BTU Calculator
A 300 sq ft room with an 8 ft ceiling needs about 8,000 BTU/h of cooling (2.34 kW, 0.67 ton). Make it very sunny, add 2 more people and a kitchen and it becomes 14,000 BTU/h.
How to use
- Enter the floor area and ceiling height of the room, in square feet and feet or square metres and metres.
- Pick how much sun the room gets, how many people usually use it, and whether it is a kitchen.
- Read the cooling capacity in BTU/h, kW and refrigeration tons, plus each adjustment. Choose a unit close to that figure.
How it works
Base capacity comes from the Energy Star room air conditioner sizing chart, which lists a BTU/h figure for each floor-area band (for example 300 up to 350 sq ft needs 8,000 BTU/h). The chart assumes an 8 ft ceiling.
Ceiling height (this tool's own assumption, not part of the chart): the base is scaled by ceiling ÷ 8 ft. Then Energy Star adjustments: heavily shaded −10%, very sunny +10%, 600 BTU/h for each person beyond two, and +4,000 BTU/h for a kitchen.
Conversions: 1 BTU/h = 0.29307 W, and 12,000 BTU/h = 1 ton of refrigeration. The result is rounded to the nearest 100 BTU/h. Energy Star suggests rounding the total down to the nearest 1,000 when you shop; both figures are shown.
Energy Star sizing chart
| Room area (sq ft) | Room area (m²) | Capacity (BTU/h) |
|---|---|---|
| 100 up to 150 | 9.3 up to 13.9 | 5,000 |
| 150 up to 250 | 13.9 up to 23.2 | 6,000 |
| 250 up to 300 | 23.2 up to 27.9 | 7,000 |
| 300 up to 350 | 27.9 up to 32.5 | 8,000 |
| 350 up to 400 | 32.5 up to 37.2 | 9,000 |
| 400 up to 450 | 37.2 up to 41.8 | 10,000 |
| 450 up to 550 | 41.8 up to 51.1 | 12,000 |
| 550 up to 700 | 51.1 up to 65.0 | 14,000 |
| 700 up to 1,000 | 65.0 up to 92.9 | 18,000 |
| 1,000 up to 1,200 | 92.9 up to 111.5 | 21,000 |
| 1,200 up to 1,400 | 111.5 up to 130.1 | 23,000 |
| 1,400 up to 1,500 | 130.1 up to 139.4 | 24,000 |
| 1,500 up to 2,000 | 139.4 up to 185.8 | 30,000 |
| 2,000 up to 2,500 | 185.8 up to 232.3 | 34,000 |
Limits of this estimate
- It is for cooling a single room. It is not a Manual J heat-load calculation, which also counts insulation, windows, climate and air leakage.
- Heating is not included. A heater needs its own sizing; do not reuse the cooling figure.
- Rooms under 100 sq ft use the smallest chart value, and rooms of 2,500 sq ft or more are outside the chart.
Sources: Energy Star sizing chart · Energy Star adjustmentsLast reviewed 2026-10-16
FAQ
How many BTU do I need per square foot?
Roughly 20 to 40 BTU/h per square foot, falling as the room gets larger (the chart gives 8,000 BTU/h for 300 to 350 sq ft and 18,000 for 700 to 1,000). That is why a chart beats a single rule of thumb.
What is the difference between BTU and BTU/h?
Air conditioners are rated in BTU per hour (BTU/h), often written just BTU. The kW figure is the rate of cooling, not the electricity used. Cooling of 1 kW is 3,412 BTU/h.
How many BTU is 1 ton of cooling?
One ton of refrigeration is 12,000 BTU/h, about 3.52 kW. An 18,000 BTU/h unit is a 1.5 ton unit.
Should I round up or down?
Energy Star advises rounding the total down to the nearest 1,000 and shopping for that size. An oversized unit cools quickly but switches off before it removes humidity, so avoid going far above the result.
Why does ceiling height change the result?
The Energy Star chart assumes an 8 ft ceiling. This tool scales the base capacity by ceiling height ÷ 8 ft as a rough allowance for the extra air. That scaling is our assumption, not an Energy Star rule.
Can I use this for heating?
No. The chart and adjustments are for cooling only. Heating load depends on climate, insulation and the temperature difference, and should be sized separately.