# BTU Calculator — AC Tonnage & Heater Sizing > Free BTU calculator: estimate the cooling load, AC tonnage and kW for a room or a whole home, get a recommended unit size with an oversizing guard, and size a heater by climate zone — sun, ceiling, occupants and insulation factored in, metric or imperial, in your browser. Estimate a room or whole-home cooling load in BTU/h, AC tonnage and kW, get a recommended unit size, and size a heater by climate zone — a rule-of-thumb estimate, not a Manual-J calculation. - **HTML page:** https://siteometry.com/en/tools/btu-calculator - **Category:** [Decorating & Interiors](https://siteometry.com/en/tools/category/decorating-interiors) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## How to size an AC or heater with this calculator 1. **Choose units and mode** — Pick metric or imperial and whether you are sizing cooling, heating or both. 2. **Add your rooms** — Enter each room's floor area, ceiling height, sun exposure, occupants and whether it is a kitchen. Add a row per room for a whole-home total. 3. **Set climate zone and insulation** — Choose your IECC climate zone and insulation quality — these apply to the whole project and drive the heating rate and both loads. 4. **Read the load and unit size** — Read the cooling load in BTU/h, tonnage and kW, the recommended unit on each ladder with its oversizing guard, and the heating load. _On the page: https://siteometry.com/en/tools/btu-calculator#howto_ ## BTU calculator Sizing an air conditioner or heater starts with the load: how much heat, in BTU per hour, the space gains in summer and loses in winter. Buy too small and it never keeps up; buy too big and it short-cycles, cools unevenly and dehumidifies poorly. This calculator turns a room's floor area, ceiling height, sun exposure, occupants and kitchen use — plus a project-wide climate zone and insulation quality — into a cooling load in BTU/h, the matching AC tonnage and kW, a recommended unit size, and a heating load. Everything runs in your browser. _On the page: https://siteometry.com/en/tools/btu-calculator#btu-guide-intro_ ## How many BTU do you need? A rough starting point for cooling is about 20 BTU/h per square foot of floor area at an 8 ft ceiling, then adjusted for the things that change the real load: taller ceilings add volume, sunny or shaded rooms shift the gain, extra occupants and a kitchen add heat, and insulation quality moves it either way. This is a rule-of-thumb estimate, not a Manual-J load calculation. A full Manual-J accounts for window area and orientation, air leakage, duct losses and local design temperatures. Treat the number here as a sensible starting point and verify with an HVAC professional before buying equipment. _On the page: https://siteometry.com/en/tools/btu-calculator#how-many-btu_ ## The cooling-load formula The cooling load is built additively so every factor is a visible part of the total: base area load, plus a ceiling-volume delta above 8 ft, plus sun and insulation percentages on the base, plus flat adders for extra occupants and a kitchen. _On the page: https://siteometry.com/en/tools/btu-calculator#cooling-formula_ ## BTU, tonnage and kW One ton of cooling is 12,000 BTU/h, and one kilowatt is 3,412 BTU/h, so a 24,000 BTU/h load is 2.0 tons or about 7.0 kW. Metric users read kW first; imperial users read BTU/h and tonnage. _On the page: https://siteometry.com/en/tools/btu-calculator#tonnage-and-kw_ ## Choosing a unit size Air conditioners come in fixed capacities, so the calculator snaps the load up to the nearest available size on three ladders — window/portable, mini-split and central tonnage — never below the load, because undersizing is the worse failure. It then flags how far above the load that size sits, with a guard tighter than ACCA Manual S. _On the page: https://siteometry.com/en/tools/btu-calculator#unit-ladder_ ## Sizing a heater Heating is estimated from the climate zone: colder zones lose more heat, so they call for more BTU/ft². Pick your IECC zone and the calculator crosswalks it to a heating band and rate. An optional AFUE step converts the heating output into the gas input a furnace draws. _On the page: https://siteometry.com/en/tools/btu-calculator#heating-load_ ## The adjustment factors Sun, ceiling height, occupants, kitchen use, insulation and climate each move the load. Change any one and the matching segment of the contribution bar and the number update immediately, so you can see what drives the size. _On the page: https://siteometry.com/en/tools/btu-calculator#factors_ ## Worked examples A few quick estimates at the default constants. Your own numbers will differ once you set the climate zone, insulation and factors to your space. _On the page: https://siteometry.com/en/tools/btu-calculator#worked-examples_ ## Frequently asked questions ### How many BTU do I need per square foot? As a rule of thumb, about 20 BTU/h per square foot for cooling at an 8 ft ceiling, before adjustments. Sun, ceiling height, occupants, a kitchen and insulation all move it, which is why this calculator adjusts the base rate rather than using a flat number. It is guidance, not a Manual-J calculation. ### What size AC (tonnage) do I need? Divide the cooling load by 12,000 to get tons — a 24,000 BTU/h load is 2.0 tons. The calculator snaps that up to the nearest real unit on the window, mini-split and central ladders and warns when a size is far above the load, since an oversized unit short-cycles and dehumidifies poorly. ### Is this a Manual-J load calculation? No. This is a rule-of-thumb estimate for a starting point. A full ACCA Manual-J accounts for window area and orientation, air leakage, duct losses and local design temperatures. Verify with an HVAC professional before purchasing equipment. ### Does it work in metric? Yes. Switch units at the top and the tool reads kW first, with BTU/h and tonnage alongside, and accepts floor area in square metres and ceiling height in metres. The physics is the same either way. _On the page: https://siteometry.com/en/tools/btu-calculator#faq_ ## Related calculators - [AC Unit Size Calculator — What Size Air Conditioner Do I Need?](https://siteometry.com/en/tools/ac-unit-size-calculator) - [Paint Coverage & Cost Calculator](https://siteometry.com/en/tools/paint) - [Tile & Flooring Calculator](https://siteometry.com/en/tools/tile) - [Flooring Calculator — Laminate, Vinyl Plank & Hardwood](https://siteometry.com/en/tools/flooring-calculator)