Furnace Size Calculator: How Many BTU Do You Need?
How Furnace Sizing Works
Furnace size is measured in BTU/h input (gas burned) and BTU/h output (heat delivered). The difference is efficiency:
Output = Input × AFUE
A 80,000 BTU input furnace with 96% AFUE delivers 76,800 BTU/h of heat. AFUE (Annual Fuel Utilization Efficiency) measures seasonal efficiency — a 96% AFUE furnace converts 96% of gas energy into usable heat.
Furnace BTU Chart by Home Size
| Home Size (sq ft) | Mild Climate (Zone 1-3) | Moderate (Zone 4-5) | Cold (Zone 6-7) | Extreme (Zone 8) |
|---|---|---|---|---|
| 800 | 24,000–32,000 | 32,000–40,000 | 40,000–52,000 | 56,000–64,000 |
| 1,000 | 30,000–40,000 | 40,000–50,000 | 50,000–65,000 | 70,000–80,000 |
| 1,200 | 36,000–48,000 | 48,000–60,000 | 60,000–78,000 | 84,000–96,000 |
| 1,500 | 45,000–60,000 | 60,000–75,000 | 75,000–97,500 | 105,000–120,000 |
| 1,800 | 54,000–72,000 | 72,000–90,000 | 90,000–117,000 | 126,000–144,000 |
| 2,000 | 60,000–80,000 | 80,000–100,000 | 100,000–130,000 | 140,000–160,000 |
| 2,500 | 75,000–100,000 | 100,000–125,000 | 125,000–162,500 | 175,000–200,000 |
| 3,000 | 90,000–120,000 | 120,000–150,000 | 150,000–195,000 | 210,000–240,000 |
Note: These are output BTU/h values for 96% AFUE furnaces. For 80% AFUE, increase input by 20%. Values assume standard insulation (R-13 walls, R-30 ceiling).
AFUE Efficiency Ratings
| AFUE Rating | Category | Annual Cost (2,000 sq ft, Zone 5) | Typical Lifespan |
|---|---|---|---|
| 80% | Standard efficiency | ~$1,800 | 15–20 years |
| 90% | High efficiency | ~$1,600 | 15–20 years |
| 95% | Very high efficiency | ~$1,515 | 15–25 years |
| 97–98% | Premium condensing | ~$1,470 | 15–25 years |
The DOE minimum AFUE for new gas furnaces is 80% (northern states require 95%+ under regional standards).
How to Calculate Furnace Size
Q = A × q × Cinsul × Cheight
- A = heated floor area (sq ft)
- q = base heating BTU/sq ft from climate zone
- Cinsul = 0.6 (super-insulated) to 1.5 (uninsulated)
- Cheight = ceiling height ÷ 8
Worked Example
Home: 2,000 sq ft in Chicago (Zone 5), 9-ft ceilings, standard insulation, 96% AFUE furnace.
Step 1: Base = 2,000 × 50 = 100,000 BTU/h
Step 2: 9-ft ceiling → 100,000 × 1.125 = 112,500 BTU/h output
Step 3: Input = 112,500 / 0.96 = 117,188 BTU/h input
Result: Install a 120,000 BTU input, 96% AFUE furnace (output: 115,200 BTU/h).
Gas vs Electric vs Heat Pump
| Feature | Gas Furnace | Electric Furnace | Heat Pump |
|---|---|---|---|
| Efficiency | 80–98% AFUE | 100% AFUE | 200–400% COP |
| Fuel Cost | $0.015–0.025/1000 BTU | $0.03–0.05/1000 BTU | $0.01–0.02/1000 BTU |
| Equipment Cost | $2,500–$6,000 | $1,500–$3,500 | $4,000–$8,000 |
| Best Climate | Any, especially cold | Mild climates | Zone 1–5 (above 25°F) |
| Lifespan | 15–25 years | 20–30 years | 15–20 years |
Common Furnace Sizing Mistakes
- Oversizing: The #1 mistake. An oversized furnace short-cycles, wastes fuel, and creates uneven temperatures. Size within 15% of Manual J load.
- Ignoring insulation upgrades: If you've added insulation or new windows, the old furnace may be 50% oversized. Recalculate before replacing.
- Using input instead of output: An 80,000 BTU input furnace at 80% AFUE only delivers 64,000 BTU output. Match output to load.
- Not considering duct capacity: A 120,000 BTU furnace needs ~1,600 CFM airflow. Existing ducts may not support this.
Standards Reference
- ACCA Manual J — Residential Load Calculation (8th Edition)
- ACCA Manual S — Equipment Selection
- ASHRAE Standard 90.1 — Energy Standard
- DOE Regional Standards — Minimum AFUE Requirements
Frequently Asked Questions
What size furnace do I need for a 1,500 sq ft house?
In a moderate climate (Zone 4–5), a 1,500 sq ft house needs approximately 60,000–75,000 BTU output (62,500–78,000 BTU input at 96% AFUE). In cold climates (Zone 6–7), increase to 75,000–97,500 BTU output. In mild climates (Zone 2–3), 45,000–60,000 BTU output may suffice.
Is a bigger furnace better?
No. An oversized furnace short-cycles (turns on/off frequently), wastes energy, creates uneven temperatures, and has a shorter lifespan. ACCA recommends sizing within 15% of the Manual J calculated heating load. A properly sized furnace runs in steady, efficient cycles.
What's the difference between 80% and 96% AFUE?
An 80% AFUE furnace converts 80% of gas energy to heat (20% goes up the flue). A 96% AFUE furnace converts 96% (only 4% wasted). For a home needing 80,000 BTU output: 80% AFUE requires 100,000 BTU input; 96% AFUE requires 83,333 BTU input. Annual savings: $200–400 depending on gas prices.
How long does a furnace last?
Average lifespan: 15–20 years for gas furnaces, 20–30 years for electric, 15–20 years for heat pumps. Proper maintenance (annual tune-ups, filter changes) extends lifespan. Signs it's time to replace: frequent repairs, rising energy bills, uneven heating, strange noises.
Can I replace my furnace with a heat pump?
Yes, but consider your climate. Heat pumps work efficiently down to about 25°F (newer models to -15°F). In Zone 1–5, a heat pump can replace a furnace entirely. In Zone 6–7, you may need a backup gas furnace (dual-fuel system). In Zone 8, keep the furnace as primary.