BTU calculator
A room's heating need in watts and BTU, or its air conditioner size
Choose heating or cooling, then describe the room. The heating example is a 4 x 3.5 m living room, 2.4 m high, with one outside wall, a 2 m² double-glazed window, filled cavity walls and a heated room above and below.
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- Watts
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- BTU per hour
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- Walls / chart size
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- Windows / sun
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- Ceiling and floor / people
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- Air / kitchen
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A sizing estimate for one room, not a heating or cooling design. It leaves out heat gained from sun, people and appliances in winter, losses through party walls to unheated rooms, thermal bridges and pipe losses, and any safety margin. A heating engineer's room-by-room survey is the reliable way to size a system.
Heating: the method
A room needs enough heat to replace what it loses on a cold day. The calculator adds two losses, the steady-state method used in CIBSE guidance and BS EN 12831:
- Through the building fabric: for each outside wall, window, roof and ground floor, U-value x area x (inside − outside temperature). A U-value is the watts lost through 1 m² for each degree of difference.
- Through air: 0.33 x air changes per hour x room volume x the temperature difference. 0.33 is the heat, in watt-hours, needed to warm 1 m³ of air by 1 °C.
The total is in watts. 1 W is 3.412 BTU per hour.
Worked example
The example room at 21 °C inside and −3 °C outside (a 24 °C difference): wall 4 x 2.4 − 2 = 7.6 m² x 0.7 x 24 = 127.7 W; window 2 x 2.0 x 24 = 96 W; air 0.33 x 1 x 33.6 m³ x 24 = 266.1 W. Total 489.8 W, about 1,671 BTU per hour.
Cooling: the method
Cooling uses the ENERGY STAR room air conditioner chart, which sizes a unit from floor area for an 8 ft ceiling, then adjusts: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU per person beyond two, and 4,000 BTU for a kitchen.
What it leaves out
The U-values are typical figures for the age of the house, not measurements of yours. Sun, people and cooking warm a room in winter; unheated neighbouring rooms, gaps and cold bridges lose more. Neither method replaces a proper survey, and the cooling chart assumes normal ceilings.
Defaults and where they come from
Every default is only a starting point. Change any box to match what you are buying or building.
| Default | Value | Source |
|---|---|---|
| Wall, window and loft U-values | Solid brick 1.7; cavity unfilled 1.5; filled 0.7; 1983 to 1995 0.6; 2003 to 2006 0.35; 2012 to 2022 0.28. Single glazing 4.8; double before 2002 2.8; 2002 or later 2.0. Loft: none 2.3; 100 mm 0.40; 200 mm 0.21; 270 mm 0.16 | RdSAP 10 Specification (BRE, 9 June 2025): Table 6 (England walls), Table 13 (solid brick 200 to 280 mm), Table 24 (windows, PVC or wood frame), Table 16 (loft insulation). |
| Ground floor U-values | Not insulated 0.6 (rough); insulated 0.25 | RdSAP works floors out from the whole floor's size (BS EN ISO 13370), which one room cannot give. 0.6 is what CIBSE Guide A's solid-floor formula gives for a typical 50 m² house floor; 0.25 is the Approved Document L (2010) limit for new floors. |
| Room temperatures | Living and dining 21 °C; bedroom 18 °C; bathroom 22 °C; hall 18 °C | MCS Heat Load Calculator defaults for homes built before 2007 (from the CIBSE Domestic Heating Design Guide). |
| Air changes per hour | Living 1; bedroom 0.5; bathroom 2; hall 1.5 | CIBSE Guide A winter design air change rates for dwellings, as reproduced in IES reference Table 12. |
| Outside design temperature | −3 °C | CIBSE Guide A (2015) Table 2.5, London Heathrow, 99.6% (via the MCS Heat Load Calculator). Colder or higher places need lower figures. |
| Cooling chart | 100 to 150 sq ft: 5,000 BTU/h ... 2,000 to 2,500 sq ft: 34,000 BTU/h | ENERGY STAR (US EPA), room air conditioner sizing chart and adjustments. |
| Waste allowance | None | This page sizes heat output; it does not order material. |
| Conversion | 1 W = 3.412142 BTU/h | 1 BTU (International Table) = 1,055.05585262 J. |