Furnace Size Calculator
Estimate furnace BTU size from floor area, ceiling height, climate, insulation, and furnace efficiency.
Results
This is a quick sizing estimate, not a Manual J load calculation. Confirm equipment sizing against project plans, local conditions, and an HVAC professional when accuracy matters.
How to use this calculator
- 01Enter the heated area using room dimensions or known floor area.
- 02Enter the ceiling height, climate, insulation, and furnace efficiency.
- 03Choose the space type.
- 04Read the heating load, furnace input BTU, recommended range, and nominal furnace size.
If you only need a room-level heating or cooling BTU estimate, use the BTU Calculator.
For blower airflow or duct velocity checks, use the CFM Calculator.
Understanding the math
The estimate starts with a climate-based area load rate. It adjusts for ceiling height, insulation, and space type, then divides the heating load by AFUE to estimate furnace input BTU.
heating load = area x load rate x ceiling x insulation x space type furnace input BTU = heating load / AFUE nominal furnace size = next common furnace input size
This is a quick sizing estimate, not a Manual J load calculation.
For a whole-home heating and cooling load estimate with Manual J-style inputs, use the HVAC Load Calculator.
If you want a heating-only envelope check from temperature difference, glazing, and air leakage, use the Heat Loss Calculator.
Furnace sizing quick reference
Use these ranges as a rough check. Real furnace sizes can move outside the table when insulation, air leakage, ceiling height, or local weather is unusual.
| Space | Area | Furnace input |
|---|---|---|
| Small home / zone | 800–1,200 sq ft | 40,000–60,000 BTU/hr |
| Average home | 1,500–2,200 sq ft | 60,000–90,000 BTU/hr |
| Large home | 2,400–3,200 sq ft | 90,000–120,000 BTU/hr |
| Garage / workshop | 400–900 sq ft | 40,000–80,000 BTU/hr |
| Basement / addition | 500–1,200 sq ft | 40,000–70,000 BTU/hr |
Frequently asked questions
How do I calculate furnace size?
Start with floor area, then adjust for ceiling height, climate, insulation, space type, and AFUE. The calculator estimates heating load first, then divides by furnace efficiency to estimate input BTU.
What size furnace do I need for a 2,000 sq ft house?
A 2,000 sq ft house often lands around 60,000 to 100,000 BTU/hr of furnace input, depending on climate, insulation, ceiling height, and AFUE. Cold climates and leaky homes push the size higher.
How many square feet will an 80,000 BTU furnace heat?
It depends on output BTU, AFUE, insulation, and climate. An 80,000 BTU furnace might cover roughly 1,500 to 2,500 sq ft in many homes, but poor insulation or very cold weather can reduce that range.
What is the difference between furnace input BTU and output BTU?
Input BTU is the fuel or electrical heat entering the furnace. Output BTU is usable heat after efficiency losses. For example, an 80,000 BTU furnace at 80% AFUE delivers about 64,000 BTU/hr of output heat.
Should I size a furnace by square footage only?
No. Square footage is only a starting point. Ceiling height, insulation, climate, and air leakage matter. For final equipment sizing, use a Manual J load calculation or an HVAC professional.
Can I use this calculator with metric measurements?
Yes. Use the Imperial / Metric toggle. Inputs, results, and reference values follow the active unit system where applicable.