How this instrument works
Every wall, roof, window, and door in a building loses heat all winter long, and the rate of that loss follows one of the oldest, most reliable formulas in building science: Q = U × A × ΔT. Heat loss in BTU per hour equals the assembly's U-value, times its area in square feet, times the temperature difference between inside and outside — a bigger area, worse insulation, or a colder night all drive the number up in direct proportion.
U-value is the reciprocal of the more familiar R-value (U = 1/R) and describes how easily heat passes through an assembly — a lower U-value means better insulation and less heat loss for the same area and temperature swing. This calculator leaves U-value fully editable rather than assuming one: a single-pane window might run U≈1.10, while a well-insulated R-20 roof assembly might sit closer to U=0.05, a difference that swamps any change in area. Look up or measure your assembly's actual U-value before trusting the output for a real decision — the 0.30 default shown here is illustrative only, not a claim about your specific wall or roof.
This is the transmission-loss term only — steady conduction through one surface at one moment — not a full Manual J or ASHRAE heating-load calculation. A real system-sizing exercise also adds infiltration and ventilation losses, sums every wall, roof, window, and door in the building separately, and often layers in equipment margins; treat this number as a single-surface building block for that larger picture, not a furnace-sizing tool on its own.
- Enter the surface area in square feet — a wall, roof, window, or whichever envelope section you're evaluating.
- Enter the assembly's U-value — look it up from a window or insulation spec sheet, or compute it as 1 ÷ R-value.
- Enter the indoor temperature you're heating to and the outdoor design temperature for your coldest expected conditions.
- Read Heat loss (BTU/hr) — the steady-state conductive loss through that surface at that temperature difference.
- Repeat for each wall, roof, window, and door, then add infiltration losses separately for a full building heating-load estimate.
Worked example — a 200 sq ft wall at U=0.30
Set Surface area to 200 sq ft, U-value to 0.30 BTU/(hr·ft²·°F) — a rough illustrative figure for an older, lightly insulated frame wall, not a universal constant — Indoor temperature to 70°F, and Outdoor design temperature to 20°F. The calculator multiplies 200 × 0.30 × (70 − 20) = 200 × 0.30 × 50 = 3,000 BTU/hr.
That 3,000 BTU/hr is what this one wall section loses at a 50°F temperature swing. Swap in a real product's U-value — a low-E double-pane window might also run near U≈0.30, while a 2×6 wall with modern batt insulation can sit closer to U=0.08 — and the result changes accordingly, since U-value drives the outcome as much as area does.
Questions
What's the difference between U-value and R-value?
They describe the same physical property from opposite directions. R-value measures resistance to heat flow — higher is better insulation — while U-value measures conductance, or how readily heat passes through — lower is better. They're reciprocals of each other (U = 1/R), so a wall assembly with R-19 insulation has a U-value of about 0.053. This calculator uses U-value directly since that's the form the Q = U×A×ΔT heat-loss formula needs.
Where do I find my wall or window's actual U-value?
Windows carry a U-factor on their NFRC label, typically ranging from about 0.20 (high-performance triple-pane) to 1.20 (old single-pane). For walls, roofs, and floors, either look up your insulation's R-value and take 1/R, or use a published assembly U-value table for your specific construction, since framing, sheathing, insulation type, and cladding all affect the number. The 0.30 default here is a placeholder for demonstration, not a stand-in for your real assembly.
Does this calculator size a furnace or heat pump for me?
No — it computes transmission heat loss through one surface only. A real heating-load calculation (commonly a Manual J in residential HVAC work) sums this transmission loss across every wall, roof, window, floor, and door in a building, adds infiltration and ventilation losses from air leakage, and often layers in equipment sizing margins. Use this tool to understand and compare individual assemblies, not as a final BTU/hr figure for buying equipment.
What outdoor temperature should I use for the calculation?
Use your location's winter design temperature, not the coldest night on record. Most HVAC design references, such as ASHRAE climate data, use a 99% or 97.5% design temperature, meaning colder conditions occur only a small fraction of winter hours. Using the record low would oversize equipment for nearly the whole heating season, so a regional design-temperature table or your local building code is the standard source to pull this figure from.
Why does a small single-pane window lose more heat than a large insulated wall?
Because U-value drives the formula as much as area does. A 30 sq ft single-pane window with U≈1.10 loses about 1,980 BTU/hr at a 60°F indoor-outdoor swing, while a 500 sq ft well-insulated roof with U=0.05 loses only about 1,700 BTU/hr at a similar swing — nearly 17 times more area losing less total heat, because its U-value is 22 times lower. This is exactly why upgrading old windows often delivers an outsized comfort and efficiency return.