BTUtally: free calculators, every figure with its source
How many BTU, kW or tons your rooms need, from published sizing charts.
Three sizing calculators for homeowners in the US and the UK: the air conditioner a room needs, the boiler and radiators a house needs, the furnace or heat pump a home needs. Published charts and rules of thumb, quoted with their source; no account, nothing stored on our servers.
The calculators
AC BTU calculator (room size)
BTU/h, tons and kW for one room from the ENERGY STAR sizing chart: area, ceiling, sun, people, kitchen, and the room unit class that matches.
Boiler and radiator size calculator (UK)
Combi or system boiler kW from bedrooms and bathrooms, whole-house heat loss from floor area and insulation, and one room's radiator output in BTU/h and W.
Heat pump / furnace size calculator (US)
Heating load in BTU/h, tons and kW from home area, climate zone 1-5 (pick your state) and insulation, with the furnace and heat pump class.
How it works
- Pick the calculator: one room to cool (US chart), a UK house and its boiler and radiators, or a US home to heat by climate zone. Every field starts with a published figure and shows its source and the date it was read; change it to your own room or home.
- Type the dimensions in feet and square feet or in metres and m². The result shows BTU per hour, tons and kilowatts together, the chart band or table row it came from, and each adjustment (sun, people, kitchen, windows, walls, insulation) separately.
- Nothing is uploaded: the numbers stay in your browser, and Pro reads them from there to build the whole-home sizing report, printed or exported to Excel, with the sources on the sheet.
Units follow the region you pick (US (sq ft, BTU/h, tons); UK / metric (m², kW)). The charts are the same in both; only the units change. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.
Where the figures come from
The room AC calculator is the ENERGY STAR room air conditioner sizing chart with its own adjustments for shade, sun, people and kitchens. The UK boiler calculator uses the bedrooms-and-bathrooms tables installers publish, a W/m² rule for heat loss and a merchant rule for radiator BTU by room volume, showing both where two publishers disagree. The US heating calculator uses the BTU per square foot by climate zone rule that HVAC suppliers and manufacturers publish. Each page lists its sources with the date they were read, and each result names the band or row it came from.
None of it replaces an installer's load calculation (Manual J in the US, a room-by-room heat loss in the UK); it gives you the figure to check a quote against and the vocabulary to read one.
Sizing the whole home? Pro puts every room on one printable report.
A room list with cooling BTU per room and the totals, the heating load, the boiler kW and the equipment classes, with the sources. Printed or exported to Excel.
Frequently asked questions
- Are the calculators free?
- Yes, all three, with no limit and no sign-up. Pro is a paid extra for people sizing a whole home who want every room on one printable report with the totals and the sources; the free tools do not change.
- Is the result what an installer would quote?
- No. It is the published chart or rule of thumb applied to your numbers: a first estimate and a check on a quote. An installer's Manual J (US) or heat loss calculation (UK) counts windows, orientation, air leakage and the local design temperature, and that is the figure equipment is bought on.
- Can I use it in the UK and the US?
- Yes. The region switch changes the units (sq ft, BTU/h, tons or m², kW). The AC chart and the radiator rule work in either; the boiler tables are UK installers' tables and the climate zones are US zones, so each of those two tools says which region it was written for.
- What does Pro add?
- A whole-home sizing report: a list of rooms with the cooling BTU, tons and kW of each from the same chart, the totals with a central system class, the heating load from the climate zone tool, the boiler kW range from the UK tool, and the sources, on one sheet. Printed or saved as PDF from the browser, or exported to Excel.