Furnace Size Calculator

As a rule of thumb, homes need roughly 25 to 60 BTU of heating per square foot depending on climate. Multiply your heated square footage by a climate heating factor — about 25 BTU/sq ft in the warm South up to 55 in the cold far North — adjust for insulation and ceiling height, add for windows and a safety margin, then divide by the furnace’s AFUE to get the input rating to shop for.

For example, a 1,500 sq ft home in a moderate climate needs about 68,750 BTU of output, which at 95% AFUE points to a standard 80,000 BTU input furnace. Confirm with a Manual J load calculation before buying.

Input BTU is the fuel a furnace consumes per hour, and output BTU is the heat it actually delivers to your home. Output equals input times the AFUE efficiency, so an 80,000 BTU input furnace at 95% AFUE delivers about 76,000 BTU of usable heat.

Furnaces are named and sold by their input rating, but the output is what has to cover your home’s heating load. That’s why a lower-efficiency furnace needs a larger input to deliver the same heat as a high-efficiency one.

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel a furnace turns into usable heat over a season. A 95% AFUE furnace delivers 95 cents of heat for every dollar of fuel, wasting only 5%; an 80% AFUE unit wastes 20%.

Higher AFUE means lower fuel bills for the same heat, though condensing high-AFUE furnaces cost more up front and need a condensate drain. Modern gas furnaces typically run 90 to 98% AFUE, while older or oil units are often 80 to 87%.

An oversized furnace heats the house fast, then shuts off — cycling on and off repeatedly instead of running steadily. This short-cycling wastes fuel, wears out the ignitor and blower, and leaves uneven, drafty temperatures between rooms.

Bigger is not better. A right-sized furnace runs longer, gentler cycles that hold an even temperature and use less fuel, which is why the goal is to match the heating load with a modest margin rather than overshoot it.

It depends on climate. Warm Southern zones need about 25 to 30 BTU per square foot, moderate mid-U.S. zones about 35 to 40, and cold Northern zones 45 to 60 or more. Insulation, air sealing, ceiling height, and window area all shift that figure.

A leaky, single-pane home in a cold climate can need well over 60 BTU/sq ft, while a tight, well-insulated home in a mild climate might need under 25. The calculator applies these factors so you don’t have to guess.

The heating load — the output BTU your home needs — is the same regardless of fuel. Fuel type mainly changes the typical efficiency and the running cost. Natural gas and propane furnaces commonly run 90 to 98% AFUE, oil furnaces 80 to 87%, and electric furnaces about 100% at the unit.

Because output equals input times AFUE, a less efficient furnace needs a larger input rating to deliver the same heat. Electricity is nearly 100% efficient at the furnace but usually costs more per BTU than gas.

No. This is a quick rule-of-thumb estimate for planning and budgeting. A contractor’s ACCA Manual J load calculation is the industry-standard method that sizes real equipment, accounting for room-by-room detail, air leakage, orientation, and local design temperatures.

Use this tool to ballpark the furnace input, output, and cost, then have an HVAC professional run a Manual J — and a Manual S to select the specific furnace and match the blower and ducts — before you buy or install anything.

Installed cost scales with the furnace’s input size and efficiency, plus local labor and any venting or ductwork changes. A common planning figure is a few hundred dollars per 10,000 BTU of input, so an 80,000 BTU furnace often lands in the low-to-mid four figures before extras.

High-AFUE condensing furnaces cost more up front than standard-efficiency units but use less fuel over their life. Get itemized quotes from licensed contractors, since regional pricing and installation complexity vary widely.