Furnace Size Calculator

Determine the right furnace size for your home in BTU output. Enter your home's square footage, climate zone, and insulation quality to calculate the heating capacity needed. A properly sized furnace heats efficiently without wasting fuel.

Enter Your Measurements

Home Square Footage(sq ft)

Climate (1=Mild, 2=Moderate, 3=Cold, 4=Very Cold)

Insulation Level (1=Poor, 2=Average, 3=Good)

Furnace Efficiency (AFUE)(%)

Results

Furnace Size (Input BTU)

100,000

BTU/hr

Required Heat Output

90,000

BTU/hr

Exact Input BTU Needed

94,737

BTU/hr

Home Heat Loss

90,000

BTU/hr

Estimated Therms/Season

947

therms

Formula

Heat Loss = Sq Ft × BTU per Sq Ft × Insulation Factor | Furnace Input = Heat Loss ÷ (Efficiency ÷ 100)

How to Use This Calculator

How to Use

1
Enter the total heated square footage of your home.
2
Select your climate zone: 1 for mild winters, 4 for very cold winters.
3
Rate your home's insulation level.
4
Enter the furnace efficiency (AFUE). Standard efficiency is 80%, high efficiency is 90-98%.

Frequently Asked Questions

What size furnace for a 2000 sq ft house?

In a moderate climate with average insulation, a 2000 sq ft home needs approximately 70,000-80,000 BTU input furnace at 95% efficiency. Cold climates may need 100,000 BTU or more.

What AFUE rating should I get?

High-efficiency furnaces (90-98% AFUE) cost more upfront but save 10-20% on fuel costs. In cold climates where the furnace runs often, a 95-96% AFUE furnace pays for itself in 5-8 years.

What is the difference between input and output BTU?

Input BTU is the total energy consumed. Output BTU is the actual heat delivered. A 100,000 BTU furnace at 95% efficiency delivers 95,000 BTU of heat, with 5,000 BTU lost up the exhaust.

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About This Calculator

This calculator uses the formula: Heat Loss = Sq Ft × BTU per Sq Ft × Insulation Factor | Furnace Input = Heat Loss ÷ (Efficiency ÷ 100). All calculations follow industry-standard methods. Results are estimates — always verify with a licensed professional for structural or code-compliant work.

Built and maintained by the CalcSmart team. Last updated March 2026.

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