Furnace Size Calculator
Furnace BTU, kW, and stage from home area and climate zone.
What it does
Furnaces size to the house and the winter. Area times a climate factor gives the planning load that quotes start from.
The tool above multiplies area by zone. Two thousand square feet in a moderate zone needs 80,000 BTU/h, or 23.4 kW, in two-stage range. The metric match is 186 m2 toward 80,083 BTU/h.
Factors track winters. Mild zones near 30, moderate near 40, and cold near 50 BTU per square foot in this guide.
Stages match comfort. Single-stage suits small loads, two-stage suits mid homes with even heat, and modulating suits large cold houses.
Area math starts quotes. Windows, insulation, and leaks move real Manual J loads, so pros measure before installing.
How to use
- Pick imperial (sq ft) or metric (m2).
- Enter heated area.
- Pick climate: moderate suits most suburbs.
- Read BTU/h first.
- Read kW next.
- Read stage and get Manual J quotes.
Formula
- area
- Heated living area.
- climate
- Mild, moderate, or cold zone.
Metric converts area at 10.7639 sq ft per m2 with shared factors. Stage bands guide quotes, not installs.
Worked example
Example: 2000 sq ft, moderate
Imperial defaults: 2000 sq ft moderate climate. These are also the default values, so the page opens on this answer.
- Home: 2000 sq ft, moderate climate.
- Factor: 40 BTU per sq ft.
- Heat: 2000 x 40 = 80,000 BTU/h.
- kW: 80000 x 0.000293071 = 23.4 kW.
- Stage: 80,000 sits two-stage.
- Tool shows 80,000 BTU/h, 23.4 kW, Two-stage suits most homes.
Reference table
| Home | Climate | BTU/h |
|---|---|---|
| 1000 sq ft | Mild | 30,000 BTU/h |
| 2000 sq ft | Moderate | 80,000 BTU/h |
| 3000 sq ft | Cold | 150,000 BTU/h |
| 186 m2 | Moderate | 80,083 BTU/h |
Limits
- Planning guide only. Manual J rules installs.
- Insulation and windows shift loads.
- Ducts need separate design.
- Oversizing wastes fuel and comfort.
Common mistakes
- Sizing to the old furnace plate.
- Ignoring new windows and insulation.
- Oversizing just in case.
- Skipping duct checks.
- One quote with no Manual J.
Try it yourself
Quick checks. Answers sit tucked under each question.
1000 sq ft mild?
1000 x 30 = 30,000 BTU/h. kW is 30000 x 0.000293071 = 8.8 kW. Stage is single.
3000 sq ft cold?
3000 x 50 = 150,000 BTU/h. Stage is modulating.
Questions
BTU for 2000 sq ft?
Multiply area by climate factor. Here 2000 x 40 = 80,000 BTU/h.
Climate factors?
Mild 30, moderate 40, cold 50 BTU per sq ft in this plan.
BTU to kW?
Multiply BTU by 0.000293071. Here 80000 x 0.000293071 = 23.4 kW.
Which stage?
Under 60k single-stage, under 100k two-stage, above modulating in this guide.
Insulation matter?
Yes. Windows, insulation, and air leaks shift real loads past area math.
Does this replace Manual J?
No. Manual J sizing needs a pro for installs.