I assume from a previous post that you have an electric water heater.

A 50 gal heater would probably be fine unless you tell me that there are more people in your family. The number of bathrooms doesn't matter as much as the number of people who get ready for school & work at the same time.

Making a few assumptions: 55 degree cold water and 130 degree heater temp as well as 105 degree bath, you could just fill your largest tub with a 50 gal heater.

If you tell me that you often entertain overnight guests, such as family members, etc. then I would suggest going with the next largest heater - in the Marathon brand, that's a 85 gal and it costs $100 more than a 50 gal.

Note: If you have gas, you might consider a tankless. An 85 gal Marathon is right around $1,000 - about the same cost as a gas tankless unit - which would solve your concerns about a leaking tank and solve the problem of not running out of hot water.

Note that Lazypup and I used slightly different numbers for our water temps to come to our conclusions.

Bottom line: If the 72 gal tub is your yardstick, then a 50/60 gal heater is JUST going to work - the 50 gal will only work if you go to 130 degree water (don't do this if there are young children or elderly in the house).

If I were considering a Marathon water heater anyway, I'd opt to spend the additional $100. Your costs may vary.

Rule of thumb in the Plumbing trade is 50 gallon minimum for soaking tubs/ jetted tubs.

A 50 gallon water heater is typically rated for a family of 4. Now if you have someone in the family who takes hollywood showers (20 minutes), then you're going to drain the water heater.

If you have an electric water heater...minimum 80 gallons because of the recovery factor. You will be sorry if you depend on a 50 gallon electric to supply all of that.

Gas water heater - minimum 50 gallons but if it were me I would install a 75 gallon in your situation. Suppose 2 people want to use each tub...then what?

However, soaking tubs are useless without a heating element, and no water heater in the world would stop the water from starting to cool.

Just my Professional opinion of course.

Thanks everyone. I got the 85 gallon Marathon. Finally made up my mind when I read that the larger tank is particularly important if you go solar, which we'd like to at some point. So we'll be ready if that ever happens.

Ozone, I take your point about soaking tubs cooling. There are tubs with heating elements?! Not in my budget, I guess. I insulated around the tub, maybe that will help. Actually, I can't even imagine having time to sit in there long enough for it to cool down.... I wonder if our radiant ceiling heat will have any effect on the water; it heats objects in the room, but water?

You couldn't add a heating element to a soaking tub if you tried. See the problem is, when the consumer buys these homes new, they have no clue that soaking tubs are an absolute waste of money, and a jetted tub with a heating element are a better option.

You would be surprised how many of my customers complain about not being able to sit in the tub for a long time cause the water gets cool quickly.

Your tub is most likely fiberglass, so I have no clue how well insulation will work for you. if your tub was cast iron...that's a whole different story, cause cast iron holds the heat.

85 Gallon Marathon is a nice choice...good luck.