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Heat Loss Calculator

Calculate a room's heat loss in BTU per hour through walls, ceiling, floor, windows and air leakage from R-values, U-factor and design temperatures to size a heater.

By CalcBuilt Editorial TeamUpdated September 11, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To estimate heat loss, multiply each surface's area by the indoor-to-outdoor temperature difference and divide by its R-value (or multiply by the U-factor for windows), then add infiltration at 0.018 x room volume x air changes per hour x the temperature difference. A 20 x 15 ft room with 8 ft ceilings, R-13 walls, an R-38 ceiling, an R-19 floor, 40 sq ft of U-0.3 windows and a 60 F design difference loses 5,837 BTU per hour, so it needs a 6,500 to 7,100 BTU heater.
Your measurements
door −20 sq ftwindow −15 sq ftHeightWall length (add all walls)
One wall: area is wall length times height, minus each door and window.
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Total heat loss5,837 BTU per hourAll results

Results

Total heat loss
5,837 BTU per hour
At a 60 F indoor-to-outdoor design difference
Walls
2,400 BTU per hour
520 sq ft of wall at R-13
Windows and doors
720 BTU per hour
40 sq ft at U-0.3
Ceiling and floor
1,421 BTU per hour
Ceiling 474 at R-38, floor 947 at R-19
Air infiltration
1,296 BTU per hour
2,400 cu ft at 0.5 air changes per hour
Heater size
6,500 to 7,100 BTU per hour
10 to 20% above the load, rounded up (1.91 to 2.08 kW electric). Planning estimate only: a Manual J calculation by an HVAC contractor governs equipment size

Heat Loss formula

Temperature difference = Indoor design temperature - Outdoor design temperature (F)
Conduction (BTU/h) = Area (sq ft) x Temperature difference / R-value  (windows: Area x U-factor x difference)
Wall area = 2 x (Length + Width) x Height - window and door area
Infiltration (BTU/h) = 0.018 x Volume (cu ft) x air changes per hour x Temperature difference

Worked example

A 12 x 12 ft bedroom with 9 ft ceilings on a slab, R-19 walls, an R-49 attic above, 25 sq ft of U-0.35 windows, kept at 68 F when it is 0 F outside, with 0.35 air changes per hour: the difference is 68 F. Wall area is 2 x (12 + 12) x 9 - 25 = 407 sq ft, so the walls lose 407 x 68 / 19 = 1,457 BTU per hour. The ceiling loses 144 x 68 / 49 = 200. The windows lose 25 x 0.35 x 68 = 595. The slab is ignored.

The room is 144 x 9 = 1,296 cubic feet, so infiltration is 0.018 x 1,296 x 0.35 x 68 = 555 BTU per hour. Total: 1,457 + 200 + 595 + 555 = 2,807 BTU per hour. Add 10 to 20% and round up for a 3,100 to 3,400 BTU heater; a 1,000 W electric baseboard delivers 3,412 BTU per hour and covers it.

Quick reference

Heat loss per 100 sq ft of surface at a 60 F temperature difference
SurfaceR-valueU-factorBTU per hour per 100 sq ft
Single-pane windowR-11.006,000
Double-pane windowR-20.503,000
Low-e double-pane windowR-3.30.301,800
Uninsulated 2 x 4 wallR-40.251,500
R-13 wallR-130.077462
R-19 wall or floorR-190.053316
R-30 atticR-300.033200
R-38 atticR-380.026158
R-49 atticR-490.020122

How to calculate heat loss

Heat leaves a room two ways: conduction through every surface that touches the cold, and air that leaks in and has to be warmed. For conduction, take each surface's area, multiply by the difference between the indoor and outdoor design temperatures, and divide by the R-value. Windows and doors are rated by U-factor instead, so multiply area by U by the temperature difference. Only count surfaces that face outdoors, an unheated attic, garage or crawlspace; a wall shared with a heated room loses nothing, so enter 0.

For infiltration, multiply the room volume by the air changes per hour to get cubic feet of cold air per hour, then by 0.018 and the temperature difference. The ACH here is the natural rate. A blower door reads at 50 pascals, and ACH50 divided by about 20 (15 in a cold, windy climate, 30 in a sheltered mild one) gives the natural figure. New homes must test at 3 ACH50 or less in zones 3 to 8 (2021 IECC R402.4.1.2), about 0.15 natural; a 1970s house at 12 ACH50 runs about 0.6. Air leakage is often a third of the total in an older room, so foam the top plates, rim joist and window trim before adding insulation.

Sizing the heater

Use the ASHRAE 99% design temperature from Manual J Table 1A, the temperature exceeded 99 percent of the hours in a year, not the record low. Roughly: 24 F in Atlanta, 27 F in Dallas and Seattle, 12 F in Boston, 0 F in Chicago, 4 F in Denver, -11 F in Minneapolis. The result is the heat a room loses on that design day, so a heater that delivers that many BTU per hour holds the set temperature. Round up 10 to 20 percent for recovery after a setback and for windier days than the design assumes, but no more; an oversized furnace or heat pump short-cycles. Electric heat converts at 3,412 BTU per hour per kilowatt, so a 5,837 BTU load is about 1.7 kW, or a 2,000 W baseboard.

This is a single-room, steady-state estimate. It ignores solar gain, internal gains from people and appliances, thermal bridging through studs, duct losses and the way rooms share heat, all of which a full ACCA Manual J calculation accounts for. Use it to compare insulation upgrades and to size a space heater or a single mini split head; let an HVAC contractor's Manual J and local code set the size of a whole-house system.

What this calculator leaves out

The estimate treats all four walls as exterior; a room with two outside walls has about half the wall loss, so scale that line. It ignores the slab edge, solar and internal gains, thermal bridging beyond the R-value you enter, duct losses, and heat shared with adjoining rooms. Not included: the heater itself and its circuit; a 2,000 W baseboard is a dedicated 240 V 15 A run with a permit. Nor a blower door test (typically $300 to $500) or the air sealing that changes the ACH input. A heat pump's output at the design temperature comes from the maker's table, not its nameplate.

Mistakes that cost money

  • Sizing to the record low instead of the 99% design temperature: Chicago's is about 0 F, not -20, and the heater comes out 30% over.
  • Guessing 0.35 ACH for an old house: blower doors on 1970s houses read 10 to 15 ACH50, about 0.5 to 0.75 natural, doubling that line.
  • Treating all four walls as exterior: a room with two outside walls has half the wall loss; halve that line or the heater is oversized.
  • Entering the batt's R-13 as the wall: studs are 25% of a 16 in OC wall at R-4, so the whole wall is nearer R-10.
  • Adding 20% and then rounding up a size: Manual S caps a furnace at 140% of load, a heat pump near 125%; stacked margins short-cycle.
  • Guessing infiltration on a whole-house job: a blower door test (typically $300 to $500) gives ACH50; divide by about 20 for the natural rate.

Key facts

  • One kilowatt equals 3,412 BTU per hour, so a 1,500 W space heater delivers about 5,100 BTU per hour.
  • U-factor is the inverse of R-value: an R-13 wall has a U of 0.077, while a single-pane window at U-1.0 loses 13 times as much heat per square foot.
  • The infiltration constant 0.018 is the density of air (0.075 lb per cubic foot) times its specific heat (0.24 BTU per lb per degree F).
  • Energy Star Version 7 (2023) requires a window U-factor of about 0.22 in the northern zone, down from 0.27 under the previous version; older double-pane windows are about 0.50 and single-pane about 1.0.
  • ASHRAE's 99% heating design temperature is the outdoor temperature exceeded 99% of the hours in a year, which is how Manual J picks the design day.

Frequently asked questions

How many BTU do I need to heat a 300 sq ft room?

With R-13 walls, an R-38 ceiling, an R-19 floor, 40 sq ft of low-e windows and a 60 F design difference, about 5,837 BTU per hour, so a 6,500 to 7,100 BTU heater. In a poorly insulated room with single-pane windows the loss can be two to three times higher.

What is the formula for heat loss through a wall?

Q = Area x temperature difference / R-value. A 520 sq ft R-13 wall at a 60 F difference loses 520 x 60 / 13 = 2,400 BTU per hour. Upgrading to R-21 drops that to about 1,486.

Will new windows cut my heat loss much?

Less than most people expect. At a 60 F difference, 40 sq ft of double-pane U-0.50 loses 1,200 BTU per hour and low-e U-0.30 loses 720. That 480 BTU saving is under 10% of a typical room, for typically $800 to $1,500 a window installed. Single-pane at U-1.0 is a different story: the same 40 sq ft loses 2,400 BTU per hour and low-e cuts it to 720. Air sealing and attic insulation almost always pay back first.

What outdoor temperature should I use?

The 99% winter design temperature for your city, published by ASHRAE and in Manual J: roughly 35 to 40 F on the Gulf Coast, 15 to 20 F in the mid-Atlantic, 0 to 10 F in the Midwest and -10 to -20 F in the northern plains. Do not use the record low.

Sources and references

Next steps for this project

  1. Insulation
  2. Blown-In Insulation
  3. Attic Ventilation
  4. Heat Loss
  5. BTU
  6. Furnace Size
  7. Mini Split

The usual order for a insulation & hvac project. See all insulation & hvac calculators.

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.