Furnace Size Calculator
Calculate recommended furnace BTU capacity by climate zone, room area, insulation, and sunlight exposure.
What is a Furnace Size Calculator?
A furnace size calculator estimates the heating capacity (BTU/h) needed for a room or home based on climate zone, floor area, insulation quality, sunlight exposure, and furnace efficiency. Choosing the right furnace size prevents wasted energy from an oversized unit or insufficient heating from an undersized one.
How to Use This Calculator
- Select your US climate zone (1 through 9).
- Enter the room or home floor area in square feet.
- Choose the insulation level and sunlight exposure.
- Enter the furnace efficiency percentage.
- Read the recommended BTU/h furnace rating range.
Furnace Sizing Formula
The recommended furnace capacity is calculated as:
$$\text{Size (BTU/h)} = \frac{\text{BTU}_{\text{rec}} \times A \times F_s \times F_i}{e}$$
Where:
- $\text{BTU}_{\text{rec}}$ is the recommended BTU/h per square foot for your climate zone
- $A$ is the floor area in square feet
- $F_s$ is the sunlight factor (0.9 sunny, 1.0 neutral, 1.1 shaded)
- $F_i$ is the insulation factor (0.9 heavy, 1.0 average, 1.1 light)
- $e$ is the furnace efficiency as a decimal (e.g. 0.90 for 90%)
For HVAC planning, also see the Square Footage Calculator.
Frequently Asked Questions
What is BTU in heating?
BTU stands for British Thermal Unit. One BTU is the heat needed to raise one pound of water by 1°F. Furnace capacity is rated in BTU per hour (BTU/h), indicating how much heat the unit can produce each hour.
How do I choose a furnace for my home?
Match furnace BTU/h to your climate zone and total heated floor area. Colder zones need more BTU per square foot. Add capacity for poor insulation and shaded rooms. Subtract for heavy insulation and sunny exposures. Account for furnace efficiency when comparing models.
Does furnace efficiency affect sizing?
Yes. A 90% efficient furnace delivers only 90% of its rated BTU as usable heat. You need a higher-rated furnace to deliver the same output as a more efficient model. Divide the required output by efficiency to get the furnace rating you should buy.
What climate zone am I in?
The US is divided into climate zones 1 through 9 based on average winter temperatures. Zone 1 covers the warmest southern areas (21°C and above). Zone 9 covers the coldest northern regions (-25°C and below). Check your local energy code or climate zone map for your exact zone.
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