BTU, tons and kW of cooling explained
One ton is 12,000 BTU/h. Here is everything that follows from that.
12,000 BTU/h equals exactly one ton of refrigeration, which is 3.52 kW of cooling capacity. A typical room needs 30-40 BTU/h per square foot.
What a BTU actually is
The British Thermal Unit is the energy needed to raise one pound of water by one degree Fahrenheit. In air conditioning it is always used as BTU per hour: that is power, not energy, even when the label just says "12,000 BTU".
Conversions worth keeping to hand
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| BTU/h | Tons | Watts | kW | Frigories/h |
|---|---|---|---|---|
| 6,000 | 0.50 | 1,758 | 1.76 | 1,512 |
| 9,000 | 0.75 | 2,638 | 2.64 | 2,268 |
| 12,000 | 1.00 | 3,517 | 3.52 | 3,024 |
| 18,000 | 1.50 | 5,275 | 5.28 | 4,536 |
| 24,000 | 2.00 | 7,034 | 7.03 | 6,048 |
| 36,000 | 3.00 | 10,551 | 10.55 | 9,072 |
Where the ton comes from
A ton of refrigeration is the cooling power needed to melt one short ton of ice in 24 hours. It dates from the era when buildings were cooled with delivered ice, and it stuck. It has nothing to do with the weight of the equipment, however much it sounds like it might.
How many BTU by room size
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| Floor area | BTU/h | Tons |
|---|---|---|
| 150 sq ft | 4,500-6,000 | 0.4-0.5 |
| 250 sq ft | 7,500-10,000 | 0.6-0.8 |
| 400 sq ft | 12,000-16,000 | 1.0-1.3 |
| 600 sq ft | 18,000-24,000 | 1.5-2.0 |
BTU is not consumption
This is the most widespread misunderstanding. BTU/h measures the heat the unit removes from the room. The electricity it uses is a different quantity and can only be derived from the efficiency rating: a 12,000 BTU/h unit with an EER of 10 draws 1,200 W, not 12,000.
Sources and methodology
Conversions use exact constants: 1 BTU/h = 0.29307107 W and 1 frigorie/h = 1 kcal/h = 1.163 W. The component sizing model (load per volume by insulation level, 120 W per person, solar gain by glazed area) is set out in full on the air conditioner size calculator. It is a guideline estimate: a professional heat-load calculation considers considerably more variables.
Frequently asked questions
How many BTU is a ton of cooling?
Exactly 12,000 BTU/h, which is also 3.52 kW of cooling capacity.
How many tons do I need for 600 square feet?
About 1.5 to 2 tons (18,000-24,000 BTU/h) at 30-40 BTU per square foot, depending on insulation and sun exposure.
How many watts is 12,000 BTU?
3,517 W of cooling capacity. Not to be confused with electricity consumption, which is far lower.
Is 2 tons enough for a house?
2 tons covers roughly 600-800 sq ft of typical construction. A whole house usually needs a room-by-room load calculation rather than a single rule.
Does BTU measure energy use?
No. It measures cooling capacity. For consumption you need the efficiency rating (EER or SEER) or the rated electrical input.