How many square feet will an 80,000 BTU furnace heat?

Bryant gives about 1,600–2,400 sq ft depending on climate and insulation. In the calculator an 80,000 BTU furnace at 95% AFUE delivers 76,000 BTU, enough for 1,500 sq ft in a moderate climate or about 1,300 sq ft in a cold one.

How many square feet will a 60,000 BTU furnace heat?

About 1,200–2,000 sq ft (Bryant, Carrier). At 95% AFUE it delivers 57,000 BTU, which covers 1,250 sq ft at 45 BTU per square foot or 1,700 sq ft in a warm climate at 32.

What size furnace do I need for a 1,500 sq ft house?

In a moderate climate with average insulation the home needs 67,500 BTU of heat. That is an 80,000 BTU furnace at 90–97% AFUE or a 100,000 BTU furnace at 80% AFUE. In a warm climate the same house drops to a 60,000 BTU unit.

What is the difference between furnace input and output BTU?

Input is the fuel the furnace burns, the number on the label. Output is the heat that reaches the ducts: input × AFUE. A 100,000 BTU furnace at 80% AFUE delivers 80,000 BTU; the same label at 96% delivers 96,000 (Lennox).

How do I know if a furnace is oversized?

Convert the nameplate to output and compare it with the home's need; the calculator does this in Check the existing furnace mode. Short cycling, uneven rooms, and a furnace that shuts off minutes after it starts are the field signs (Carrier). More than 25% over the need is oversized.

When do I need a Manual J instead of the chart?

For a replacement with ductwork changes, a permit that asks for it, an addition, or a house with large glass. ACCA Manual J is the ANSI-recognized standard and is required by most jurisdictions. The chart is for a first-visit number.

Free Furnace Sizing Calculator | Tofu Field Service

Tofu

Free calculators

Furnace sizing calculator

Find the heat a home needs, the furnace size that delivers it at your AFUE, and whether the old furnace is oversized.

4.8

How to size a furnace

Heat output needed = home area × BTU per sq ft for the climate × ceiling factor

A 1,500 sq ft home in a moderate climate needs 67,500 BTU of heat. At 95% AFUE that is an 80,000 BTU furnace; at 80% AFUE, a 100,000 BTU one.

Sizing

Home sq ft

AFUE of the new furnace

Ceiling

Your numbers

Insulation BTU per sq ft instead of the chart Extra margin on the load %

Your load Bryant chart

Furnace size

80,000 BTU

Nameplate output76,000 BTU

Headroom+13%

Per sq ft45 BTU

Quote this furnace Send it with Tofu – free

Furnace size chart by square feet and climate

Bryant sizes by climate: 30–35 BTU per square foot in the warm South and Southwest, 40–50 in the moderate Midwest and Mid-Atlantic, 50–60 in the cold Northeast and northern Midwest. HomeGuide uses the same 30–60 BTU band. The chart gives the heat a home needs, not the number on the furnace label.

Home size	Warm	Moderate	Cold
1,000 sq ft	30,000 – 35,000 BTU	40,000 – 50,000 BTU	50,000 – 60,000 BTU
1,500 sq ft	45,000 – 52,500 BTU	60,000 – 75,000 BTU	75,000 – 90,000 BTU
2,000 sq ft	60,000 – 70,000 BTU	80,000 – 100,000 BTU	100,000 – 120,000 BTU
2,500 sq ft	75,500 – 87,500 BTU	100,000 – 125,000 BTU	125,000 – 150,000 BTU

Source: Bryant. The calculator takes the low end of each range for a well-insulated home, the middle for average, and the top for poor insulation and drafty windows.

Input, output and AFUE

The chart above is heat output. The furnace label shows input, the fuel it burns. AFUE is the share that becomes heat: a standard furnace delivers about 80%, high-efficiency models 90% and up, the best 98–99% (Carrier, Lennox). ENERGY STAR requires 95% AFUE in the South and 97% in the North. The same 67,500 BTU of heat needs a different label at each efficiency.

AFUE	Input needed for 67,500 BTU of heat	Standard furnace
80%	84,375 BTU	100,000 BTU
90%	75,000 BTU	80,000 BTU
95%	71,053 BTU	80,000 BTU
97%	69,588 BTU	80,000 BTU

Quote the 80% unit without this step and you under-deliver by a full size.

What changes the number

Ceilings above 8 ft add air to heat: Bryant notes sizing is about volume, not floor area, so the calculator scales by height. Poor insulation, drafty windows, and air leaks push a home to the top of its climate range. Beyond that the chart stops being reliable. ACCA Manual J is the ANSI-recognized standard for sizing loads and is required by most building codes; Carrier sends every final size through it. Use the chart for a first-visit number, Manual J for the contract.

Is the old furnace oversized

Type the nameplate input and AFUE into Check the existing furnace mode: the calculator converts the label to output and compares it with the need. More than 25% over is oversized. Carrier describes what that does: the furnace “heats spaces too quickly, causing it to cycle on and off frequently”, with uneven temperatures and extra wear. An undersized one runs non-stop and never catches up.

What a furnace install costs

HomeGuide prices installed gas furnaces by size and by efficiency, not counting ductwork.

Furnace size	Home size	Published installed cost
40,000 – 50,000 BTU	up to 1,200 sq ft	$3,800 – $6,700
50,000 – 60,000 BTU	1,200 – 1,600 sq ft	$4,200 – $7,200
60,000 – 70,000 BTU	1,600 – 2,000 sq ft	$4,700 – $8,000
70,000 – 90,000 BTU	2,000 – 2,500 sq ft	$5,100 – $9,500
90,000 – 120,000 BTU	2,500 – 3,000+ sq ft	$5,800 – $12,000

By efficiency: 80–89% AFUE $3,800–$6,200, 90–95% $5,200–$8,800, 96% and up $7,500–$12,000. Removing the old furnace adds $150–$400 when it is not included, duct sealing $300–$1,500, and a full duct replacement $1,400–$5,600 or more (HomeGuide).

From size to quote

Here is the default 80,000 BTU replacement, as the customer sees it:

Line item	Price
80,000 BTU gas furnace, 95% AFUE – equipment	$3,000
Installation labor – replace in place	$1,800
Venting and materials	$600
Total	$5,400

$5,400 sits inside HomeGuide’s $5,100–$9,500 band for a 70,000–90,000 BTU install. Tofu Field Service keeps the size, the quote, and the payment on one job.

Furnace replacement estimate in Tofu with equipment, labor and venting lines

Check out more free resources

🌡️

HVAC software

Schedule service calls, quote installs, and invoice from the truck.

See the software

→

📝

HVAC estimate templates

Free estimate templates for installs and repairs.

Get the templates

→

🧾

HVAC invoice templates

Free invoice templates with equipment and labor lines.

Get the templates

→

❄️

Mini split sizing calculator

Size one zone or a multi-zone system in BTU.

Open the calculator

→

Furnace sizing FAQ

Size the furnace at the kitchen table. Send the quote before you leave.

Try Tofu for free