Free

Calculators

Every calculator here runs entirely in your browser. No account, no email, no data leaves the page — there is no server to send it to. Each one shows the arithmetic it used and links the source of every figure, and each carries an embed snippet so you can put it on your own site for free.

Disclosure. This page contains affiliate links; we may earn a commission on qualifying purchases at no cost to you. As an Amazon Associate I earn from qualifying purchases. Every figure on this site is reproduced from a published manufacturer manual, standard or government document and linked so you can check it yourself. We do not own or test the products we write about, and we say so on every page rather than implying otherwise.
Sauna heater sizerRoom dimensions in, kW out using Harvia's published rule — then the breaker and wire the manuals specify alongside what the NEC continuous-load formula gives, because they disagree at every size. Water softener and whole-house filter sizerGrain capacity as a range across both published conventions, peak flow in GPM built from federal fixture figures, and the NSF/ANSI certifications to ask a seller for in writing. Air purifier CADR and air-change calculatorMinimum smoke CADR from EPA's table and AHAM's rule, plus the useful trick: enter a unit's CADR and its claimed coverage area and it converts that claim into air changes per hour. Running cost by stateSauna, purifier and softener running costs at EIA's published residential rate for your state, with all fifty-one ranked — and the reason that rate is not what one more kilowatt-hour actually costs you.

Putting one on your own site

Each calculator carries a copyable iframe snippet at the bottom of its page. They are free to embed with no attribution required beyond the link the embed already carries, and no permission needed. They are single self-contained files with no external requests, so an embed will not slow your page down or load anything from a third party.

Why these are free. A calculator that reproduces published figures and shows its arithmetic is checkable by anyone, which is the point. We would rather it spread and be audited than sit behind an email form.