Furnace Size Calculator
Calculate the ideal furnace BTU capacity for your home based on climate, area, and insulation
Calculate Furnace Size
Climate Information
Select your climate zone based on average annual temperature
Room Details
Total floor area to be heated
Modern furnaces: 80-98%, older: 60-80%
Environmental Factors
Factor: Light (1.1), Average (1.0), Heavy (0.9)
Factor: Sunny (0.9), Average (1.0), Shaded (1.1)
Furnace Size Calculation Results
Current Climate Zone: 5
Temperature Range: 23-42°F (-4 to 5°C)
BTU Recommendation: 55-70 BTU/h per sq ft
Description: Cool Climate
Applied Factors
0 sq ft
1.0x
1.0x
100%
Calculation Formula:
Sizing Analysis
Example Calculation
Heavily Insulated Room (Competitor Example)
Floor Area: 1,500 sq ft
Climate Zone: Zone 5 (cool climate)
Insulation: Heavy (factor 0.9)
Sunlight: Sunny room (factor 0.9)
Efficiency: 90%
Calculation Results
Min: 55 BTU/h per sq ft × 1,500 sq ft × 0.9 × 0.9 ÷ 0.90 = 74,250 BTU/h
Max: 70 BTU/h per sq ft × 1,500 sq ft × 0.9 × 0.9 ÷ 0.90 = 94,500 BTU/h
This matches the competitor's example exactly!
Recommended range: 74,250 - 94,500 BTU/h
Climate Zone Reference
BTU recommendations by climate zone and temperature range:
| Zone | Temperature Range | BTU/h per sq ft | Description |
|---|---|---|---|
| 1 | 70°F and up (21°C+) | 30 - 35 | Very Hot Climate |
| 2 | 61-69°F (16-20°C) | 35 - 40 | Hot Climate |
| 3 | 52-60°F (11-15°C) | 40 - 45 | Warm Climate |
| 4 | 43-51°F (6-10°C) | 45 - 55 | Moderate Climate |
| 5 | 23-42°F (-4 to 5°C) | 55 - 70 | Cool Climate |
| 6 | 16-22°F (-9 to -5°C) | 70 - 80 | Cold Climate |
| 7 | 2-15°F (-19 to -10°C) | 80 - 90 | Very Cold Climate |
| 8 | -11 to 1°F (-24 to -20°C) | 90 - 100 | Extremely Cold Climate |
| 9 | -12°F and below (-25°C-) | 100 - 110 | Arctic Climate |
BTU requirements increase with colder climates. Factor in insulation and sunlight exposure for accurate sizing.
BTU Guide
Small Furnace
800-1,200 sq ft
Apartments, small homes
Medium Furnace
1,200-2,000 sq ft
Average homes
Large Furnace
2,000-3,000 sq ft
Large homes
Efficiency Tips
Choose 90%+ efficiency furnaces for lower operating costs
Properly size - oversized furnaces waste energy
Improve insulation to reduce furnace size needed
Regular maintenance maintains efficiency
Consider backup heating for extreme climates
Understanding Furnace Sizing
What is BTU?
British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by 1°F. In heating, we measure furnace capacity in BTU per hour (BTU/h).
Why Proper Sizing Matters
- •Undersized: Inadequate heating, runs constantly
- •Oversized: Short cycling, poor humidity control, energy waste
- •Properly Sized: Efficient operation, even heating, optimal comfort
Sizing Formula
Key Factors
- •Climate: Colder areas need more BTUs
- •Insulation: Better insulation reduces requirements
- •Sunlight: Direct sun reduces heating needs
- •Efficiency: Higher efficiency = lower operating costs
Professional Recommendations
- • Get a Manual J load calculation for precise sizing
- • Consider whole-house insulation improvements before furnace replacement
- • Size based on actual loads, not rule-of-thumb estimates
- • Account for ductwork condition and proper sealing
- • Choose high-efficiency models for long-term savings