Energy Prices by StateHome energy prices, read from the meter
Menu

Furnace size calculator

This furnace size calculator estimates your home's heat loss, then picks the furnace whose heat output is 100–140% of it, the range in ACCA Manual S. A 2,000 sq ft two-story home with average insulation at a 5°F design temperature loses about 32,656 BTU per hour, so it needs a 40,000 BTU input furnace at 95% AFUE.

By the Energy Prices by State team · Updated · 3 sources

Furnace size calculator

Heat loss · AFUE

Describe the house and the winter design temperature. The result is the furnace input rating printed on the nameplate.

Try:
sq ft
ft
%
°FThe 99% heating design temperature for your county (local code or Manual J tables), not the record low.
%On the yellow EnergyGuide label: 80% for a standard furnace, 90–98% for a condensing one.

Furnace size (input rating)

40,000 BTU/h

38,000 BTU/h of heat out at 95.0% AFUE

  • Heat loss at design temperature32,656 BTU/h
  • Output vs heat loss116% (Manual S range 100–140%)
  • Input range that fits34,374–48,124 BTU/h
  • Per sq ft16.3 BTU/h heat loss

At 5°F outside, this 2,000 sq ft home loses about 32,656 BTU per hour. A 40,000 BTU input furnace at 95.0% AFUE puts out 38,000 BTU per hour, 116% of the load, inside the 100–140% range. Simplified estimate, not a Manual J.

Not a Manual J. A contractor's Manual J works room by room with your county's design data and duct losses. Use this result to check a quote.

On this page
  1. How the furnace size calculator works
  2. Heat loss calculator: where the heat goes
  3. What size furnace do I need for my square footage?
  4. Climate changes the answer more than size
  5. Insulation and air sealing shrink the furnace
  6. Common sizing mistakes
  7. FAQ
  8. Sources

How the furnace size calculator works

A furnace is sized in two steps: first the house's heat loss on the design day, then the equipment whose output covers it without overshooting. The calculator does both:

  1. Heat loss. Walls, windows, ceiling, floor and air leakage, each as U × area × (70°F − design temperature), the same simplified model as the HVAC size calculator. ACCA's Manual J is the full version of this step.[acca]
  2. Output range. Manual S, ACCA's equipment selection standard, limits a gas or oil furnace's heat output to 100–140% of the Manual J heating load.[manuals][cb_manuals] For a 56,000 BTU/h load that means between 56,000 and 78,400 BTU/h of output.
  3. Input rating. Furnaces are sold by input (fuel burned). Output = input × AFUE, so the input range is the output range ÷ AFUE. The calculator picks the smallest common size (40k, 60k, 80k, 100k, 120k, 140k) whose output reaches the load and says whether it stays under 140%.

Heat loss calculator: where the heat goes

The first number the calculator shows is the house's heat loss on the design day, in BTU per hour. For the example 2,000 sq ft, two-story house with average insulation and double-pane windows at a 5°F design temperature, the heat loss calculator adds up to 32,656 BTU/h:[acca]

Heat loss by part of the house, 2,000 sq ft example at 5°F (U × area × ΔT; air leakage 0.018 × ACH × volume × ΔT)
PartBTU/hShare
Walls8,61926%
Windows9,55529%
Ceiling and attic2,1677%
Floor2,9559%
Air leakage9,36029%
Total heat loss32,656100%

The biggest shares are where insulation, window upgrades and air sealing pay off; the effect of each on the furnace size is in the insulation table below. For cooling as well as heating, run the same heat loss and heat gain model in the HVAC size calculator.

What size furnace do I need for my square footage?

Square footage alone does not set furnace size, but for the same construction it gives a starting point. With average insulation, double-pane windows, average air leakage and a 5°F design temperature:

Furnace size by floor area (average construction, 5°F design temperature)
HouseHeat loss BTU/hInput range at 95% AFUE95% AFUE size80% AFUE size
1,000 sq ft (1 story)18,88819,882–27,83540,00040,000
1,500 sq ft (1 story)26,94028,358–39,70140,00040,000
2,000 sq ft (2 story)32,65634,374–48,12440,00060,000
2,500 sq ft (2 story)39,48441,562–58,18760,00060,000
3,000 sq ft (2 story)46,19848,629–68,08160,00060,000
4,000 sq ft (2 story)59,38362,509–87,51280,00080,000

The 80% column is larger because a standard-efficiency furnace sends a fifth of its fuel's heat up the flue; for the same delivered heat it needs a bigger input rating. A rule of thumb like "40 BTU per square foot" would put the 2,000 sq ft house at 80,000 BTU, about twice what this example needs, which is how many older homes ended up with oversized furnaces.

Climate changes the answer more than size

The winter design temperature is the 99% value for your location: the temperature the outdoor air stays above for all but about 88 hours of the year, not the record low. The same 2,000 sq ft house at different design temperatures, 95% AFUE:

Same 2,000 sq ft house, different winter design temperatures
ClimateHeat loss BTU/hBTU/h per 1,000 sq ftFurnace inputOutput vs load
Mild (30°F design)20,09610,04840,000189%
Moderate (15°F)27,63213,81640,000138%
Cold (5°F)32,65616,32840,000116%
Very cold (−10°F)40,19120,09660,000142%
Extreme (−25°F)47,72723,86460,000119%

In mild climates even the smallest common furnace is well above 140% of the load. That is where a two-stage or modulating furnace, or a heat pump, fits better; the yearly running costs are compared on heat pump vs furnace cost.

Insulation and air sealing shrink the furnace

2,000 sq ft two-story house at 5°F: shell changes
House shellHeat loss BTU/hFurnace input at 95%
As described32,65640,000
Good insulation27,28840,000
Tight, air-sealed27,97640,000
Poor insulation, leaky56,69160,000

Sealing and insulating before replacing a furnace can drop it a size, which lowers the purchase price and the fuel bill. Today's state fuel prices are on natural gas prices, propane prices and heating oil prices.

Common sizing mistakes

  • Replacing like for like. The old furnace may have been oversized from the start, and the house has probably been tightened since.
  • Mixing input and output. A "100,000 BTU" furnace at 80% AFUE delivers 80,000 BTU/h. Compare output with heat loss.
  • Using the record low. Designing for the coldest day ever recorded oversizes the furnace for every other hour.
  • Ignoring airflow. The blower and ducts must move the air the furnace and the air conditioner need; Manual S also checks the cooling side.

Frequently asked questions

What size furnace do I need for a 2,000 sq ft house?

About 40,000 BTU input at 95% AFUE for the average-insulation example at 5°F (32,656 BTU/h heat loss); 60,000 BTU at 80% AFUE. Colder climates and leaky houses need more.

How many BTU furnace do I need per square foot?

There is no fixed number: in the tables above heat loss ranges from under 10 to over 25 BTU/h per sq ft depending on climate and construction. Calculate the heat loss instead.

How do I calculate heat loss for a house?

Add U × area × (indoor − outdoor design temperature) for walls, windows, ceiling and floor, plus air leakage. The example 2,000 sq ft house loses 32,656 BTU/h at 5°F; the calculator above does the sum for your house.

Is a bigger furnace better?

No. Manual S caps furnace output at 140% of the heating load; beyond that the furnace short-cycles, wastes fuel and heats unevenly.

Is the furnace size the input or the output?

Furnaces are labeled by input. Multiply by AFUE to get output: 60,000 BTU input at 95% AFUE is 57,000 BTU/h of heat.

Sources

  1. Manual J Residential Load Calculation, 8th Edition (ANSI/ACCA 2 Manual J - 2016) · Air Conditioning Contractors of America (ACCA), accessed Nov 14, 2026
  2. Manual S Residential Equipment Selection (ANSI/ACCA 3 Manual S): fossil-fuel furnace output 100% to 140% of the Manual J heating load · Air Conditioning Contractors of America (ACCA), accessed Nov 14, 2026
  3. An Introduction to ACCA Manual S (worked furnace example: 56,000 Btu/h load, furnace output no larger than 78,400 Btu/h) · Contracting Business, accessed Nov 14, 2026