Roomtherm is built and maintained by Alexander Polekha, an independent developer in Armenia.
It answers one question: how much cooling or heating does this room, garage or shop need? You give the floor size and a few facts about the room; it gives the answer in BTU per hour and in watts, the next common unit size to look for, and the steps that produced the number. The rules come from published sources: the ENERGY STAR room air conditioner chart, a heater maker's sizing sheet, a university extension bulletin and the radiator makers' correction method. Where those sources leave a choice open, the site says which reading it took and shows the other one on How we calculate.
What it does not do: Roomtherm is not an HVAC contractor and does not replace a load calculation for a whole house. It does not recommend brands, products or prices.
Roomtherm sizes heat output only. It does not cover wiring, circuits, gas or other fuel-burning heaters; those follow the heater's instructions and local code.
There are no accounts and nothing to sign up for. The calculation runs in your browser, and the numbers you type are kept in the page's link so that the link brings the same room back; anyone you share it with sees the same room. The site does not store them, and the page address it sends to analytics has the query removed (see the privacy policy). If a result looks wrong or a sentence is unclear, the contact form reaches the person who maintains the site.