Furnace Size Calculator: What Size Furnace Do I Need?

A furnace has one job: replace the heat your house loses on the coldest days. To size one, you need the heat loss at your winter design temperature, and then you pick a furnace whose output, input times efficiency, covers it without going much over. This calculator estimates the heat loss from your home’s size, insulation, windows and air leakage, using NOAA climate data for your state, and picks the matching input rating.

How it works

Heat loss is the house’s loss rate times the difference between 70 °F inside and the design temperature outside:

heat loss = (Σ U × area + F × perimeter + 1.08 × CFM) × (70 °F − design)
          walls, windows, ceiling   slab edge     air leakage
        + 15% if ducts run through unconditioned space
input     = heat loss ÷ AFUE
furnace   = smallest common input rating (40k–140k) whose output covers it

Design temperature. It’s the lowest hourly 10th-percentile temperature from December through February in NOAA’s 1991–2020 normals, at the station nearest a major city in your state. It’s close to the 99% design temperature HVAC designers use, which means it’s colder than all but a few dozen hours in a normal winter.

Insulation and windows set the U-values, the rate heat passes through each square foot for each degree of difference. Air leakage is often the biggest loss in older homes. Winter air leakage runs about 30% higher than summer because wind and the temperature difference push harder.

No credits. Heating loads ignore sun, people and appliances, because the coldest hours are usually at night.

AFUE is the share of the gas that becomes heat in the house. The calculator divides the heat loss by AFUE to get the input rating you’d see on the furnace’s data plate.

Worked example

A two-story, 2,400 sq ft house in Chicago has average insulation, double-pane windows and a drafty older envelope, with ducts in the basement and attic. NOAA normals at Midway give a design temperature of 3.9 °F, a 66 °F difference from indoors.

• Air leakage: about 20,800 BTU/h, the largest single loss
• Windows: 11,900. Walls: 9,800. Slab edge: 5,000. Ceiling: 2,600. Ducts: 7,500
• Total heat loss: about 57,700 BTU/h
• At 96% AFUE that needs 60,100 BTU/h of input. A 60,000 BTU furnace falls short by less than 0.2%, so the calculator picks an 80,000 BTU furnace, which delivers 76,800 BTU/h, 33% above the loss and within the 140% limit
• Air seal the same house to “tight” and the loss drops to about 42,300 BTU/h, which a 60,000 BTU furnace covers easily

In practice, a contractor would probably choose the 60,000 BTU unit and seal the house. The calculator’s result is the same tradeoff, done in numbers.

Furnace size by state: reference table

The same 2,000 sq ft home in every state at 96% AFUE.

Tips and common mistakes

Don’t replace like for like. Many older furnaces are oversized, and the house may have been insulated or re-windowed since. Calculate the load before you buy.

Check the blower too. The furnace’s blower has to move enough air for the air conditioner on the same ducts, about 400 CFM per ton. A smaller furnace may need a variable-speed blower to serve a large AC.

Condensing furnaces need a drain. High-efficiency furnaces make acidic condensate and vent through plastic pipe. Plan the drain and the vent route before you choose a location.

Size gas pipe and venting with the furnace. A larger input rating can need a larger gas line and a different vent. Those are code items for your installer.

Seal ducts in unheated spaces. Duct leaks in a crawlspace or attic waste heat before it reaches the rooms, which is why the calculator adds 15% when ducts run outside.