Furnace Size Calculator

Enter Calculation

Home Area (sq ft)

Climate Zone Zone 1 — Hot (Southern US) 30-35 BTU Zone 2 — Warm 35-40 BTU Zone 3 — Moderate 40-45 BTU Zone 4 — Cool 45-50 BTU Zone 5 — Cold 50-60 BTU Zone 6/7 — Very Cold (Northern US/Canada) 60 BTU

Formula

Show calculation steps (1)

1. Equivalent Heating Tons

   

   Rough heat-equivalent in tons = BTU divided by 12000

Results

Recommended Furnace Output

60,000

BTU per hour

Home area	1,500 sq ft
Heating factor	40 BTU/sq ft
Heat equivalent	5 tons

What Is a Furnace Size Calculator?

This tool estimates the heating capacity, measured in BTUs (British Thermal Units) per hour, that a furnace needs to comfortably heat your home. It is designed for homes in the US and Canada, where furnace ratings and climate-zone heating factors are commonly expressed in BTU per square foot. An undersized furnace runs constantly and never reaches temperature, while an oversized unit short-cycles, wastes fuel, and wears out faster — so getting the size right matters.

How to Use It

Enter your home's heated floor area in square feet, then pick the climate zone that best matches where you live. Colder zones require a higher BTU-per-square-foot factor. The calculator multiplies the two values to give a recommended furnace output, plus a rough heat equivalent in tons.

The Formula Explained

The core equation is $$\text{BTU} = \text{Area} \times \text{Heating Factor}$$. The heating factor ranges from about 30 BTU/sq ft in hot southern climates up to 60 BTU/sq ft in very cold northern climates. These factors assume average insulation, standard 8-foot ceilings, and typical window area. Homes with poor insulation, high ceilings, or many windows should use the higher end of a zone.

Heating factor rises with colder climate zones (BTU per square foot increases northward).

Worked Example

A 2,000 sq ft home in a cold climate (factor 50 BTU/sq ft) needs: $$2{,}000 \times 50 = 100{,}000 \text{ BTU/hr}$$ That equals roughly \(100{,}000 \div 12{,}000 \approx 8.3\) tons of heat output. You would shop for a furnace rated near 100,000 BTU output.

Multiply home area by the climate heating factor to estimate required furnace BTU.

FAQ

Is this BTU input or output? It is recommended output (heating) capacity. Furnace nameplates often list input BTU; divide output by the efficiency (AFUE) to find input — e.g. 100,000 BTU output at 95% AFUE needs ~105,000 BTU input.

Should I round up to the next furnace size? Yes, round up to the nearest commonly available size, but avoid going far beyond your estimate to prevent short-cycling.

Does this replace a Manual J load calculation? No. This is a quick estimate. For a final purchase, have an HVAC pro perform an ACCA Manual J calculation that accounts for insulation, windows, infiltration, and orientation.