Furnace sizing calculator
This furnace sizing calculator turns your heating load into the output range and the input BTU per hour size to look for.
Where the heat loss comes from
Input and output are not the same
A furnace has two sizes. The input is the fuel it burns each hour. The output is the heat that reaches the house. The model number, like 80,000, is usually the input. Your house needs output.
Why there is a range
Output below the load means the house cannot hold its temperature on the coldest night. Output far above the load means short cycles. Common sizes come in steps of 20,000 BTU/h of input, from 40,000 to 120,000, so we pick the smallest common size that lands in the range.
A two stage or modulating furnace can run low most of the time. That helps comfort when the size is on the large side.
Check the airflow too
A furnace needs enough air moving through the ducts. If the ducts are small, a bigger furnace will not help. The ductulator on this site sizes ducts from CFM, and the app shows the heating airflow for your house.
Get the full version in the app
The web page gives a fast estimate. The iPhone app does the whole job.
- Every window, wall and duct, not just six choices
- Load split by room with CFM and duct sizes
- A PDF report with your logo
- Works with no signal. Your jobs stay on your phone.
Questions people ask
What is AFUE?
AFUE is the share of the fuel that becomes heat for the house. A furnace with 80 percent AFUE sends 80 percent of its input into the house as heat. The rest goes up the flue.
What is the difference between input and output BTU/h?
Input is the fuel the furnace burns, and it is the number on the model name, like 80,000. Output is the heat it gives the house. Output equals input times AFUE. The load is matched to the output.
Why not buy a furnace that is much bigger than the load?
A big furnace heats the house fast, then shuts off. It cycles on and off, wastes fuel, makes uneven heat and wears parts faster. A common limit is 140 percent of the load.
Does the cooling size change the furnace size?
The air handler and ducts are shared. A big AC with small ducts can force a bigger fan. Size the furnace from the heating load and then check that the airflow works for both.
More free tools
An estimate in the style of ACCA Manual J. It does not replace a certified room by room design. Manual J is a trademark of ACCA. HVAC Load is not affiliated with ACCA.