Furnace Size Calculator
Estimate furnace BTU input from heating load and efficiency.
This page is an editable estimator with realistic defaults so you can model your own situation in minutes.
Furnace input BTU = heating load divided by furnace efficiency.
Treat the result as a planning estimate and verify important numbers with the relevant official source before acting.
| Area | 1800 sq ft |
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Assumptions
- This is a rough sizing estimate, not a Manual J load calculation.
- Insulation, windows, air leakage, climate, orientation, occupancy, and duct design can change HVAC sizing.
- Confirm HVAC sizing with a qualified contractor.
How this calculator works
Formula used
Furnace input BTU = heating load divided by furnace efficiency.
Example calculation
Plug in your own numbers and the result updates instantly. Furnace input BTU = heating load divided by furnace efficiency.
Furnace Size Calculator FAQ
Is this furnace size calculator exact?
No. It is an estimating tool only. Actual quantities can change with field measurements, waste, layout, product dimensions, building codes, soil conditions, and installer methods.
Should I add extra material?
Usually yes. Many projects use a waste factor for cuts, breakage, overlap, spillage, grading, or mistakes. The best waste factor depends on the material and layout.
Should I verify before ordering?
Yes. Confirm quantities with your contractor, supplier, plans, local code requirements, and product installation instructions before purchasing materials.