How It Works
Furnace sizing starts with BTU per square foot by climate: mild climates need ~28 BTU/ft², moderate ~34, cold ~42, and very cold ~52. Ceiling height adds 10% per foot above 8 ft, and insulation quality adjusts the result by ±10–20%.
Calculation Explanation
Example: 1,500 sq ft, four-season climate, 8-ft ceilings, average insulation → 1,500 × 34 = 51,000 output BTU. A 60,000 input BTU furnace at 92% AFUE (≈55,000 output) is a good match.
Pair this with the HVAC Load Calculator for a full heating + cooling estimate, or the BTU Calculator for single rooms.
Furnace Size Quick Reference
| Home Size (sq ft) | Output BTU (moderate) | Output BTU (cold) |
|---|---|---|
| 1,000 | 30,000–35,000 | 40,000–45,000 |
| 1,500 | 45,000–55,000 | 60,000–65,000 |
| 2,000 | 60,000–70,000 | 80,000–90,000 |
| 2,500 | 80,000–90,000 | 100,000–110,000 |
| 3,000 | 95,000–105,000 | 120,000–130,000 |