Furnace Size Calculator — BTU Heating Load
Find the right furnace size for your home: enter the heated floor area, your climate, ceiling height, insulation quality, and the furnace's AFUE rating to get the recommended output and input BTU/hr.
sq ft
Climate zone
ft
Insulation quality
%
Heat delivered to the home — compare to furnace output rating
- 1
Climate factor
45BTU/hr per sq ft for your climate zone (ASHRAE rule of thumb). - 2
Height adjustment
8 ft ÷ 8 ft = 1Scales the load proportionally to ceiling height relative to the 8 ft standard. - 3
Insulation multiplier
1Poor = 1.25× • Average = 1.0× • Good = 0.85× - 4
Heat output needed
1,500 ft² × 45 × 1 × 1 = 67,500 - 5
Recommended furnace output
68,000Rounded to the nearest 1,000 BTU/hr — furnaces are sold in discrete sizes.
How does this calculator work?
Multiply your floor area by a climate factor (25–65 BTU/sq ft), adjust for ceiling height and insulation, and that gives the output BTU/hr your furnace must deliver. Divide by the AFUE fraction to get the input rating to compare with product specs. These are estimates — a Manual J calculation gives exact sizing.
Formula
How this is calculated
Furnace sizing starts from the heat load — the rate at which your home loses heat on the coldest day of the year, measured in BTU/hr. The classic rule of thumb multiplies heated floor area by a climate factor, typically 25 BTU/sq ft for hot climates up to 65 BTU/sq ft for very cold ones. These figures come from ASHRAE Manual J load calculations simplified for quick estimates at a standard 8-foot ceiling.
Two adjustments refine the estimate: ceiling height scales the load proportionally (a 10-foot ceiling has 25% more air volume than an 8-foot one), and insulation quality changes how fast heat leaks through walls and windows — poor insulation adds roughly 25% to the load, while a well-insulated modern build reduces it by about 15%.
The AFUE (Annual Fuel Utilization Efficiency) rating converts the required heat output into the furnace's rated input capacity. A 95 AFUE furnace delivers 95% of the fuel energy as heat, so you need slightly more input than output. Furnace manufacturers list their equipment by input BTU/hr, so divide output by AFUE to compare with product ratings. These are rule-of-thumb estimates for initial sizing — a licensed HVAC contractor performing a full Manual J calculation will give a more accurate result.
Frequently asked questions
AFUE (Annual Fuel Utilization Efficiency) measures what percentage of the fuel's energy a furnace converts into usable heat. A 95 AFUE furnace wastes only 5% up the flue. Higher AFUE means lower operating costs but usually a higher purchase price. Modern high-efficiency furnaces run 90–98% AFUE; older units can be as low as 60–70%.
Oversizing a furnace causes short-cycling — the unit heats the space too quickly, shuts off, then restarts repeatedly. This wastes energy, reduces comfort, and shortens equipment life. Aim to match the load estimate; going 10–15% larger for extreme cold snaps is acceptable, but do not double the size "just in case".
This tool gives a useful first estimate for comparing options and understanding the range of sizes you need. For a final purchasing decision, have a licensed HVAC professional perform a full Manual J heat-load calculation, which accounts for window area, orientation, local design temperatures, duct losses, and other factors.
Also known as
TG we-Calculate Editorial Team. (2026). Furnace Size Calculator — BTU Heating Load [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/furnace-size-calculator
TG we-Calculate Editorial Team. "Furnace Size Calculator — BTU Heating Load." TG we-Calculate. 2026. https://we-calculate.com/calculator/furnace-size-calculator.
TG we-Calculate Editorial Team, "Furnace Size Calculator — BTU Heating Load," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/furnace-size-calculator
@misc{wecalculate_furnace_size_calculator, title = {Furnace Size Calculator — BTU Heating Load}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/furnace-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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