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Furnace Size Calculator

Result
—BTU output
◆ LIVE 3D PREVIEW
Input BTU
—
Furnace Tonnage
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CalcBuild › Furnace Size Calculator

How to Calculate Furnace Size

Use this free furnace size calculator to get instant, accurate results for your construction project. Simply enter your measurements above and the calculator will automatically compute the result using industry-standard formulas.

Pro Tip: Always add 10-15% extra material to account for waste, cuts, and job-site conditions. This is standard practice recommended by experienced contractors.

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💡 Pro Tips — Furnace Size

❓ Frequently Asked Questions

+How many BTU per square foot?
20-25 BTU/sq ft for mild climates, 30-40 for moderate, 40-60 for extreme heat or cold.
+What size AC do I need?
Rough guide: 1 ton per 500-600 sq ft. Actual sizing requires Manual J load calculation for accuracy.
+How often to change filters?
1" filters: every 1-3 months. 4" filters: every 6-12 months. Homes with pets: change more frequently.
+What SEER rating should I get?
Minimum is 14-15 SEER (varies by region). 16-20 SEER offers good efficiency. 20+ SEER is premium with highest savings.