Furnace Size Calculator

Furnace Size Calculator (BTU Estimator)

Zone 1 (Hot – Southern Florida, Texas) Zone 2 (Warm – Coastal SE, SoCal) Zone 3 (Moderate – Virginia, Missouri) Zone 4 (Cold – NY, Illinois, Colorado) Zone 5 (Very Cold – MN, ND, Canada)

Poor (Old windows, thin walls) Average (Standard modern home) Excellent (High efficiency, sealed)

Recommended Furnace Capacity

0 BTUs

(This is the estimated heating output required for your home)

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Understanding Furnace Sizing (BTUs)

Choosing the right furnace size is critical for home comfort and energy efficiency. Furnace capacity is measured in BTUs (British Thermal Units). A BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

The Importance of Proper Sizing

If a furnace is too small, it will run constantly during winter, failing to reach the thermostat setpoint and wearing out components prematurely. Conversely, an oversized furnace will "short cycle," turning on and off rapidly, which wastes fuel and creates uneven temperatures throughout the house.

Climate Zones Explained

The climate where you live significantly impacts how many BTUs per square foot you need:

• Zones 1 & 2: Requires roughly 30-35 BTUs per sq. ft. These areas rarely see freezing temperatures.
• Zone 3: Requires 40-45 BTUs per sq. ft. Typical for the middle of the United States.
• Zones 4 & 5: Requires 50-60 BTUs per sq. ft. Necessary for harsh northern winters where temperatures stay below zero for extended periods.

Calculation Example

If you have a 2,000 square foot home in Zone 4 with average insulation, the math works as follows:

2,000 sq. ft. × 45 BTU Factor = 90,000 BTUs

Note: Always check the "Output BTU" rating of a furnace rather than the "Input BTU." For example, a 100,000 BTU input furnace with 80% efficiency only provides 80,000 BTUs of actual heat.