Furnace Size Calculator

Please fill in the required fields with values greater than 0.
–heating needed (BTU/hr)
–furnace input needed (BTU/hr)
–nearest standard input size
–heating per sq ft (BTU)

Rule-of-thumb estimate. A proper Manual J load calculation by an HVAC contractor gives the exact size. An oversized furnace wastes energy.

How to use this calculator

Enter the area of the home you want to heat, choose your climate, the insulation level and the efficiency (AFUE) of the furnace. Press Calculate to see the heating load and the furnace size to look for.

The formula

Heating needed = area × BTU per sq ft for your climate × insulation factor
Furnace input = heating needed ÷ AFUE

Example: A 2,000 sq ft home in a moderate climate with average insulation needs about 90,000 BTU/hr of heat. A 95% efficient furnace needs an input of about 94,737 BTU/hr, so look for a 100,000 BTU/hr furnace.

Understanding your furnace size estimate

This calculator gives two numbers: the heat your home needs and the furnace input rating that delivers it. Furnaces are labeled by input BTU, but some of that energy goes up the vent. The efficiency rating (AFUE) tells you how much of the input becomes useful heat.

Output vs. input

The “heating needed” figure is the heat your home loses on a cold day. The “furnace input” figure is that number divided by efficiency. For example, a 1,500 sq ft home in a cold climate with average insulation needs about 82,500 BTU/hr. With a 95% AFUE furnace, the input needed is about 86,842 BTU/hr. With an 80% furnace, it rises to about 103,125 BTU/hr. The home needs the same heat either way, but the less efficient furnace burns more fuel to deliver it.

Main factors that change the size

  • Climate: Colder winters raise the BTU needed per square foot.
  • Insulation and air sealing: A tight, well-insulated home can need a much smaller furnace.
  • Windows: Large areas of older single-pane glass lose a lot of heat.
  • Ceiling height and layout: Tall ceilings and open two-story spaces add volume to heat.
  • Ductwork: Leaky ducts in an unheated attic or crawl space waste heat before it reaches the rooms.

Tips and common mistakes

  • Do not simply copy the size of your old furnace. Many older homes have oversized units, and insulation or window upgrades may have lowered the load.
  • Do not oversize “to be safe.” An oversized furnace short-cycles, which can mean uneven temperatures and more wear.
  • Count only the heated area. Leave out unheated garages and attics.
  • Make sure the furnace blower can move enough air for any central air conditioner sharing the ducts.

Buying and planning advice

Use this estimate as a starting point to judge contractor quotes. Ask each contractor to do a Manual J load calculation, which accounts for your windows, insulation, air leakage and local weather. High-efficiency furnaces cost more up front but use less fuel. Check whether your utility offers rebates. Gas lines, venting and electrical work must meet local building codes, so have a licensed HVAC contractor size and install the furnace. Install carbon monoxide detectors in any home with fuel-burning appliances.

Frequently asked questions

What size furnace do I need for my house?
Multiply the area by the BTU per square foot for your climate, then divide by the furnace efficiency.
How many BTU per square foot do I need for heating?
About 30 to 35 in warm climates, 40 to 45 in moderate climates and 50 to 60 in cold climates.
What is AFUE?
AFUE shows how much of the fuel becomes heat. A 95 percent furnace turns 95 percent of the fuel into heat.
Is a bigger furnace better?
No. An oversized furnace short-cycles, wastes energy and heats unevenly.
Should I get a professional load calculation?
Yes. A Manual J calculation looks at windows, insulation, air leakage and local weather for the most accurate size.
How long does a furnace last?
Many gas furnaces last about 15 to 20 years with regular maintenance, though some last longer. Signs a furnace may be near the end include rising repair costs, uneven heating, frequent cycling and higher energy bills. Annual tune-ups and regular filter changes help a furnace run safely and last longer.
What is the difference between a single-stage and two-stage furnace?
A single-stage furnace runs at full output every time it turns on. A two-stage furnace has a low setting for mild days and a high setting for cold days. Running on low most of the time gives steadier temperatures, quieter operation and fewer on-off cycles. Two-stage and modulating furnaces usually cost more to buy.
How often should I change my furnace filter?
Basic 1-inch filters are typically changed every 1 to 3 months. Thicker media filters often last 6 to 12 months. Change them more often if you have pets, allergies or a lot of dust. A clogged filter restricts airflow, makes the furnace work harder and can cause it to overheat and shut off.