.
At the equal-use shower length, bath minus shower should be zero. Doubling shower time with unchanged flow must double shower litres.
Common questions
What does the shower water result represent?
It is the water flowing through the showerhead during the entered running minutes, expressed in litres.
What should I check before relying on this result?
Measure actual bath fill and shower flow, and distinguish water volume from the energy or money needed to heat it.
Inputs
- Bath fill volume (L)
- Shower flow (L/min)
- Shower running time (min)