How many BTUs do I need?
Enter the floor area, ceiling height and a few details about the space — you'll get the cooling or heating capacity in kW and BTU/hr, worked out from the standard rule of thumb, instantly.
Assumptions (tap to fine-tune)
Cooling uses the ~20 BTU/hr per sq ft rule of thumb, scaled for ceiling height; the sun, occupant and kitchen adjustments follow US EPA ENERGY STAR room-AC guidance.
To cool about 35 m² you need roughly 2.2 kW of cooling capacity. Treat this as a starting point and size the actual unit at or just above it.
Rule-of-thumb estimate — for an installed system, confirm with a professional Manual J load calculation.
Capacity = floor area × a BTU-per-area figure × a ceiling-height scale. The figures are labelled planning rules of thumb (cooling adjustments follow ENERGY STAR guidance), not code values, and there are no prices.
Common questions
- How do I figure out how many BTUs I need?
- As a rule of thumb, multiply the floor area you want to condition by about 20 BTU/hr per square foot for cooling, then adjust for ceiling height, sun and how many people use the space. Heating is more climate-dependent — roughly 30–60 BTU/hr per square foot from a mild to a very cold climate. The calculator above does this for you; for an installed system a professional confirms it with a Manual J load calculation.
- How many BTU do I need to cool a 20x20 room?
- A 20 × 20 ft room is 400 square feet. At the ~20 BTU/hr per square foot rule of thumb that is about 8,000 BTU/hr (roughly 2.3 kW). Add capacity if the room is very sunny, has high ceilings, holds more than two people, or includes a kitchen, and trim it if it is heavily shaded.
- How many BTUs do I need for a 12x12 room?
- A 12 × 12 ft room is 144 square feet, so the rule of thumb gives roughly 2,900 BTU/hr of cooling — small window and portable units are typically sold from 5,000 BTU/hr, so the smallest common unit comfortably covers it unless the room is very sunny or a kitchen.
- How many sq ft does a 12000 BTU AC cool?
- Working the rule of thumb backwards, 12,000 BTU/hr ÷ 20 BTU/hr per sq ft is about 600 square feet of average space — for example a large living room or a small open-plan area. A 12,000 BTU/hr unit is also called a 1-ton air conditioner. Sunny rooms, high ceilings or a kitchen reduce the area it comfortably handles.
- What happens if the BTU rating is too high?
- An oversized air conditioner cools the air quickly but short-cycles — it switches off before it has run long enough to pull moisture out, so the room feels cold and clammy and the compressor wears faster. Going moderately under is usually better than going far over; size at or just above the figure the calculator returns rather than rounding up to the next big unit.
- How many BTUs are needed for a 2000 sq ft house?
- At the ~20 BTU/hr per square foot cooling rule of thumb a 2,000 sq ft home works out to roughly 40,000 BTU/hr (a little over 3 tons), before adjusting for climate, insulation and ceiling height. Whole-house heating in a cold climate can need considerably more — which is exactly why a Manual J calculation, not a rule of thumb, sizes a central system.
Want the full explanation? Read the BTU sizing guide → Sizing an on-demand water heater instead? Try the first hour rating calculator →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-06.