Heat Loss Calculator
A heat loss calculator multiplies surface area by the temperature difference and divides by R-value: 2,000 sq ft at R-13 and a 50 degree gap loses 7,692 BTU/hr.
Heat loss calculator formula
A heat loss calculator estimates how fast a building loses heat through its outer surfaces on a cold day. Heat loss in BTU per hour = area ÷ R-value × (indoor temperature − outdoor temperature). This one takes the exterior surface area, an average R-value, and the two design temperatures.
The formula in plain words
R-value measures how well a material resists heat flow, so heat flow through a surface is one divided by R for each square foot and each degree of difference. Multiply that by the number of square feet. Multiply again by how many degrees colder it is outside. The answer is BTU per hour, the heat your system must replace to hold the indoor temperature.
Worked examples
These match the calculator exactly.
- Small house in a cold snap: 2,000 sq ft of exterior surface, average R-13, 70 F inside and 20 F outside
- Temperature difference: 70 − 20 = 50 degrees
- Heat flow per sq ft per degree: 1 ÷ 13 = 0.0769
- Heat loss: 2,000 × 0.0769 × 50 = 7,692 BTU/hr
- The same shell with better insulation: average R-19
- Heat loss: 2,000 ÷ 19 × 50 = 5,263 BTU/hr
- That is 2,429 BTU/hr less than example 1
- Smaller shell in a harsher climate: 1,500 sq ft, R-19, 70 F inside and 0 F outside
- Temperature difference: 70 degrees
- Heat loss: 1,500 ÷ 19 × 70 = 5,526 BTU/hr
Notice that example 3 loses less than example 1 even though it is colder outside. The surface is smaller and better insulated.
Heat lost per 100 square feet
This table holds the temperature difference at 50 degrees.
| Surface | R-value | BTU/hr per 100 sq ft |
|---|---|---|
| Single pane window | about 1 | 5,000 |
| Double pane window | about 2 | 2,500 |
| Wall with R-13 insulation | 13 | 385 |
| Wall or floor with R-19 | 19 | 263 |
| Ceiling with R-30 | 30 | 167 |
| Attic with R-38 | 38 | 132 |
The window rows explain why a house full of glass leaks so much. One hundred square feet of single pane glass loses about thirteen times what the same area of R-13 wall does.
How to measure the surface area
Add up every surface that touches the outdoors or unheated space. That means exterior walls, the ceiling under the attic, and the floor over a crawl space or garage. Measure each wall as length times height and subtract nothing for windows, then handle windows separately if you want a better answer.
For a better average R-value, weight each surface by its area. If 1,800 sq ft is R-13 wall and 200 sq ft is R-2 window, the average is 2,000 ÷ (1,800/13 + 200/2) = 2,000 ÷ 238.5 = about R-8.4. Enter 8.4 and the tool shows a heat loss close to 11,900 BTU/hr, far above the 7,692 you saw with a flat R-13.
What this tool leaves out
Warm air that leaks out through gaps is often a large share of real heat loss. Ducts that run through a cold attic lose more. Wind, sun and thermal bridging through studs also change the result. Because of this, treat the output as a lower bound for the conduction part of your load.
Turning the result into a check on equipment
A furnace is rated in BTU per hour of input, and its output is lower by the efficiency. A unit with 80 percent efficiency and 100,000 BTU/hr input delivers 80,000 BTU/hr. Compare that to a load like the 7,692 above and you can see the margin. A big gap usually means the unit is oversized, which can cause short cycling. A pro load calculation settles it.
Common mistakes
- Entering the insulation’s R-value for the whole wall. Studs and windows lower the average.
- Using the record low. Pick the winter design temperature for your area.
- Mixing interior and exterior area. Use surface area facing the outdoors only.
- Sizing equipment from this number. Ask a licensed HVAC contractor for a Manual J load.
Related pages
Compare the result against the furnace size calculator. Convert the output with the BTU calculator. If you are choosing between fuels, see the heat pump vs gas furnace calculator.
Frequently asked questions
How do you calculate heat loss through a wall?
Divide the area in square feet by the R-value, then multiply by the indoor to outdoor temperature difference in degrees F. A 2,000 sq ft envelope at R-13 with 70 F inside and 20 F outside loses 2,000 ÷ 13 × 50 = 7,692 BTU per hour.
Does this calculator include air leaks and ducts?
No. It counts heat conducted through surfaces with one average R-value. Air leakage, duct losses, and different R-values for windows, doors and walls are left out, so a real load is often higher than this figure.
Can I use this to size a furnace?
Use it as a rough check only. Furnace and heat pump sizing should come from a full room-by-room load calculation, often called Manual J, done by a licensed HVAC contractor. An oversized or undersized unit wastes money and comfort.
What R-value should I enter?
Use an area-weighted average of your walls, roof, floor and windows. A single pane window is near R-1 and a double pane window is near R-2, so windows pull the average down a lot. Insulation labels such as R-13 or R-19 describe the insulation only.
What outdoor temperature should I use?
Use the winter design temperature for your area, which local HVAC contractors or building departments can supply. It is usually a cold night that occurs only a few hours a year, not the record low.
Estimates only. Verify quantities with your supplier before purchasing.