About

I'm Alan, an independent developer in Utah. I build small, free calculators under the name Firebuilt Development, and Home Spa Calc is one of them.

What it is for

Owning a sauna or a hot tub raises a few money questions. What does a session cost? What will the bill look like in winter? What size heater does this room need? How long before the water is ready? Each calculator answers one of those from numbers you type in.

The calculators are free and need no account. Your numbers stay in your browser.

Where the numbers come from

Some figures are physics, like how much energy it takes to warm water. Others are trade rules of thumb that vary by maker. The sources page lists both and says which is which. The methodology page shows how each calculator works out its answer and where it stops being reliable.

Where the honest answer is a range, you get a range.

What it cannot do

These are planning estimates. They cannot size your wiring or sign off an install. Anything involving circuits or structure needs a qualified person who has seen the job.

Found a mistake?

Email [email protected]. Say which calculator, what you entered and what you expected. I fix wrong numbers and publish the correction. More on the contact page.