About the Bath vs Shower Calculator
This calculator compares how much water a shower uses against how much water a bath uses, then scales both by how often you wash to show the weekly and yearly difference. It uses two well-established relationships: a shower's water use is its flow rate multiplied by how long it runs, while a bath's water use is simply the volume of water sitting in the tub when you get in.
The formula
- Water per shower (gallons) = flow rate (GPM) × duration (minutes). A showerhead delivers a roughly constant number of gallons every minute it runs, so multiplying the rate by the time gives total gallons used.
- Water per bath (gallons) = the volume you fill the tub to. There is no universal formula for this — it depends on your tub's size and how full you run it — so you enter it directly. A typical filled bath uses somewhere around 30-50 gallons.
- Weekly and annual totals multiply each per-use figure by how many times a week you wash, then by 52 weeks to project an annual difference. The calculator assumes a constant flow rate for the whole shower and the same bath fill volume every time.
How to read the results
If the weekly/annual difference is positive, showering at your settings uses less water than bathing, and switching would save that many gallons. If it is negative, your shower — because of a high flow rate or a long duration — actually uses more water than your bath, and the reverse switch would save water. Either direction is possible depending on your specific numbers; there is no rule that showers always win.