Room Heat Loss Calculator
Calculate the design heat loss of a room the way heating engineers do: conduction through each surface (U × area × temperature difference) plus the heat carried away by air leakage. Use it to size a radiator, baseboard heater, mini-split head or furnace branch, in imperial or metric units.
Last updated:
0.35–0.5 tight, 0.5–1 average, 1–2 leaky.
From the window label: ~0.25–0.35 double low-e, ~0.5 old double, ~1.0 single pane (IP).
How to use the Room Heat Loss Calculator
- Choose units and whether you know R-values or U-factors for walls, ceiling and floor.
- Enter the indoor and outdoor design temperatures. The outdoor value is the cold design temperature for your area (the 99% winter design condition), not the record low.
- Enter the room's floor area, ceiling height and air changes per hour (0.35–0.5 for a tight modern home, 1–1.5 for an older drafty one).
- Enter each exterior surface: net wall area (without windows and doors), window and door area, ceiling and floor area, with their insulation values.
- Read the total heat loss and the breakdown, then size the heater at or a little above that figure.
The heat loss formulas
Conduction through each wall, window, door, ceiling and floor:
Q = U × A × ΔT with U = 1 ÷ R
Infiltration (air leaking in and out):
- Imperial: Q (BTU/h) = 0.018 × volume (ft³) × ACH × ΔT (°F) (0.018 BTU/ft³·°F is the heat capacity of air)
- Metric: Q (W) = 0.33 × volume (m³) × ACH × ΔT (K)
Example (imperial, ΔT = 70 − 10 = 60 °F): R-13 walls of 186 ft² lose 186 ÷ 13 × 60 = 858 BTU/h; 30 ft² of U-0.30 windows lose 540 BTU/h; an R-38 ceiling of 180 ft² loses 284 BTU/h; and 1,440 ft³ at 0.5 ACH leaks 0.018 × 1,440 × 0.5 × 60 = 778 BTU/h. Total ≈ 2,460 BTU/h (721 W).
Typical U-factors and R-values
| Element | R-value (IP) | U (BTU/h·ft²·°F) | U (W/m²·K) |
|---|---|---|---|
| Uninsulated 2×4 wall | ~4 | ~0.25 | ~1.4 |
| 2×4 wall, R-13 batts (nominal) | 13 | 0.077 | 0.44 |
| 2×6 wall, R-21 batts (nominal) | 21 | 0.048 | 0.27 |
| Attic, R-38 insulation | 38 | 0.026 | 0.15 |
| Attic, R-49 insulation | 49 | 0.020 | 0.12 |
| Single-pane window | ~1 | ~1.0–1.1 | ~5.7–6.2 |
| Older double-pane window | ~2 | ~0.5 | ~2.8 |
| Double-pane low-e, argon | ~3.3 | ~0.30 | ~1.7 |
| Triple-pane window | ~5 | ~0.20 | ~1.1 |
| Insulated steel door | ~3–6 | ~0.17–0.35 | ~1.0–2.0 |
Conversions: U(SI) = U(IP) × 5.678; RSI = R(IP) × 0.1761; 1 W = 3.412 BTU/h. Framing lowers the effective R-value of walls below the insulation's nominal value.
For slab-on-grade floors and basement walls, heat loss depends mostly on the exposed perimeter and the ground temperature, which this simple room method does not model in detail. Treat the result as a room-level estimate; whole-house equipment sizing should follow a full load calculation such as ACCA Manual J.
Frequently asked questions
How is room heat loss calculated?
Each surface loses Q = U × A × ΔT, where U is the heat transfer coefficient (1 ÷ R), A the area and ΔT the indoor minus outdoor design temperature. Air leakage adds Q = 0.018 × volume (ft³) × ACH × ΔT in BTU/h, or 0.33 × volume (m³) × ACH × ΔT in watts. The room's heat loss is the sum of all surfaces plus infiltration.
What is the difference between R-value and U-factor?
R-value measures resistance to heat flow (higher is better) and U-factor measures how easily heat flows (lower is better). U = 1 ÷ R. In US units R is in ft²·°F·h/BTU; in metric, RSI is in m²·K/W. To convert, RSI = R(IP) × 0.1761 and U(SI) = U(IP) × 5.678.
What outdoor temperature should I use?
Use the winter design temperature for your location, usually the 99% or 99.6% heating dry-bulb temperature published by ASHRAE or in local building codes. It is the temperature that is only colder for a few hours a year. Sizing for the record low oversizes the system.
What air change rate (ACH) should I use?
Air changes per hour describe how often the room's air is replaced by outdoor air. New, airtight construction is often around 0.3 to 0.5 ACH under natural conditions, average older homes 0.5 to 1.0 and leaky homes 1.0 to 2.0. A blower-door test gives the most accurate value.
Should I use the nominal wall R-value?
Nominal values (such as R-13 for 2×4 walls with batts) ignore framing, which conducts more heat. A whole-wall effective R-value is often 10% to 25% lower, so using it, or adding a margin, gives a more realistic result.
How big should the heater be?
Choose heating equipment with an output at or slightly above the calculated design heat loss. A small margin of about 10% to 15% covers warm-up after setbacks; much larger oversizing wastes energy and causes short cycling in furnaces and heat pumps.
Is my data saved?
Only locally in your browser so the form is filled in next time. The calculation runs on your device and nothing is uploaded.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.