Air conditioner size for a room in BTU/h, tons and kW from floor area, ceiling height, insulation, sun and occupants, beside the Energy Star chart.
By AdminDrafted with AI writing toolsUpdated Checked examples: 4
By the rule of thumb the room needs about 6,000 BTU/h (0.5 tons, 1.76 kW), so look at 6,000 BTU/h units. The Energy Star chart suggests 8,000 BTU/h for the same floor area.
About 20 BTU/h per ft² for an 8 ft ceiling (27.87 m²), this calculator's rule of thumb. The Energy Star chart gives 8,000 BTU/h for this area, 26.7 per ft².
Scaled by room volume; the insulation factors (good 0.9, average 1.0, poor 1.2) are this calculator's assumption.
The cooling load starts at 20 BTU/h per square foot for an 8 ft ceiling, scales with ceiling height and an insulation factor, then takes Energy Star's room air conditioner adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h for a kitchen. The load is also given in tons (12,000 BTU/h each) and kW, with the next common unit size.
A 300 ft² room with an 8 ft ceiling and average conditions comes to 6,000 BTU/h, half a ton or 1.76 kW. Energy Star's sizing chart, based on 8 ft ceilings, lists 8,000 BTU/h for 300 up to 350 ft², so its figure is shown alongside for rooms of 100 to 2,500 ft².
Both are estimates for one room. For a whole house, ACCA describes its Manual J as the ANSI-recognised method for equipment sizing loads.
For one room, start at 20 BTU/h per square foot with an 8 ft ceiling and scale by ceiling height. Subtract 10% for heavy shade, add 10% for strong sun, 600 BTU/h per regular occupant beyond two and 4,000 BTU/h for a kitchen. A 300 ft² room with average conditions needs about 6,000 BTU/h, half a ton.
The sun, occupant and kitchen adjustments come from ENERGY STAR. The 20 BTU/h base and the insulation factors are a rule of thumb, and ENERGY STAR's own chart asks for more in small rooms: 8,000 BTU/h for 300 to 350 ft².
BTU/h = 20 × area × (ceiling ÷ 8) × insulation × sun + 600 × occupants beyond two + 4,000 for a kitchen, with area in square feet and height in feet.
The insulation factor is 0.9 for good, 1.0 for average and 1.2 for poor; sun is 0.9 for heavy shade and 1.1 for strong sun. A room with fewer than two occupants gets no reduction.
For a sunny 400 ft² kitchen with poor insulation, an 8 ft ceiling and four regular occupants:
The allowances are fixed amounts added last, so ceiling height does not scale them. A 600 ft² kitchen with a 10 ft ceiling gives 20 × 600 × 10 ÷ 8 = 15,000 BTU/h, plus 4,000 for the kitchen: 19,000 BTU/h, not 20,000.
ENERGY STAR lists these capacities for rooms with 8 ft ceilings. Each band includes its lower bound, so exactly 300 ft² falls in the 8,000 BTU/h band.
Apply the same sun, occupant and kitchen adjustments to the chart figure. The chart has no insulation or ceiling-height factor.
At 300 ft² the chart's 8,000 BTU/h is about 27 BTU/h per square foot. By 1,000 ft² it is 21, close to the rule of thumb, so the two methods disagree most in small rooms.
Default is the 300 ft² room above, and tall kitchen is the 600 ft² kitchen with a 10 ft ceiling. Metric shaded is 20 m² (215.28 ft²) under a 2.4 m ceiling with good insulation and heavy shade.
Poor sunny room is the step-by-step example, and tall room is the default with a 12 ft ceiling. The last column is the ENERGY STAR chart figure with the same sun, occupant and kitchen adjustments.
| Scenario | Modeled load, BTU/h | Next listed size, BTU/h | Area-band comparison, BTU/h |
|---|---|---|---|
| Default | 6000.000000 | 6000.000000 | 8000.000000 |
| Tall kitchen | 19000.000000 | 24000.000000 | 18000.000000 |
| Metric shaded | 3432.585605 | 5000.000000 | 5400.000000 |
| Poor sunny room | 15760.000000 | 18000.000000 | 16200.000000 |
| Tall room | 9000.000000 | 10000.000000 | 8000.000000 |
The tall room shows where the methods part: a 12 ft ceiling raises the estimate by half, to 9,000 BTU/h, while the chart assumes 8 ft and stays at 8,000.
One ton of refrigeration is 12,000 BTU/h, or 3,516.853 W (NIST SP 811). So 6,000 BTU/h is half a ton or 1.76 kW of cooling, and 24,000 BTU/h is 2 tons or 7.03 kW.
These kilowatts measure heat removed, not electricity used; the electrical draw depends on the unit's efficiency. Take the input watts from the nameplate for the electricity cost calculator or the electrical load calculator.
To measure an irregular room first, split it into rectangles with the room area calculator.
Checked against: 300 × 20 = 6,000 BTU/h ÷ 12,000 = 0.5 ton × 0.29307 = 1.758 kW; Energy Star chart row 300–350 ft² = 8,000
Checked against: 400 × 20 × 1.25 × 1.2 × 1.1 + 2 × 600 + 4,000 = 18,400; Energy Star 10,000 × 1.1 + 1,200 + 4,000 = 16,200
Checked against: 215.28 ft² × 20 × (7.874/8) × 0.9 × 0.9 = 3,432.6 BTU/h; one occupant adds nothing; chart 6,000 × 0.9
Checked against: Energy Star chart: 150 ft² falls in the “150 up to 250” row (6,000 BTU/h); rule of thumb 150 × 20 = 3,000
Energy Star's chart for 8 ft ceilings lists 5,000 BTU/h for 100–150 ft², 6,000 for 150–250 ft², 8,000 for 300–350 ft², 10,000 for 400–450 ft², 12,000 for 450–550 ft² and 18,000 for 700–1,000 ft². The 20 BTU/h per ft² rule gives less for small rooms, 6,000 against 8,000 at 300 ft², so the chart figure is shown beside the result.
This calculator uses 20 BTU/h per square foot for an 8 ft ceiling. Energy Star's chart works out higher for small and mid-sized rooms: 8,000 BTU/h for 300 ft² is about 27 per ft², and 12,000 BTU/h for 450 ft² is also about 27. At 1,000 ft² the chart's 21,000 BTU/h is 21 per ft², close to the rule.
One ton of refrigeration is 12,000 BTU/h, which NIST SP 811 lists as 3,516.853 W; one BTU/h is 0.2930711 W. A 6,000 BTU/h room unit is therefore half a ton or 1.76 kW of cooling, and a 24,000 BTU/h split system is 2 tons or 7.03 kW.
No. Energy Star says an oversized unit costs more to buy, wastes energy and does not cool better: it cools the room before it has time to remove the humidity, leaving the air damp and clammy. Choose the unit size that matches the load rather than rounding up by a full size or more for safety.
Energy Star's chart assumes 8 ft ceilings, and this calculator scales the load in proportion to height, so a 10 ft ceiling adds 25%. Energy Star advises 10% less capacity for a heavily shaded room and 10% more for a very sunny one, plus 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h in a kitchen.
This calculator includes 4 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.