BTU calculator: what size air conditioner does this room need?
This BTU calculator estimates the cooling or heating a room needs, in BTU per hour. Enter the room size, ceiling height and climate, then sun, people and a kitchen for cooling. A 20 by 20 ft room with average conditions needs about 8,000 BTU/h of cooling. Every factor, its source and the math are printed with the result.
Rooms
Hot, South and Southwest. Moderate, Midwest and Mid-Atlantic. Cold, Northeast and northern Midwest. These examples are a guide.
BTU/h
Waiting for a room size.
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The base factor stays in BTU/h per sq ft in both units. Switching units converts the sizes you entered and keeps the exact values.
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How is the number worked out?
It starts with floor area times a base figure. For cooling we use 20 BTU/h per sq ft, the rule of thumb Forbes Home gives. Bryant's mini-split guide puts it at 20 to 25, and we take the low end. Then each factor multiplies it, and two adders go on last.
Take a 20 by 20 ft room, 400 sq ft, with an 8 ft ceiling, a moderate climate, average insulation, normal sun and two people:
- 400 x 20 = 8,000 BTU/h.
- Every multiplier is 1.00 for this room, so it stays 8,000.
- That's 0.67 ton, at 12,000 BTU/h per ton.
Put three people in the room and it's 8,600. Add a kitchen and it's 12,600, or 1.05 tons.
Which factors change the answer?
Climate. Cold is x 0.80 and hot is x 1.25 for cooling. They come from HomeGuide's figures of 16 to 20 BTU per sq ft in cooler climates and 25 to 30 in hotter ones, taking the low end of each. The example room reads 6,400 in a cold climate and 10,000 in a hot one, and the page shows all three side by side.
Ceiling height. Add 10% for each foot above 8 ft, also from HomeGuide. A 12 ft ceiling takes the example room to 11,200 BTU/h. There is no reduction below 8 ft.
Insulation. Poor insulation is x 1.10. HomeGuide says 10% to 20% more, and we take the low end. We found no published credit for good insulation, so the page doesn't give one.
Sun, people and kitchens. These three come from ENERGY STAR's room air conditioner guidance: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h for each person over two, and 4,000 BTU/h for a kitchen. They're cooling factors. The order is this page's choice: multipliers first, then the two adders. ENERGY STAR lists its adjustments without an order.
A hot, very sunny room of 400 sq ft with poor insulation comes to 12,100 BTU/h.
Why do BTU calculators disagree?
Because they start from different base figures and factors. When we tried one 400 sq ft room on several online calculators in September 2026, the answers ran from 8,300 to 12,000 BTU/h. Rules of thumb vary. So this page prints each factor with its source and the date we read it, and you can change the base figure, the sun values, the per-person and kitchen figures and the poor insulation value if you have a better number. Set the base to 25 and the example room reads 10,000.
What size heater does a room need?
Switch to heating and the base figure becomes 40 BTU/h per sq ft, the low end of the 40 to 50 range Bryant gives for moderate climates. Hot climates are x 0.75 and cold ones x 1.25, from Bryant's ranges of 30 to 35 and 50 to 60. Bryant's figures are for sizing a whole-home furnace, and applying them to one room is our choice. We also apply the ceiling and insulation rules to heating, and that's our choice too.
The 400 sq ft room needs 16,000 BTU/h of heat, or 4,690 W. In a cold climate that's 20,000 BTU/h (5,862 W), and in a hot one 12,000 BTU/h (3,517 W). Sun, people and kitchens don't count in heating mode. Watts are the exact total divided by 3.412, rounded up.
Can you size several rooms at once?
Yes. Add each room and the page gives its own figure plus a total. A 150 sq ft room and a 250 sq ft room come to 3,000 and 5,000 BTU/h, 8,000 in all. The total is worked out from the unrounded room figures, so it's sometimes a little less than the rounded room figures added up.
What this doesn't do
It doesn't do a Manual J load calculation, look up your climate zone, or recommend a unit, brand or model. It doesn't size ducts or a whole-house system, or estimate running costs. There is no ZIP code, address or map. More tools are on our home calculators page.
Before you buy
This is a rule-of-thumb estimate. A contractor's Manual J load calculation is the standard for sizing a system. An oversized air conditioner short-cycles and leaves a room humid, and an undersized one runs without catching up. For a whole-house system, get a Manual J from a contractor. For a heater, follow the maker's clearance and circuit ratings.
BTU calculator FAQ
How many BTU do I need for a 20 by 20 room?
About 8,000 BTU/h of cooling in a moderate climate with average conditions, or 16,000 BTU/h of heating, using whole-home furnace figures applied to one room.
How many square feet does a 12,000 BTU air conditioner cool?
At 20 BTU/h per sq ft and average conditions, about 600 sq ft. That's 1 ton.
How many BTU per square foot?
We start cooling at 20 and heating at 40, then adjust for climate, ceiling, insulation, sun, people and kitchens.
Does the number of people matter?
For cooling, yes. Add 600 BTU/h for each person over two.
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