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Heat Loss Calculator

Calculate heat loss and furnace sizing for your home

Total Heat Loss

21,018 BTU/hr

Furnace Size

26k BTU

ΔT

60°F

Annual Cost

$1387

Typical: 0.25 (tight), 0.5 (average), 1.0 (leaky)

Total Heat Loss

21,018 BTU/hr

Furnace Size

26k BTU

Temp Difference

60°F

Annual Cost

$1387

Heat Loss Breakdown (BTU/hr)

Walls (26%)5,538 BTU
Windows (29%)6,000 BTU
Roof (14%)3,000 BTU
Infiltration (31%)6,480 BTU

Component Details

Wall Loss (R-13)5,538 BTU/hr
Window Loss (R-2)6,000 BTU/hr
Roof Loss (R-30)3,000 BTU/hr
Infiltration (0.5 ACH)6,480 BTU/hr
Total Heat Loss21,018 BTU/hr

Furnace Sizing

Heat Loss21,018 BTU/hr
Safety Factor1.25x (25%)
Heating Degree Days11,000
Recommended Furnace26k BTU
BTU per Sq Ft14.0 BTU/ft²

Example Calculations

1Average 1,500 sq ft Home (Cold Climate)

Inputs

Floor Area1,500 sq ft
Wall Area1,200 sq ft
Window Area200 sq ft
Roof Area1,500 sq ft
Wall R-ValueR-13
Window R-ValueR-2
Roof R-ValueR-30
Indoor Temp70°F
Outdoor Temp10°F
ACH0.5

Result

Total Heat Loss15,878 BTU/hr
Wall Loss5,538 BTU/hr (35%)
Window Loss6,000 BTU/hr (38%)
Roof Loss3,000 BTU/hr (19%)
Infiltration1,340 BTU/hr (8%)
Furnace Size20k BTU

ΔT = 70 - 10 = 60°F. Walls: 1,200 × 60/13 = 5,538. Windows: 200 × 60/2 = 6,000. Roof: 1,500 × 60/30 = 3,000. Infiltration: 0.018 × 1,500 × 8 × 0.5 × 60 = 6,480. Total = 21,018. Furnace = 21,018 × 1.25 = 26,273 BTU.

2Poorly Insulated Older Home

Inputs

Floor Area2,000 sq ft
Wall Area1,600 sq ft
Window Area300 sq ft
Roof Area2,000 sq ft
Wall R-ValueR-4
Window R-ValueR-1
Roof R-ValueR-11
Indoor Temp70°F
Outdoor Temp0°F
ACH1.0

Result

Total Heat Loss71,509 BTU/hr
Wall Loss28,000 BTU/hr (39%)
Window Loss21,000 BTU/hr (29%)
Roof Loss12,727 BTU/hr (18%)
Infiltration20,160 BTU/hr (28%)
Furnace Size89k BTU

ΔT = 70°F. Walls: 1,600 × 70/4 = 28,000. Windows: 300 × 70/1 = 21,000. Roof: 2,000 × 70/11 = 12,727. Infiltration: 0.018 × 2,000 × 8 × 1.0 × 70 = 20,160. Total = 81,887. Furnace = 102,359 BTU.

Frequently Asked Questions

Q

How do I calculate heat loss for my home?

Heat loss = Area × Temperature Difference / R-value for each surface (walls, windows, roof). Add infiltration loss: 0.018 × floor area × ceiling height × air changes per hour × temperature difference. Sum all components for total heat loss in BTU/hr.

  • Calculate each surface separately: walls, windows, doors, roof/ceiling, and floor
  • Use your local 99% ASHRAE design temperature for the outdoor value
  • Infiltration typically adds 15–30% on top of conduction losses
  • A 1,500 ft² home in a cold climate usually totals 40,000–80,000 BTU/hr
  • Online Manual J calculators automate this—but you still need accurate R-values
Q

What R-value do my walls have?

Uninsulated walls: R-4. Walls with fiberglass batts: R-11 to R-15. Walls with spray foam: R-13 to R-21. Exterior foam board adds R-5 to R-10. Check your insulation type and thickness to determine R-value.

  • Heat loss values assume ΔT of 60°F (e.g., 70°F indoors, 10°F outdoors)
  • Check your insulation by removing a switch plate on an exterior wall
  • Adding exterior foam board over existing insulation is the least disruptive upgrade
  • Spray foam seals air gaps AND insulates—fiberglass batts only insulate
  • R-value per inch: fiberglass 3.2, cellulose 3.7, closed-cell foam 6.5
Wall AssemblyR-ValueHeat Loss (BTU/hr per 100 ft²)Notes
Uninsulated 2×4R-41,500Pre-1960s construction
2×4 + fiberglass battR-13462Standard since 1980s
2×4 + spray foamR-15 to R-21286–400Best retrofit option
2×6 + fiberglass battR-19316Cold climate standard
2×6 + foam board outsideR-24 to R-29207–250High-performance wall
Q

What size furnace do I need?

Calculate total heat loss in BTU/hr, then add 25% safety factor. A 1,500 sq ft home with average insulation in a cold climate typically needs 60,000-80,000 BTU/hr. Oversizing wastes fuel; undersizing leaves rooms cold.

  • Furnace output BTU (not input BTU) must exceed your calculated heat loss
  • A 95% AFUE furnace rated at 100,000 input delivers 95,000 BTU output
  • Two-stage furnaces run at 60–70% capacity most of the time for better comfort
  • Oversized furnaces short-cycle and create hot/cold spots—aim for right-sized
  • Add 25% safety factor for extreme cold snaps and equipment aging
Home SizeMild Climate (30°F)Moderate (10°F)Cold (−10°F)
1,000 ft²30,000–40,000 BTU45,000–60,000 BTU60,000–80,000 BTU
1,500 ft²40,000–60,000 BTU60,000–80,000 BTU80,000–100,000 BTU
2,000 ft²55,000–75,000 BTU80,000–100,000 BTU100,000–120,000 BTU
2,500 ft²70,000–90,000 BTU100,000–120,000 BTU120,000–140,000 BTU
Q

Where does a home lose the most heat?

Windows are the biggest heat loss per square foot (R-2 vs R-13 for walls). But walls have the largest total area, often accounting for 30-40% of total loss. Air infiltration typically causes 20-30% of heat loss. Attic/roof accounts for 15-25%.

  • Air sealing is the cheapest fix—$200–$400 in materials can cut 15% of heat loss
  • Windows lose 6× more heat per ft² than insulated walls (R-2 vs R-13)
  • Attic insulation upgrades from R-11 to R-38 can cut roof losses by 70%
  • A blower-door test reveals hidden air leaks invisible to the eye
  • Prioritize fixes by cost per BTU saved: sealing > attic insulation > windows
Heat Loss Source% of Total LossR-ValueBest Fix
Walls30–40%R-4 to R-19Add insulation, seal gaps
Windows15–25%R-1 to R-5Upgrade to double/triple-pane
Air infiltration20–30%N/ACaulk, weatherstrip, seal
Roof / attic15–25%R-11 to R-60Blow in attic insulation
Floors / foundation5–15%R-0 to R-25Insulate crawlspace
Q

What is outdoor design temperature?

The outdoor design temperature is the coldest temperature your heating system must handle, typically the 99% ASHRAE design temperature for your area. Examples: Minneapolis -12°F, Chicago 0°F, Atlanta 22°F, Phoenix 34°F.

  • Use the 99% design temp, not the record low—your system handles 99% of winter hours
  • A higher ΔT (indoor minus outdoor) means more BTU/hr needed from your furnace
  • Heating degree days (HDD) estimate annual fuel usage—higher HDD = higher bills
  • Look up your exact city's design temp in ASHRAE Fundamentals or local building codes
City99% Design Temp (°F)Heating Degree DaysClimate Zone
Minneapolis, MN−12°F7,876Zone 6 (Cold)
Chicago, IL0°F6,498Zone 5 (Cold)
Denver, CO1°F6,128Zone 5 (Cold)
New York, NY12°F4,871Zone 4 (Mixed)
Atlanta, GA22°F2,827Zone 3 (Warm)
Phoenix, AZ34°F1,125Zone 2 (Hot)

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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