Find out how many BTUs your room needs before you buy a window unit, mini-split, portable air conditioner or space heater. Enter the room dimensions and a few details about sun, people and insulation, and the calculator gives the cooling or heating load in BTU per hour, the unit size to shop for, and the equivalent in tons and kilowatts.
Cooling needed
6,000
BTU/h
Buy a unit rated
6,000
BTU
In kilowatts
1.8 kW
Room size
300
sq ft
300 sq ft × 20 BTU = 6,000 BTU/h.
📌 Based on the Energy Star sizing method (20 BTU per sq ft, ±10 % for sun/shade, +600 BTU per extra person, +4,000 BTU for kitchens) and typical heating loads of 35–60 BTU per sq ft by climate. A full Manual J load calculation by an HVAC contractor is the standard for central systems.
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Air conditioner sizing chart (Energy Star)
Room area (sq ft)
Capacity needed (BTU/h)
Tons
100–150
5,000
0.42
150–250
6,000
0.5
250–300
7,000
0.58
300–350
8,000
0.67
350–400
9,000
0.75
400–450
10,000
0.83
450–550
12,000
1
550–700
14,000
1.17
700–1,000
18,000
1.5
1,000–1,200
21,000
1.75
1,200–1,400
23,000
1.92
1,400–1,500
24,000
2
1,500–2,000
30,000
2.5
2,000–2,500
34,000
2.83
Worked example: a 20 × 15 ft living room (300 sq ft) with 8 ft ceilings, average sun, two people: 300 × 20 = 6,000 BTU/h → buy a 6,000–7,000 BTU window unit. The same room with a 10 ft ceiling, west-facing glass and four people: 6,000 × 1.25 × 1.1 + 1,200 = 9,450 BTU → a 10,000 BTU unit. Make it an open-plan kitchen and add 4,000 more.
Heating: BTUs per square foot by climate
Heating loads depend far more on climate than cooling loads do. Typical rules of thumb for a reasonably insulated home: 30–35 BTU/sq ft in the Deep South and coastal California, 40–45 across the mid-South and Mid-Atlantic, 45–50 in the Midwest and Northeast, and 50–60 in the northern tier and mountain states. Well insulated new construction can need 20–30 % less; a drafty older house 20 % more. For a single room, a 1,500 W electric space heater delivers about 5,100 BTU/h — enough for roughly 100–150 sq ft in a cold climate.
About 20 BTU per square foot of floor area for air conditioning in an average room with 8-foot ceilings — so a 300 sq ft bedroom needs roughly 6,000 BTU and a 20 × 15 ft living room 6,000–7,000. Add 10 % for a very sunny room, subtract 10 % for a shaded one, add 600 BTU for each person beyond two, and 4,000 BTU for a kitchen. Heating needs 35–60 BTU per square foot depending on climate.
Too small runs constantly and never reaches the set temperature on hot days. Too big is the more common mistake: it cools the air quickly and shuts off before it has removed humidity, leaving the room clammy, and the short cycles waste energy and wear out the compressor. Pick the size closest to the calculated load, rounding up only a little.
One ton of cooling is 12,000 BTU per hour (the heat needed to melt a ton of ice in a day), so a 3-ton central AC is 36,000 BTU. One kilowatt is 3,412 BTU/h, which is how portable and European units are rated. A 12,000 BTU unit is therefore 1 ton or about 3.5 kW of cooling capacity.
Yes. Single-hose portables pull cooled room air out through the exhaust, so their effective cooling is much lower than the label. Under the newer DOE (SACC) test a “12,000 BTU” portable often delivers 7,000–8,000 BTU. Use the SACC figure on the yellow EnergyGuide label, or size a single-hose portable 30–50 % above the calculator’s number.
Multiply floor area by 35–60 BTU per square foot depending on climate: about 40 in the South, 45–50 in the middle of the country, 55–60 in the northern states. A 2,000 sq ft home in Chicago needs roughly 100,000 BTU of output — and since furnaces are rated by input, divide by the efficiency (100,000 ÷ 0.95 ≈ 105,000 BTU input for a 95 % AFUE furnace).