A 12 × 14 ft corner bedroom with 8 ft ceilings, 2×4 walls with R-13 batts, 20 ft² of double-pane windows, an exterior door, a slab floor and an R-38 attic loses 4,368 Btu/h when it is 70 °F inside and 0 °F outside — 26 Btu/h per square foot. A heater with a 20 % margin is 5,242 Btu/h, or about 6 ft of electric baseboard. The calculator above works this out for any room and wall.
How the calculator works
Enter the floor area, ceiling height and the area of the exterior walls, windows and doors, pick the wall, glazing, floor and ceiling, then set the indoor and outdoor design temperatures and how tight the room is. The result is the loss through each surface, the total in Btu/h and kW, the heater size and the baseboard length.
Where the heat goes in the example
| Surface | R-value | Btu/h |
|---|---|---|
| Walls, 168 ft² net | R-11.9 (2×4, R-13) | 993 |
| Windows, 20 ft² | U 0.50 | 704 |
| Door, 20 ft² | R-2.8 | 493 |
| Slab floor, 168 ft² | R-11.4 | 1,036 |
| Ceiling, R-38 attic | R-39.7 | 296 |
| Air leakage, 0.5 ACH | — | 846 |
| Total | 4,368 |
Walls
A wood frame wall is not as good as its batts: about a quarter of it is studs at R-1.25 per inch. With drywall, sheathing, siding and air films, a 2×4 wall with R-13 works out to R-11.9, R-15 to R-12.7, and a 2×6 wall with R-21 to R-17.1 (R-22.9 with R-5 foam outside). Solid walls — brick, concrete, aerated concrete, logs, SIPs — are counted by thickness and conductivity, plus any insulation you add.
Windows and design temperature
Windows lose the most per square foot: changing the example to low-e glass saves 352 Btu/h, single pane adds 746. The outdoor temperature is the 99 % heating design temperature for your location, not the record low: at 20 °F outside the same room loses 3,120 Btu/h.
Better walls and attic cut the loss before you buy equipment — see the Insulation Calculator. For summer, size the cooling with the AC BTU Calculator; fresh air is in the CFM Calculator, and room areas in the Square Footage Calculator.