About the Room Heat Loss Calculator
This room heat loss calculator estimates how much heat a room loses on a cold design day, in BTU per hour and watts — the number you need to size a baseboard heater, radiator, mini-split head or space heater. It adds up conduction through each surface (walls, windows, doors, the ceiling and the floor) using their areas and insulation values, plus the heat carried away by outside air leaking in.
It is for homeowners choosing a heater for an addition, garage conversion or cold bedroom, and for DIYers comparing the payoff of new windows or extra attic insulation. Use it room by room; a whole-house load for furnace or heat pump sizing should be done with a full Manual J calculation by an HVAC professional.
Use whole-assembly R-values where you can: a 2x4 wall with R-13 batts performs closer to R-11 once studs are counted. Window and door performance is entered as a U-factor (from the NFRC label; U = 1 ÷ R). The outdoor design temperature is the cold temperature your heating should handle — typically the 99% heating design temperature for your area, which local HVAC contractors or ASHRAE climate data can give you.
With the default inputs, the total heat loss is 2,460 BTU/hr. Change any value above to recalculate instantly.
How to use the room heat loss calculator
- 1Enter the room’s floor area, ceiling height and gross exterior wall area.
- 2Enter the wall R-value and the window and door areas with their U-factors.
- 3Turn on the ceiling or floor if they face an unheated attic, crawlspace or garage and add their R-values.
- 4Choose an air-leakage rate and your indoor and outdoor design temperatures.
- 5Read the total heat loss and the suggested heater size with margin.
Formula and method
Heat flows through a building surface at a rate equal to its area times its U-factor times the temperature difference across it; the U-factor is the inverse of the R-value (U = 1/R). The calculator applies this to the net wall area (gross wall minus windows and doors) and, if they face unheated space, the ceiling and floor using their R-values, and to windows and doors using their U-factors.
Air leakage is the heat needed to warm outside air that replaces indoor air: room volume × air changes per hour × 0.018 Btu/ft³·°F (the density of air, 0.075 lb/ft³, times its specific heat, 0.24 Btu/lb·°F) × ΔT. All components are added to give the total design heat loss. It is a steady-state estimate that ignores solar and internal gains (which only help) and below-grade or slab-edge losses, so add a margin when choosing equipment.
- A
- Surface area (ft²)
- R, U
- Thermal resistance and transmittance (U = 1/R)
- ΔT
- Indoor minus outdoor design temperature (°F)
- ACH
- Air changes per hour from infiltration
- V
- Room volume (ft³)
Worked examples
12 × 15 ft corner bedroom under an attic
With 70 °F inside and 10 °F outside (ΔT = 60), the 186 ft² of net wall at R-13 loses 186 ÷ 13 × 60 = 858 BTU/hr, 30 ft² of windows at U 0.30 lose 540, the R-38 ceiling 284, and air leakage 0.018 × 0.5 × 1,440 × 60 = 778. The total is about 2,460 BTU/hr (721 W); with 10% margin, a 2,700 BTU/hr (≈ 790 W) heater.
Older room with single glazing and leaky air
Poorly insulated R-7 walls lose 186 ÷ 7 × 60 = 1,594 BTU/hr, single-glazed windows 30 × 1.0 × 60 = 1,800, and 1 ACH of leakage 1,555, plus the ceiling’s 284 — over twice the heat loss of the modern room at about 5,234 BTU/hr.
Room over a garage in a very cold climate
ΔT is 80 °F. The net wall shrinks to 166 ft² (1,022 BTU/hr), the 20 ft² door adds 640, windows 720, ceiling 379, the R-19 floor over the garage 758 and leakage 1,037, for about 4,555 BTU/hr (1,335 W).
Frequently asked questions
How do I calculate heat loss for a room?+
For each outside surface multiply its area by its U-factor (1 ÷ R-value) and by the temperature difference between inside and the outdoor design temperature, then add the air-leakage loss: volume × air changes per hour × 0.018 × ΔT. The sum is the room’s heat loss in BTU per hour.
How many BTUs do I need to heat a room?+
It depends on insulation, windows and climate, which is why a heat loss calculation beats rules of thumb. Well-insulated rooms in cold climates often need about 10–20 BTU/hr per square foot, while older, leaky rooms can need 30–50 BTU/hr per square foot or more.
How do I convert BTU/hr to watts for a heater?+
Multiply BTU/hr by 0.293 to get watts (1 W = 3.412 BTU/hr). A 2,500 BTU/hr heat loss is about 730 W, so a 750 W or 1,000 W electric heater would cover it; a 1,500 W plug-in heater supplies about 5,100 BTU/hr.
What outdoor design temperature should I use?+
Use your area’s 99% heating design temperature — the outdoor temperature that is only colder for about 88 hours a year. It is published in ASHRAE climate data and local code tables; for example roughly 20–35 °F across much of the US South and around 0 to −20 °F in the northern states.
Is this the same as a Manual J calculation?+
No. Manual J, the ACCA standard used to size whole-house heating and cooling equipment, accounts for many more factors such as duct losses, slab edges, orientation and internal gains. This calculator is a simpler room-by-room conduction and infiltration estimate, good for sizing a room heater or comparing upgrades.