Furnace sizing: what size do I need?
Sizing is the most important spec on a furnace install, more important than brand, more important than AFUE. An oversized furnace short-cycles, wastes fuel, and burns out faster. An undersized one runs constantly and never reaches setpoint on the coldest days.
BTU sizing calculator
BTU calculator
1970-2000, decent insulation
Recommended capacity
80,000 BTU
Stock sizing tier
Calculated range
70,000–90,000 BTU
Furnaces are sold in fixed sizes (40k, 60k, 80k, 100k, 120k). Round up to the next stock size, but only by one tier.
This is a rule-of-thumb estimate. For a final quote, your contractor should run a Manual J load calculation that factors in window orientation, air infiltration, and your specific construction.
Manual J: the gold standard
If a contractor sizes by square footage alone, get a different quote
Manual J is the ACCA-published heat load calculation. It accounts for window area, orientation, insulation R-values, air infiltration, and occupant load. A proper Manual J takes 30 to 60 minutes and costs $100 to $300 if billed separately. Most reputable HVAC firms include it in the quote.
Why oversizing is the bigger mistake
Old-school HVAC contractors often over-size furnaces by 25 to 50% on the theory that bigger is safer. It is not. An oversized furnace fires up, blasts the house with heat, hits setpoint in 4 minutes, and shuts off. Five minutes later, the house has cooled and it fires again. This pattern (called short-cycling) creates four problems:
- • Wastes fuel. Each ignition cycle uses gas before the heat exchanger reaches steady state.
- • Wears out the furnace faster. Ignitor, blower motor, and gas valve all rated by cycles, not run hours.
- • Hot/cold rooms. The blower never runs long enough to push air to the furthest registers.
- • Poor humidity control. Short cycles leave moisture in the home in summer (when AC shares the blower).
Quick sizing reference
| Home size | Mild climate | Moderate | Cold | Very cold |
|---|---|---|---|---|
| 1,000 sq ft | 30,000 BTU | 40,000 | 50,000 | 60,000 |
| 1,500 sq ft | 45,000 BTU | 60,000 | 75,000 | 90,000 |
| 2,000 sq ft | 60,000 BTU | 80,000 | 100,000 | 120,000 |
| 2,500 sq ft | 75,000 BTU | 100,000 | 125,000 | 150,000 |
| 3,000 sq ft | 90,000 BTU | 120,000 | 150,000 | 180,000 |