BTU Calculator — what size air conditioner do I need?

How it works

BTU (British Thermal Units per hour) measures an air conditioner's cooling capacity. Too small and it can't keep up; too large and it cools the air quickly without running long enough to remove humidity, leaving the room cold and clammy. Energy Star sizes a room unit from a capacity chart keyed to floor area, where each row covers an area up to the next (so 300 sq ft starts the 8,000 BTU row, not the 7,000 one):

• Sun: +10% for a very sunny room, −10% for a heavily shaded one
• People: +600 BTU for each person beyond two who is regularly in the room
• Kitchen: +4,000 BTU, because of the heat from cooking

Air conditioners are sold in fixed sizes (5,000, 6,000, 8,000 BTU and up), so round up to the next available capacity. The chart runs higher than the popular "20 BTU per square foot" rule — a 300 sq ft room maps to 8,000 BTU, about 27 per square foot — so that rule tends to undersize. Above 2,500 sq ft a room unit is the wrong tool: use a professional load calculation (a Manual J).

Worked example

A 300 sq ft bedroom, average sun, two people:

• Chart band 300 up to 350 → 8,000 BTU

A 450 sq ft living room, very sunny, four people:

• Chart band 450 up to 550 → 12,000 × 1.10 = 13,200, + 2 extra people × 600 = 14,400 BTU

Frequently asked questions

What size air conditioner do I need for my room?

Measure the room's length × width for the area, then read the Energy Star chart: 5,000 BTU up to 150 sq ft, 6,000 for 150–250, 7,000 for 250–300, 8,000 for 300–350, rising to 12,000 around 450–550. Then adjust for sun, occupants and kitchen use, and round up to the next size sold.

Is a bigger air conditioner better?

No. An oversized unit short-cycles — it cools the air fast, shuts off, and never runs long enough to pull out humidity, so the room feels cold and damp. Right-sizing is more comfortable and efficient.

Why does a kitchen need more BTU?

Ovens, stoves and refrigerators give off a lot of heat, so Energy Star adds about **4,000 BTU** for a kitchen on top of the chart figure.

Is it really 20 BTU per square foot?

That popular rule **undersizes** against the Energy Star chart, which runs closer to 25–27 BTU per square foot in the mid range — a 300 sq ft room maps to 8,000 BTU, not the 6,000 a strict 20/sq ft rule gives. Use the chart bands, which is what this calculator does.

Does this size a central AC or heat pump?

Not precisely. This is for **room/window units**, up to about 2,500 sq ft. Whole-home systems should be sized with a Manual J load calculation that factors in insulation, windows, ductwork and your climate.

What about heating BTU?

Heating capacity depends heavily on climate and insulation, so it uses a different method (often 30–60 BTU per square foot by climate zone). This calculator is for cooling.