Furnace size is measured in BTU per hour
A furnace's size is its heating capacity in BTU per hour. Residential gas furnaces are commonly made in steps of 20,000 BTU/hr, from about 40,000 to 120,000. The right size is the smallest one that keeps your house warm on a design cold day, the cold temperature your area reaches in a typical winter.
A bigger furnace does not make a house warmer. It heats in short bursts, shuts off, and starts again a few minutes later. This short-cycling wastes fuel, wears parts and leaves rooms uneven.
The quick estimate: BTU per square foot
For a first estimate, multiply the heated floor area by a figure for your climate. These are rules of thumb, assuming 8 ft ceilings and average insulation. They err on the generous side, and a Manual J calculation for a newer or well-sealed house often comes in well below them, so treat the table as an upper ballpark, not a size to buy.
| Climate | BTU/hr per sq ft | 1,500 sq ft | 2,000 sq ft | 2,500 sq ft |
|---|---|---|---|---|
| Hot | 30 | 45,000 | 60,000 | 75,000 |
| Warm | 35 | 52,500 | 70,000 | 87,500 |
| Moderate | 40 | 60,000 | 80,000 | 100,000 |
| Cold | 50 | 75,000 | 100,000 | 125,000 |
| Very cold | 60 | 90,000 | 120,000 | 150,000 |
Adjust for your house. Ceilings higher than 8 ft add volume: multiply by the height divided by 8. A 2,000 sq ft house with 9 ft ceilings in a moderate climate comes to 2,000 × 40 × 9 ÷ 8 = 90,000 BTU/hr. A tight, well-insulated house may need 15 percent less than the table; an older, drafty one may need 30 percent more. The heating BTU calculator applies these adjustments for you.
Input vs. output: read the label carefully
Gas furnaces carry two numbers. The input rating is how much fuel energy they burn per hour. The output (heating capacity) is how much heat reaches your ducts. The ratio between them is the efficiency, shown as AFUE.
output BTU/hr = input BTU/hr × AFUE
| Input rating | Output at 80% AFUE | Output at 96% AFUE |
|---|---|---|
| 60,000 | 48,000 | 57,600 |
| 80,000 | 64,000 | 76,800 |
| 100,000 | 80,000 | 96,000 |
Compare your heat-loss estimate with the output, not the input. An 80,000 BTU furnace at 80 percent efficiency delivers only 64,000 BTU/hr of heat.
Worked example
A 2,000 sq ft house with 8 ft ceilings and average insulation in a moderate climate: 2,000 × 40 = 80,000 BTU/hr needed.
- A 96 percent AFUE furnace needs an input of at least 80,000 ÷ 0.96 = 83,333 BTU/hr to deliver that.
- An older 80 percent furnace would need an input of 80,000 ÷ 0.8 = 100,000 BTU/hr.
Before you buy based on this, remember the estimate is rough. Many existing furnaces turn out to be oversized when a proper calculation is done, especially after windows, insulation or air sealing have been upgraded.
Why installers should do a Manual J
In the US, the industry standard for sizing is ACCA Manual J, a room-by-room heat-loss calculation that uses your local design temperature, wall and attic insulation, window area and type, air leakage and duct location. Canada and Australia have their own equivalent methods. A good installer measures the house rather than copying the size of the old furnace. Ask to see the calculation.
The duct system matters too. A furnace can only deliver what the ducts can carry, so a larger blower on small ducts can be noisy and inefficient. Manual D is the companion method for ducts.
Signs your current furnace is the wrong size
- It runs for only a few minutes at a time, even on cold days. Likely oversized.
- It runs nonstop on the coldest nights and the house still falls below the setpoint. Possibly undersized, or the ducts, filter or house leaks are the real problem.
- Some rooms are much colder than others. Often a duct or air-sealing issue rather than furnace size.
Cut the load before you size
If you plan insulation or window upgrades, do them first. Bringing an attic up to a modern R-value and sealing leaks can lower the heat loss enough to allow a smaller, cheaper furnace. See the insulation calculator for depths and bag counts.
Safety
Gas furnaces must be installed, vented and inspected by qualified technicians, with permits where local code requires them. A cracked heat exchanger or blocked flue can leak carbon monoxide, so put CO alarms on every level of the home and have the furnace serviced before each heating season.
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
As a rough rule of thumb, 60,000 to 120,000 BTU/hr of output depending on climate: about 80,000 in a moderate climate and 100,000 in a cold one with average insulation. A Manual J calculation gives the real figure.
Is a bigger furnace better?
No. An oversized furnace short-cycles, which wastes fuel, wears components faster and heats unevenly. Choose the smallest size that meets the calculated heat loss.
What does AFUE mean?
Annual fuel utilization efficiency, the share of the fuel's energy that becomes heat for the home. A 96 percent AFUE furnace turns 96 of every 100 BTU of gas into usable heat.
Do the math for your own project.
Open the btu calculator