Estimate the BTU/hr needed to heat or cool a room based on size, insulation, climate, and occupancy.
| Room Size | BTU/hr (cooling) | Tons |
|---|---|---|
| 150 ft² | 5,000 | 0.4 |
| 250 ft² | 6,000 | 0.5 |
| 300 ft² | 7,000 | 0.6 |
| 350 ft² | 8,000 | 0.7 |
| 400 ft² | 9,000 | 0.8 |
| 450 ft² | 10,000 | 0.8 |
| 550 ft² | 12,000 | 1.0 |
| 700 ft² | 14,000 | 1.2 |
| 1000 ft² | 18,000 | 1.5 |
| 1200 ft² | 21,000 | 1.8 |
| 1400 ft² | 23,000 | 1.9 |
| 1600 ft² | 24,000 | 2.0 |
| 1 Ton (AC) | 12,000 BTU/hr |
| 1 kW | 3,412 BTU/hr |
| 1 BTU/hr | 0.000293 kW |
One 20 by 15 foot room does not have one answer. With the dropdowns set as kindly as possible (excellent insulation, a cold climate, a shaded north-facing room) the calculator asks for 3,024 BTU/hr of cooling. Set them the other way and the same 300 sq ft wants 10,296, which is 3.4 times as much. Floor area puts you in the right neighborhood; insulation, climate and sun exposure decide which house in it you are on.
Take that same 20 × 15 ft room and make it a difficult one: cooling mode, poor insulation, hot climate, high sun, five occupants and six windows.
Area = 20 × 15 = 300 sq ft. Cooling starts at 20 BTU/hr per sq ft, so the base is 300 × 20 = 6,000 BTU/hr. Heating mode would start at 30 per sq ft.
The formula is BTU = base × insulation × climate × sun. Poor insulation is 1.3, a hot climate is 1.2 and high sun is 1.1, so 6,000 × 1.3 × 1.2 × 1.1 = 10,296 BTU/hr.
Then come two flat additions: 600 BTU/hr for each occupant beyond two, which is 3 × 600 = 1,800, and 1,000 for each window beyond two, which is 4 × 1,000 = 4,000. Total: 10,296 + 1,800 + 4,000 = 16,096 BTU/hr.
Conversions follow: 16,096 ÷ 12,000 = 1.34 tons and 16,096 × 0.000293071 = 4.72 kW. In heating mode the climate factor is flipped to 2 minus the cooling value, so a cold climate means 1.2 instead of 0.8, and the default room needs 9,000 BTU/hr. The BTU figure is rounded to a whole number.
In the current version the ceiling height box is accepted but not used; the estimate is built from floor area. For a room with a much taller ceiling than 8 or 9 feet, choose a size above the result rather than at it.
The table is a generic room-size chart, while the calculator starts at 20 BTU per sq ft and applies your inputs. With every dropdown on 'average' you get 6,000. A chart is a starting point, not a second opinion.
No. It is the thermal power, the heat to be moved. Electrical draw is BTU/hr ÷ efficiency; a 6,000 BTU/hr air conditioner with an EER of 10 draws about 600 W.
To the nearest standard size at or slightly above the result, yes. A badly oversized air conditioner cools quickly, shuts off, and leaves the air humid, which is worse than being a touch under.
No. Furnaces are rated by input, and the heat delivered is that figure times the efficiency. This number is closer to the heat the room needs delivered, and it does not replace a proper load calculation.
Yes, by hand. Room air-conditioner sizing guidance from ENERGY STAR, for example, adds capacity for a kitchen, and a room under a hot roof deserves extra margin. This page covers occupants and windows only.