Turn a room size into the heating or cooling capacity you need — cooling BTU and tonnage with a suggested AC or mini-split size, a heating estimate by climate, or a UK radiator output in watts. It adjusts for ceiling height, sun, occupants and kitchen heat instead of just multiplying by 20. A rule-of-thumb sizing aid, not a Manual J load calculation.
Room size
ft²
Standard is 8 ft (2.4 m). Taller ceilings scale the load up proportionally.
Sun exposure
Shaded trims the load ~10%; a sun-drenched room adds ~10%.
Adds 600 BTU each beyond two.
Adds ~4,000 BTU for appliance heat.
Room size
ft²
Climate
Sets the heating load per square foot, from ~30 (hot) to ~55 (cold).
Radiator output is sized from room volume. Enter the room dimensions in metres.
Room type
Sets the heat factor in watts per cubic metre.
A rule-of-thumb sizing aid. For equipment you are buying — especially a central furnace, heat pump or whole-home AC — use a Manual J load calculation that accounts for the real insulation, windows, orientation and local design temperatures.
Most quick BTU answers stop at “square feet times 20”, which is a fine starting point and badly wrong for any room that isn’t average. The base cooling load here is about 20 BTU per square foot of floor area, and then four well-known adjustments are layered on top so the number reflects your actual room:
Ceiling height — the load scales with volume, so the base figure is multiplied by ceiling height ÷ 8 ft. A 10-foot ceiling adds 25%.
Sun exposure — a heavily shaded room is reduced about 10%; a very sunny, west-facing room is increased about 10%.
Occupants — the base assumes two people; each additional regular occupant adds roughly 600 BTU.
Kitchen — cooking appliances add a lot of heat, so a kitchen gets about 4,000 BTU extra.
cooling BTU = area × 20 × (ceiling ÷ 8 ft) × sun + 600 × (occupants − 2, if positive) + 4000 (if kitchen) tons = BTU ÷ 12,000
Tonnage and picking a unit
Cooling capacity is also measured in tons, where one ton equals 12,000 BTU per hour — a historical hangover from the cooling power of a ton of melting ice. Divide the BTU figure by 12,000 to get tons. Window and portable units are sold directly in BTU (commonly 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000), while mini-splits and central systems are quoted in tons or their BTU equivalents (9,000 / 12,000 / 18,000 / 24,000 and up).
Pick the nearest standard size at or just above your figure — and resist the urge to round way up. An oversized AC cools the air quickly and then switches off before it has run long enough to wring humidity out of the room, so you end up cold and clammy while the compressor short-cycles itself toward an early grave. Slightly under is often more comfortable than badly over.
Heating BTU by climate
Heating needs more energy per square foot than cooling, and how much more depends heavily on climate. As a rough guide the heating load ranges from about 30 BTU per square foot in a hot region to 50–60 in a cold one, again scaled by ceiling height. A furnace is rated by input BTU, so its usable output is the input times its efficiency (AFUE) — a 50,000 BTU furnace at 95% AFUE delivers about 47,500 BTU of heat. Heat pumps are rated differently and lose capacity as it gets colder, which is exactly where a load calculation earns its keep.
UK radiator sizing
British radiator sizing works from room volume rather than floor area. Multiply length by width by height for the volume in cubic metres, then multiply by a heat factor for the room: roughly 50 W/m³ for a living room, 40 for a bedroom, 30 for a kitchen (which gets help from appliances) and 75 for a bathroom (where you want it toasty). That gives the required output in watts; multiply by 3.412 to read it in BTU. Add about 15% for a north-facing or draughty room. One catch worth knowing: radiator catalogue outputs are quoted at a temperature difference (ΔT) of 50°C between the radiator and the room, so a lower-temperature system needs a physically larger radiator for the same output.
BTU by room size (cooling, 8 ft ceiling, average sun)
A quick chart for the base cooling load before any sun, occupant or kitchen adjustments — useful for a sanity check on window-unit shopping.
Room size
Cooling BTU
Tons
Typical unit
100 ft²
2,000
0.17
Small window unit (5,000)
150 ft²
3,000
0.25
5,000 BTU window unit
250 ft²
5,000
0.42
5,000–6,000 BTU
300 ft²
6,000
0.50
6,000 BTU / 0.5-ton mini-split
400 ft²
8,000
0.67
8,000–9,000 BTU
500 ft²
10,000
0.83
10,000 BTU
600 ft²
12,000
1.00
12,000 BTU / 1-ton mini-split
800 ft²
16,000
1.33
14,000–18,000 BTU
1,000 ft²
20,000
1.67
18,000–24,000 BTU
1,200 ft²
24,000
2.00
24,000 BTU / 2-ton
These are unadjusted base figures. A sunny room, tall ceiling, crowded space or kitchen pushes the requirement up; strong shade and good insulation pull it down. Treat the chart as a floor for a typical room, not a final spec.
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Frequently asked questions
How many BTU do I need per square foot?
For cooling, a common rule of thumb is about 20 BTU per square foot of floor area with a standard 8-foot ceiling, so a 300 sq ft room is roughly 6,000 BTU. Heating runs higher and varies by climate, from about 30 BTU per square foot in a mild region to 50–60 in a cold one. These are starting points; raise them for tall ceilings, strong sun, poor insulation or many occupants, and use a Manual J load calculation for a precise figure.
How do I convert BTU to tons of cooling?
One ton of cooling equals 12,000 BTU per hour, so divide the BTU figure by 12,000. A 6,000 BTU room is half a ton; 12,000 BTU is one ton; 24,000 BTU is two tons. Tonnage describes central AC and heat pumps; window and portable units are usually rated directly in BTU.
What size air conditioner or mini-split do I need for a room?
Work out the cooling BTU for the room, then pick the nearest common unit size at or just above it — window units come in roughly 5,000, 6,000, 8,000, 10,000, 12,000 and 14,000 BTU, and mini-splits in 9,000, 12,000, 18,000 and 24,000 BTU. Avoid going much larger than needed: an oversized unit cools fast then shuts off without running long enough to pull humidity out, leaving the room cold and clammy and short-cycling the compressor.
Why shouldn’t I just use square feet times 20?
Square feet times 20 ignores everything else that drives the load. A tall or vaulted ceiling adds volume to condition, a sun-drenched west-facing room runs hotter than a shaded one, each person beyond two adds heat, and a kitchen adds several thousand BTU from appliances. Insulation, window area and climate matter too. This tool layers those adjustments on top of the base figure, but for equipment you are buying, a Manual J calculation from the actual building is the reliable method.
How do I size a radiator in watts or BTU?
Radiator sizing starts from room volume rather than floor area. Multiply length by width by height for the volume in cubic metres, then multiply by a heat factor for the room type — roughly 50 W/m³ for a living room, 40 for a bedroom, 30 for a kitchen and 75 for a bathroom — to get the required output in watts. Convert to BTU by multiplying watts by 3.412. Add about 15% for a north-facing or poorly insulated room, and remember radiator catalogue outputs are quoted at a ΔT of 50°C.
Is this BTU estimate accurate enough to buy equipment?
It is a rule-of-thumb estimate, good for sanity-checking a window unit or comparing rooms, but it is not a load calculation. Permanent central systems should be sized with a Manual J calculation that accounts for the real insulation, windows, orientation, infiltration and local design temperatures. Treat the number here as a ballpark and confirm with an HVAC professional before installing fixed equipment.