Bathtub Water Volume Calculator
Size the bath before you size the water heater. Enter your tub's length, width, and depth plus a typical fill level, and this calculator tells you how many gallons of water the tub holds — in gallons, liters, and cubic feet.
Enter your numbers
Your results
Estimate only — your body displaces water, so expect to use 10–20% less than the tub's full volume.
How to use this calculator
Measure the inside of your tub: length, width, and depth from the floor of the tub up to where you'd fill it (the overflow drain is a good reference). Then enter the fill level as a percentage of that depth.
Compare the gallon figure to your water heater's capacity — if a full bath needs 75 gallons and your tank holds 50, you'll be mixing cold water in unless the heater recovers fast.
How it works
Water volume in cubic inches = length × width × depth × fill%. Divide by 231 (cubic inches per US gallon) for gallons, multiply gallons by 3.78541 for liters, and divide cubic inches by 1728 for cubic feet.
For example, a 60 × 30 × 14 in tub at 70% fill holds 60 × 30 × 14 × 0.7 = 17,640 cubic inches, or about 76.4 gallons. Real tubs slope inward and curve at the corners, so the true volume runs a little lower — and once you're in it, displacement drops the actual fill by another 10–20%.
Frequently asked questions
How many gallons does a standard tub hold?
A typical 60 × 30 in alcove tub holds about 70–80 gallons at normal fill levels.
Why is the real fill less than calculated?
Your body displaces water — expect to use 10–20% less than the tub's full volume.
How does this help with a water heater?
Compare the gallons to your water heater's capacity to see if one tank covers a full bath.
What fill level should I use?
60–80% is typical; overflow drains usually sit around 70–80% of tub depth.