About the BTU Calculator
This BTU calculator estimates the cooling capacity a room needs, in British thermal units per hour, so you can pick the right window, portable or mini-split air conditioner. Enter the room’s length and width, the ceiling height, how sunny it is, how many people normally use it and whether it is a kitchen, and it returns the BTU/hr requirement, the equivalent in tons and kilowatts, and the nearest common unit size to shop for.
The starting point is the ENERGY STAR room air conditioner sizing chart, which maps floor area to capacity (for example 7,000 BTU for 250–300 sq ft). ENERGY STAR’s adjustments are then applied: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU for each regular occupant beyond two and +4,000 BTU if the unit serves a kitchen. ENERGY STAR notes that its chart assumes 8 ft ceilings but gives no correction for taller rooms, so this calculator makes its own assumption: for ceilings above 8 ft it scales the capacity in proportion to height (height ÷ 8). The suggested unit is the nearest widely sold size at or above the requirement; in-between sizes such as 7,000 or 9,000 BTU exist but are less common.
An oversized unit cools quickly but short-cycles and leaves the air clammy; an undersized one runs constantly. For whole-house central systems a professional Manual J load calculation, which accounts for insulation, windows and climate, is the right tool — use this as a quick room-level estimate.
With the default inputs, the cooling capacity needed is 7,000 BTU/hr. Change any value above to recalculate instantly.
How to use the btu calculator
- 1Measure the room’s length and width in feet and enter the ceiling height.
- 2Choose how sunny the room is.
- 3Enter how many people normally use the room, and switch on "kitchen" if it is one.
- 4Read the required BTU/hr and buy the suggested unit size (or the nearest size above).
Formula and method
The base capacity comes from the ENERGY STAR sizing chart for room air conditioners, which assigns a capacity to each band of floor area (5,000 BTU for 100–150 sq ft up to 34,000 BTU for 2,000–2,500 sq ft). Above 2,500 sq ft the last band is extended proportionally.
Adjustments follow ENERGY STAR guidance: multiply by 0.9 for heavily shaded rooms or 1.1 for very sunny rooms, add 600 BTU per regular occupant beyond two, and add 4,000 BTU for a kitchen. ENERGY STAR states that the chart assumes 8 ft ceilings but publishes no height correction; as an approximation this calculator scales taller rooms by height ÷ 8 (i.e. by room volume). One ton of refrigeration is 12,000 BTU/hr and 1 BTU/hr ≈ 0.2930711 W (NIST SP 811).
- area
- Floor area = length × width (sq ft)
- H
- Ceiling height in feet
- sun factor
- 0.9 shaded, 1.0 average, 1.1 very sunny
- people
- Number of regular occupants
Worked examples
15 × 18 ft bedroom
270 sq ft falls in the 250–300 sq ft band, which needs 7,000 BTU/hr (0.58 tons, about 2.05 kW). 7,000 BTU window units are sold but less common, so the suggestion is the next widely sold size, 8,000 BTU; any unit rated at 7,000 BTU/hr or more fits.
Sunny 20 × 25 ft living room, 4 people
500 sq ft needs 12,000 BTU. +10% for sun gives 13,200, and two extra occupants add 1,200 BTU, for 14,400 BTU/hr — a 15,000 BTU unit.
Shaded 12 × 12 ft office, 1 person
144 sq ft starts at 5,000 BTU; the 10% shade reduction brings it to 4,500 BTU/hr, so the smallest 5,000 BTU unit is plenty.
Open-plan kitchen/living, 30 × 40 ft with 10 ft ceilings
1,200 sq ft is in the 1,200–1,400 band (23,000 BTU). 10 ft ceilings scale it by 1.25 to 28,750, and the kitchen adds 4,000 for 32,750 BTU/hr — roughly a 3-ton system.
Frequently asked questions
How many BTU do I need per square foot?+
A common rule of thumb is about 20 BTU per square foot for rooms with 8 ft ceilings, which is close to the ENERGY STAR chart for mid-size rooms. Small rooms need a bit more per square foot, large open spaces a bit less.
What size air conditioner do I need for a 12×12 room?+
A 144 sq ft room needs about 5,000 BTU/hr under the ENERGY STAR chart — the smallest common window unit. Add capacity if the room is very sunny or regularly holds more than two people.
How many BTU is a ton of air conditioning?+
One ton of cooling equals 12,000 BTU per hour — the heat needed to melt one ton of ice in 24 hours. A 3-ton central air system delivers about 36,000 BTU/hr.
Is it bad to oversize an air conditioner?+
Yes. An oversized unit cools the air quickly and shuts off before removing much humidity, so the room feels damp and the unit cycles more, wasting energy and wearing components.
Can I use this for heating?+
Roughly, for a room heater in a moderate climate. Heating needs depend strongly on outdoor design temperature and insulation, so cold-climate rooms often need 30–60 BTU per square foot.