🌡️ BTU Calculator
Enter a room's size, ceiling height, climate, sun exposure, and occupancy to estimate the heating and cooling BTU it needs — plus a recommended AC size.
🌡️ Room & Conditions
What is a BTU Calculator?
It estimates how much heating and cooling power a room needs, measured in BTUs per hour. Starting from the floor area, it adjusts for ceiling height, climate, sun exposure, and how many people use the room, then recommends an air-conditioner size rounded to the nearest 1,000 BTU — the units you actually shop for.
Use it to size a window or portable AC, sanity-check an HVAC quote, or compare rooms before you buy. The figures are estimates for planning — a professional Manual J load calculation, which also weighs insulation and air leakage, is the standard for sizing a whole-home system.
❓ Frequently Asked Questions
How does the calculator estimate BTUs?
Cooling starts from a base of 20 BTU per square foot, scaled for ceiling height (relative to a standard 8 feet) and for sun exposure — sunny rooms add 10%, shaded rooms subtract 10% — with 600 BTU added for each occupant beyond two. Heating uses a climate-specific figure per square foot, from about 25 BTU in hot regions to 45 in cool ones, also scaled by ceiling height.
What size air conditioner should I buy?
The recommended AC size rounds the room's cooling load to the nearest 1,000 BTU, which matches how units are commonly sold. A window or portable AC is typically 5,000 to 12,000 BTU for a single room. Avoid heavily oversizing — an oversized unit cools the air fast but short-cycles, leaving the room clammy because it doesn't run long enough to remove humidity.
Why do sun exposure and occupancy matter?
A room with large sunny windows gains heat all day and needs more cooling, while a shaded north-facing room needs less — hence the ±10% adjustment. People add heat too, roughly 600 BTU each beyond the first two, which is why kitchens and busy living rooms run warmer. Accounting for both gives a more realistic load than floor area alone.
Is this a substitute for a professional load calculation?
No — treat it as a quick planning estimate. Sizing a whole-home HVAC system properly uses a Manual J load calculation that also weighs insulation, air leakage, window type, and duct losses. Use this tool to ballpark a room unit or sanity-check a quote, but rely on an HVAC professional for a full system.