Furnace Size Calculator
Estimate the furnace BTU output and input a house needs from its size, climate zone, insulation and the furnace's AFUE, as a rule of thumb before a load calculation. It gives the heating load in BTU per hour with the range for your climate zone, the input rating to look for and the nearest size as sold.
Your numbers
Every heated floor, finished basement included.
Not sure? The DOE and PNNL county guide in the sources lists every county's zone.
On the furnace's rating plate or spec sheet. 80% is the federal minimum for indoor (non-weatherized) gas furnaces made since 2015.
Heating load estimate (output)
–
- Range for zone 4
- –
- Furnace input at 80% AFUE
- –
- Nearest size as sold
- –
- BTU per square foot used
- –
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How the furnace size estimate works
Multiply the heated floor area by the BTU per square foot for the climate zone, choosing the low end of the range for a tight, well insulated house, the middle for an average one and the high end for a leaky or older one. That is the heating load: the furnace output the house needs on a cold day. Divide it by the AFUE to get the input rating, then pick the smallest furnace as sold whose output covers the load.
Worked example: 2,000 sq ft in zone 4, average insulation, 8 ft ceilings. The range is 40 to 50 BTU per square foot, so the middle is 45: 2,000 × 45 = 90,000 BTU per hour of output, within a range of 80,000 to 100,000. At 80% AFUE, 90,000 ÷ 0.80 = 112,500 BTU of input. A 100,000 input furnace puts out 80,000, too little; a 120,000 input furnace puts out 96,000, which covers it.
Ceiling height: the sources do not say what ceiling their figures assume, so the calculator takes them as 8 ft and scales the load by height ÷ 8. A 9 ft ceiling adds 12.5%.
BTU per square foot by climate zone
Bryant's figures by climate band, matched to the IECC climate zones by this calculator. Zone 5 takes in Chicago (Midwest, Bryant's moderate band) and Boston (Northeast, the cold band), so it uses both: 40 to 60. The example places come from the DOE Building America county table.
| Zone | For example | BTU per sq ft | 2,000 sq ft |
|---|---|---|---|
| 1 | Miami | 30 to 35 | 60,000 to 70,000 |
| 2 | Houston, Phoenix | 30 to 35 | 60,000 to 70,000 |
| 3 | Dallas, Atlanta, Las Vegas | 30 to 35 | 60,000 to 70,000 |
| 4 | Washington DC, New York City, St. Louis, Seattle | 40 to 50 | 80,000 to 100,000 |
| 5 | Chicago, Boston, Denver | 40 to 60 | 80,000 to 120,000 |
| 6 | Minneapolis, Burlington VT | 50 to 60 | 100,000 to 120,000 |
| 7 | Fargo, Duluth, northern Maine | 50 to 60 | 100,000 to 120,000 |
Input, output and AFUE
A furnace is sold by its input rating, the fuel it burns per hour. What heats the house is the output, input × AFUE. Bryant puts it simply: a furnace with 80% AFUE delivers 80% of its input BTUs as heat. Goodman's GM9S80 spec sheet lists the 80,000 BTU input models at 64,000 BTU of natural gas output.
Federal rules (10 CFR 430.32) set the minimum AFUE for non-weatherized gas furnaces at 80% for those made from 19 November 2015 and 95% for those made from 18 December 2028. Type the AFUE of the furnace you are looking at; the input needed falls as the AFUE rises.
Furnace sizes as sold
Gas furnaces are sold in steps of input. Goodman's GM9S80 line runs 40,000, 60,000, 80,000, 100,000 and 120,000 BTU per hour of input, and Bryant says most residential furnaces fall between 40,000 and 120,000 or more. The same sheet's model code lists 80, 92, 96 and 97% AFUE lines; the table shows the output at 80% and 96%. Other makers and lines have other steps; the calculator picks from these five.
| Input | Output at 80% | Output at 96% |
|---|---|---|
| 40,000 | 32,000 | 38,400 |
| 60,000 | 48,000 | 57,600 |
| 80,000 | 64,000 | 76,800 |
| 100,000 | 80,000 | 96,000 |
| 120,000 | 96,000 | 115,200 |
Why bigger is not better
An oversized furnace heats the air fast, shuts off, and starts again: short cycling. Bryant lists the result as uneven temperatures and higher energy bills. The Department of Energy's building science pages say rules of thumb too often lead to oversized systems, with more cost, wasted energy and too-frequent on-and-off cycling.
That is why the calculator shows the range for the zone and flags a size whose output is above the top of it. If the only size that covers the estimate is well above it, that is a question for the load calculation, not a reason to round up.
Where Manual J comes in
BTU per square foot cannot see the windows, the duct runs, the attic or the local design temperature. ANSI/ACCA Manual J can: it is the standard method for residential heating and cooling loads, worked room by room, and Manual S then selects the equipment. This page is not a substitute for that calculation. Use it to get a first number, to check a quote that looks far off, and to know what to ask the contractor.
For HVAC reps: what to write down on the consult
The size comes from the load calculation. The sale comes from what the homeowner tells you: which rooms are cold, what the winter bills look like, how old the furnace is, whether they want a higher AFUE and who else decides. Write it down while you are in the house, and read it back before you send the quote.
Sources, each read on 2026-10-26. All results are rule-of-thumb estimates:
- Bryant: What size furnace do I need? (BTU per square foot by climate, AFUE, oversizing, Manual J and S)
- 10 CFR 430.32(e): federal minimum AFUE for residential furnaces (Cornell LII)
- Goodman GM9S80/GC9S80 spec sheet, 8/21 (input and output by model; dealer-hosted copy)
- U.S. Department of Energy: Proper sizing of HVAC systems (rules of thumb, Manual J)
- DOE Building America: Guide to Determining Climate Regions by County, Vol. 7.3 (example counties)
- PNNL: Guide to Determining Climate Zone by County, Building America and IECC 2021 Updates (look up your county)
Sizing the air conditioner too? The BTU and AC size calculator does the cooling side, and the CFM calculator checks the airflow.
More for HVAC and home-services reps: HVAC sales, HVAC sales training, Home improvement sales.
The cold rooms get mentioned on the in-home visit
So do the square footage, the bills, the age of the old furnace and who else decides. Record the consult in ParrotNotes and get a structured note, a summary and the action items, so the load calculation and the quote start from what the homeowner said. Free for 100 minutes of recording a month.
Get Started FreeRelated tools
- BTU and AC Size CalculatorEstimate the BTUs and air conditioner tonnage a room or home needs from its size, sun, shade and occupants, as a rule of thumb before a load calculation.Open the tool
- CFM and Duct Size CalculatorWork out the CFM a room needs from air changes per hour, tons of cooling, bathroom size or whole-house ventilation, and size a round or rectangular duct at a chosen velocity.Open the tool
- Insulation CalculatorWork out how much insulation an attic, wall or floor needs from its area and target R-value, in batts, rolls or bags, with cost.Open the tool
- Square Footage CalculatorWork out square footage for rectangles, L-shaped rooms, triangles and circles, add rooms together, and convert to square yards, square meters and acres.Open the tool