hot water heater capacity for bath tub

Depends on the size of the tub, and whether you are using any cold water at all while filling.

I suggest an experiment, using old milk cartons (or other known measurement) of water to pour into the tub to the desired level. That will tell you the total number of gallons of water needed. If that is not practical, then measure the tub in cubic feet (Length x Width x Desired depth - ie .25 to .33) and multiply by 7 to get the approximate total number of gallons. If you really want to fill the tub with hot water, than this is the answer to your question.

If you are going to be using some cold water, then you can figure out how much hot water will be needed with the

formula

Hot Gallons = Total Gallons X (Desired Temp - Cold Water Temp)/(Hot Water Temp - Cold Water Temp)

For instance, if you need 6 gallons at 83 degrees, and your hot water is 100 degrees and your cold water is 50 degrees, then you will need 6 X (83 - 50)/(100 - 50) or 4 gallons of hot water (with 2 gallons of cold, of course).

This does not, as you might think, leave 2 gallons of hot water for other usage. As you removed 4 gallons of hot water from the tank, 4 gallons of cold water rushed in to take its place, lowering the temperature of that 2 gallons. How long does it take to recover? I'll leave that question for people out there with more experiance than I.