This is only an initial estimate. The final furnace size should be based on an ACCA Manual J heating-load calculation that considers climate, insulation, windows, air leakage, ceiling height and building construction. Manual J is the ANSI-recognized residential HVAC load-calculation standard used for equipment sizing.

How to Use the Furnace BTU Calculator

Enter the information requested by the calculator.

1. Enter the Heated Square Footage

Use only the floor area that the furnace will actively heat.

Include:

Finished living rooms
Bedrooms
Finished basements connected to the system
Heated hallways
Heated offices
Finished additions served by the furnace

Do not automatically include:

Unheated garages
Unfinished attics
Open porches
Unconditioned crawlspaces
Detached buildings using another heater

For multiple floors:

Total heated area = Floor 1 + Floor 2 + Finished basement + Other heated areas

2. Select the Climate Zone

Colder locations require more heating capacity per square foot because the temperature difference between indoors and outdoors is greater.

Use the climate or heating zone that best reflects your local winter conditions rather than the average yearly temperature. Proper HVAC design uses a local winter design temperature, not merely the coldest temperature ever recorded. ENERGY STAR recommends using the nearest weather station’s 99% heating-season design temperature when performing a Manual J calculation.

3. Select the Insulation Level

Insulation and air sealing directly affect heat loss.

Choose:

Good: Modern insulation, efficient windows and limited air leakage
Average: Typical insulated house with standard windows
Poor: Older construction, little insulation, leaky windows or significant drafts

A well-insulated 2,000-square-foot home can need a smaller furnace than a poorly insulated 1,600-square-foot home in the same city.

4. Enter the Ceiling Height

Most simple BTU-per-square-foot rules assume approximately 8-foot ceilings.

For higher ceilings, a preliminary adjustment is:

Ceiling factor = Actual ceiling height ÷ 8

Example for 10-foot ceilings:

10 ÷ 8 = 1.25

If the unadjusted requirement is 60,000 BTU:

60,000 × 1.25 = 75,000 BTU

Cathedral ceilings, open stairwells and multi-story spaces require more careful heat-loss calculations than this simple adjustment.

5. Enter the Furnace Efficiency

Gas and oil furnaces lose some fuel energy through combustion and exhaust. Their efficiency is expressed as AFUE.

Use:

Required furnace input = Required heat output ÷ AFUE

For a house requiring 60,000 output BTU:

At 80% AFUE:

60,000 ÷ 0.80 = 75,000 input BTU

At 96% AFUE:

60,000 ÷ 0.96 = 62,500 input BTU

The Department of Energy defines AFUE as the ratio of useful seasonal heat output to fuel-energy input.

Furnace Sizing Formula

A practical preliminary formula is:

Output BTU = Area × Climate factor × Insulation factor × Ceiling factor

Where:

Area = Heated floor area in square feet
Climate factor = Suggested output BTU per square foot
Insulation factor = Adjustment for building efficiency
Ceiling factor = Ceiling height divided by 8

Suggested preliminary insulation factors:

Insulation Condition	Planning Factor
Excellent insulation and airtight construction	0.85–0.90
Good insulation	0.90–0.95
Average construction	1.00
Below-average insulation	1.10
Poor insulation or major air leakage	1.15–1.25

These adjustments are useful for early planning. They do not replace a room-by-room heating-load calculation.

Heating BTU per Square Foot Chart

Use this chart for preliminary furnace estimates based on output BTU, not necessarily the number printed as the furnace’s input capacity.

General Heating Zone	Typical Conditions	Output BTU per Square Foot
Zone 1	Hot winters	30–35
Zone 2	Warm winters	35–40
Zone 3	Moderate winters	40–45
Zone 4	Cool winters	45–50
Zone 5	Cold or very cold winters	50–60

Example Locations

The exact zone depends on local design temperature, elevation and building conditions, but broad examples include:

Hot: Southern Florida, Hawaii and similar climates
Warm: Gulf Coast, southern Texas and southern California
Moderate: Parts of the Southeast, Mid-Atlantic and Pacific Northwest
Cool: Midwest, Northeast and mountain regions
Cold: Northern states, high elevations and severe winter climates

The United States energy-code system uses eight formal IECC climate zones, including separate moisture classifications. A simplified five-zone BTU chart should therefore be treated as a calculator shortcut, not a substitute for the official climate data used by HVAC designers.

Furnace BTU by Square Footage

The following chart assumes average insulation and standard ceiling height.

Heated Area	Hot Climate	Warm Climate	Moderate Climate	Cool Climate	Cold Climate
400 sq ft	12,000–14,000	14,000–16,000	16,000–18,000	18,000–20,000	20,000–24,000
600 sq ft	18,000–21,000	21,000–24,000	24,000–27,000	27,000–30,000	30,000–36,000
800 sq ft	24,000–28,000	28,000–32,000	32,000–36,000	36,000–40,000	40,000–48,000
900 sq ft	27,000–31,500	31,500–36,000	36,000–40,500	40,500–45,000	45,000–54,000
1,100 sq ft	33,000–38,500	38,500–44,000	44,000–49,500	49,500–55,000	55,000–66,000
1,500 sq ft	45,000–52,500	52,500–60,000	60,000–67,500	67,500–75,000	75,000–90,000
1,700 sq ft	51,000–59,500	59,500–68,000	68,000–76,500	76,500–85,000	85,000–102,000
2,000 sq ft	60,000–70,000	70,000–80,000	80,000–90,000	90,000–100,000	100,000–120,000
2,500 sq ft	75,000–87,500	87,500–100,000	100,000–112,500	112,500–125,000	125,000–150,000
3,000 sq ft	90,000–105,000	105,000–120,000	120,000–135,000	135,000–150,000	150,000–180,000

These values represent required heat output before converting to furnace input capacity.

Furnace Input BTU Versus Output BTU

This is one of the most important parts of furnace sizing.

Input BTU

Input capacity is the amount of fuel energy entering the furnace each hour.

A furnace advertised as “80,000 BTU” commonly refers to input capacity.

Output BTU

Output capacity is the usable heat delivered after efficiency losses.

Use:

Output BTU = Input BTU × AFUE

Example Output Chart

Furnace Input	80% AFUE Output	90% AFUE Output	96% AFUE Output
40,000 BTU	32,000	36,000	38,400
60,000 BTU	48,000	54,000	57,600
80,000 BTU	64,000	72,000	76,800
90,000 BTU	72,000	81,000	86,400
100,000 BTU	80,000	90,000	96,000
120,000 BTU	96,000	108,000	115,200

A 96%-efficient 80,000 BTU furnace provides more usable heat than an 80%-efficient unit with the same input rating.

ENERGY STAR describes AFUE as the percentage of incoming fuel converted into space heat instead of being lost. Current ENERGY STAR criteria require at least 90% AFUE for qualifying southern U.S. gas furnaces and 95% for qualifying northern-region models.

Furnace Output Calculator

When the input rating and AFUE are known:

Furnace output = Input BTU × AFUE decimal

Example:

Input capacity: 90,000 BTU
Efficiency: 96%

90,000 × 0.96 = 86,400 output BTU

When the required output is known:

Required input = Heating load ÷ AFUE decimal

Example:

Required heat: 72,000 BTU
Furnace efficiency: 96%

72,000 ÷ 0.96 = 75,000 input BTU

The nearest available model might be an 80,000 BTU furnace, subject to its certified output and Manual S equipment selection.

How Many BTUs Are Needed for a 2,000-Square-Foot House?

For an average 2,000-square-foot home:

Climate	Estimated Output Requirement
Hot	60,000–70,000 BTU
Warm	70,000–80,000 BTU
Moderate	80,000–90,000 BTU
Cool	90,000–100,000 BTU
Cold	100,000–120,000 BTU

For a moderate climate requiring 85,000 output BTU:

At 80% AFUE:

85,000 ÷ 0.80 = 106,250 input BTU

At 96% AFUE:

85,000 ÷ 0.96 = 88,542 input BTU

The high-efficiency option could therefore meet the same output requirement with a lower input rating.

Furnace Size for a 1,700-Square-Foot House

A 1,700-square-foot home may require approximately:

Hot climate: 51,000–59,500 output BTU
Warm climate: 59,500–68,000 output BTU
Moderate climate: 68,000–76,500 output BTU
Cool climate: 76,500–85,000 output BTU
Cold climate: 85,000–102,000 output BTU

In a moderate climate, a high-efficiency furnace with approximately 70,000 to 80,000 input BTU may be suitable, depending on its AFUE and the actual heating load.

A poorly insulated house in a cold location may need significantly more. A modern airtight home may need less.

Furnace Size for 900 to 1,100 Square Feet

900 Square Feet

Estimated output:

Hot: 27,000–31,500 BTU
Moderate: 36,000–40,500 BTU
Cold: 45,000–54,000 BTU

1,100 Square Feet

Estimated output:

Hot: 33,000–38,500 BTU
Moderate: 44,000–49,500 BTU
Cold: 55,000–66,000 BTU

A 40,000 BTU input furnace may be enough only when its output matches the home’s calculated load. At 96% AFUE, it supplies about 38,400 output BTU. At 80% AFUE, it supplies only 32,000.

How Much Space Will Common Furnace Sizes Heat?

The chart below assumes the listed number is input capacity and the furnace is 90% efficient.

Furnace Input	Approximate Output	Hot Climate Coverage	Moderate Climate Coverage	Cold Climate Coverage
40,000 BTU	36,000 BTU	1,030–1,200 sq ft	800–900 sq ft	600–720 sq ft
60,000 BTU	54,000 BTU	1,540–1,800 sq ft	1,200–1,350 sq ft	900–1,080 sq ft
80,000 BTU	72,000 BTU	2,060–2,400 sq ft	1,600–1,800 sq ft	1,200–1,440 sq ft
90,000 BTU	81,000 BTU	2,310–2,700 sq ft	1,800–2,025 sq ft	1,350–1,620 sq ft
100,000 BTU	90,000 BTU	2,570–3,000 sq ft	2,000–2,250 sq ft	1,500–1,800 sq ft
120,000 BTU	108,000 BTU	3,090–3,600 sq ft	2,400–2,700 sq ft	1,800–2,160 sq ft

Actual coverage can fall outside these ranges because square footage is only one part of heat loss.

Factors That Change Furnace Size

Outdoor Design Temperature

The colder the winter design temperature, the greater the heat loss through walls, ceilings, windows and air leakage.

Insulation

Attic, wall, basement and floor insulation reduce the rate at which heat escapes.

Air Leakage

Drafts around windows, doors, plumbing penetrations, attic hatches and ductwork increase the heating load.

Windows and Doors

Large glass areas and older single-pane windows generally lose more heat than insulated walls.

Ceiling Height

A house with 10- or 12-foot ceilings contains more air volume and often more exposed wall area than a home with standard ceilings.

Building Shape

A compact two-story house can lose less heat than a long single-story house with the same floor area because it has less exterior surface area relative to its living space.

Duct Location and Leakage

Ducts passing through an attic, crawlspace or garage can lose useful heat before it reaches the rooms.

ENERGY STAR recommends that contractors inspect duct leakage, duct insulation and airflow while evaluating a replacement system.

Solar Exposure

Windows receiving winter sunlight can reduce daytime heating demand, while heavily shaded rooms may lose more heat.

Basement Condition

A finished basement inside the thermal envelope behaves differently from an unfinished or poorly insulated basement.

Ventilation

Bathroom fans, kitchen exhaust, fireplaces and mechanical ventilation remove indoor air that must be replaced and reheated.

Why a Manual J Calculation Is Better

A BTU-per-square-foot calculator is useful for:

Early budgeting
Comparing furnace capacities
Checking whether a quote appears reasonable
Estimating a garage or workshop heater
Understanding input and output ratings

It should not be the final basis for purchasing expensive equipment.

A Manual J calculation evaluates:

Local heating design temperature
Wall and ceiling construction
Insulation values
Window sizes and performance
Exterior doors
Air infiltration
Room orientation
Duct gains and losses
Building dimensions
Internal and solar gains

ACCA states that Manual J is the ANSI-recognized national standard for residential heating and cooling load calculations. Equipment is then selected using Manual S and matched to suitable ductwork using Manual D.

Is It Better to Oversize or Undersize a Furnace?

Neither is ideal.

Oversized Furnace Problems

An oversized furnace can:

Heat the house too quickly
Turn on and off frequently
Create uneven room temperatures
Produce excessive airflow or noise
Reduce comfort
Increase equipment wear
Make zoning and duct problems more noticeable

The Department of Energy recommends installing the smallest furnace capacity that can meet the calculated heating load.

Undersized Furnace Problems

A significantly undersized furnace may:

Run continuously during cold weather
Fail to maintain the thermostat setting
Recover slowly after a temperature setback
Leave distant rooms cold
Rely excessively on supplemental heat

A properly sized furnace should run for meaningful heating cycles and meet the design load without requiring extreme excess capacity.

Single-Stage, Two-Stage and Modulating Furnaces

Capacity staging does not replace correct sizing, but it affects how the furnace operates.

Single-Stage Furnace

Runs at full capacity whenever heating is requested.

It is usually less expensive but may produce larger temperature swings if oversized.

Two-Stage Furnace

Runs at a lower capacity during mild weather and switches to high capacity when necessary.

It can provide longer, quieter cycles and more consistent comfort.

Modulating Furnace

Adjusts its output over a wider range.

It can operate close to the home’s current heating demand rather than repeatedly using full output.

Even variable or modulating equipment should be selected from a proper heating-load calculation.

Electric Furnace Size Calculator

Electric resistance furnaces convert nearly all electricity used at the appliance into heat.

Use:

Required electric kW = Required BTU per hour ÷ 3,412

Example:

60,000 ÷ 3,412 = 17.59 kW

A furnace would therefore need approximately 18 kW of electric heating capacity before any design adjustment.

Electrical service capacity must also be checked. Large electric furnaces can require substantial amperage and multiple high-current circuits.

Do not confuse an electric furnace with a heat pump. A heat pump transfers heat and uses different performance ratings and sizing procedures.

Gas Furnace Size Calculator

For a gas furnace:

Estimate the required output BTU.
Divide by the selected furnace’s AFUE.
Compare the result with available furnace input capacities.
Verify the model’s certified output.
Confirm the choice through Manual J and Manual S.

Example:

Heating load: 70,000 BTU
Furnace: 95% AFUE

70,000 ÷ 0.95 = 73,684 input BTU

An available 80,000 BTU input furnace would provide:

80,000 × 0.95 = 76,000 output BTU

That model may be appropriate if its performance data and airflow match the project.

Garage and Shop Furnace Sizing

Garages and workshops often require more BTU per square foot than finished living areas because they may have:

High ceilings
Large overhead doors
Concrete floors
Limited insulation
Frequent door opening
Significant outdoor-air leakage
Intermittent heating schedules

Do not estimate a garage heater from floor area alone.

At minimum, consider:

Length
Width
Ceiling height
Insulation
Door size
Desired indoor temperature
Outdoor design temperature
Frequency of door opening

A detailed heat-loss calculation is particularly important for metal buildings, pole barns and tall workshops.

Heating BTU and Cooling BTU Are Not the Same

Do not use a furnace heating chart to size an air conditioner.

Heating and cooling loads respond differently to:

Solar gain
Humidity
Window orientation
Occupancy
Lighting
Appliances
Ventilation
Outdoor temperature

The furnace and air conditioner may share a blower and duct system, but their required capacities must be calculated separately.

ENERGY STAR recommends proper sizing through Manual J for both heating and cooling systems.

Frequently Asked Questions

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

First determine whether 80,000 BTU refers to input or output.

An 80,000 BTU input furnace provides approximately:

64,000 output BTU at 80% AFUE
72,000 output BTU at 90% AFUE
76,800 output BTU at 96% AFUE

At 90% AFUE, its approximate coverage is:

Hot climate: 2,060–2,400 sq ft
Moderate climate: 1,600–1,800 sq ft
Cold climate: 1,200–1,440 sq ft

Insulation, windows, ceiling height and air leakage can change the result substantially.

How do I calculate the BTU requirements for my furnace?

Use:

Required output BTU = Square footage × Climate BTU per square foot

Adjust for insulation and ceiling height.

Then convert the required output into furnace input:

Required input BTU = Required output ÷ AFUE

For final equipment selection, obtain an ACCA Manual J heating-load calculation.

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

At 90% AFUE, a 60,000 BTU input furnace provides approximately 54,000 output BTU.

Its rough coverage is:

Hot climate: 1,540–1,800 sq ft
Moderate climate: 1,200–1,350 sq ft
Cold climate: 900–1,080 sq ft

A 60,000 output BTU furnace would cover more than a 60,000 input BTU furnace.

How much space can a 120,000 BTU furnace heat?

At 90% AFUE, a 120,000 BTU input furnace delivers approximately 108,000 output BTU.

Its rough coverage is:

Hot climate: 3,090–3,600 sq ft
Moderate climate: 2,400–2,700 sq ft
Cold climate: 1,800–2,160 sq ft

A 120,000 BTU unit is too large for many average homes and should not be selected from square footage alone.

Is it better to undersize or oversize a furnace?

The correct choice is a properly sized furnace.

Slightly smaller equipment that still meets the calculated design load often provides longer and more consistent heating cycles than significantly oversized equipment. A furnace that is too small, however, may fail to maintain temperature during design weather.

The Department of Energy recommends selecting the smallest capacity that can meet the actual load.

Which size is better, a 60,000 or 70,000 BTU furnace?

Choose the model whose output capacity meets the Manual J heating load.

Example:

Home heating load: 55,000 BTU
60,000 input furnace at 96% AFUE: 57,600 output BTU
70,000 input furnace at 80% AFUE: 56,000 output BTU

Despite the larger input number, both models provide similar usable output.

Also compare airflow, blower performance, staging, duct capacity and manufacturer performance data.

What is the most reliable furnace brand?

There is no single brand that is best for every home, installer and region.

Consumer Reports’ January 2026 gas-furnace guide identified AirEase, Trane and Payne among the brands with standout reliability ratings.

Brand ranking should not be the only factor. Also compare:

Local parts availability
Installer experience
Warranty terms
Specific model history
Correct equipment sizing
Installation quality
Local service support

A correctly sized mid-range furnace installed by a skilled contractor can be a better purchase than a premium model installed badly.

What size furnace is needed for a 1,700-square-foot house?

A preliminary output estimate is:

Hot climate: 51,000–59,500 BTU
Warm climate: 59,500–68,000 BTU
Moderate climate: 68,000–76,500 BTU
Cool climate: 76,500–85,000 BTU
Cold climate: 85,000–102,000 BTU

Convert the required output to input capacity using the furnace AFUE.

What is the installed cost of a 60,000 BTU furnace?

As of July 2026, Angi lists a typical equipment range of approximately $2,500 to $4,200 for a 60,000 BTU gas furnace, with installation labor commonly adding approximately $500 to $2,000. Complete gas-furnace replacement projects broadly range from about $3,800 to $10,000, depending on efficiency, ductwork, permits and installation complexity.

A straightforward 60,000 BTU replacement may therefore cost approximately $3,000 to $6,500, while difficult or high-efficiency installations can cost more.

How many BTUs are needed to heat 400 square feet?

A preliminary estimate is:

Hot climate: 12,000–14,000 BTU
Moderate climate: 16,000–18,000 BTU
Cold climate: 20,000–24,000 BTU

A garage, poorly insulated room or high-ceiling space may require more.

What does BTU mean on a furnace?

BTU stands for British thermal unit.

Furnace capacity is normally expressed as BTU per hour, representing the rate at which the furnace consumes fuel or provides heat.

Always determine whether the listed rating is:

Input BTU
Output BTU
Heating capacity

How many square feet will a 40,000 BTU furnace heat?

At 90% AFUE, a 40,000 BTU input furnace produces approximately 36,000 output BTU.

Rough coverage:

Hot climate: 1,030–1,200 sq ft
Moderate climate: 800–900 sq ft
Cold climate: 600–720 sq ft

How many square feet will a 100,000 BTU furnace heat?

At 90% AFUE, a 100,000 BTU input furnace produces approximately 90,000 output BTU.

Rough coverage:

Hot climate: 2,570–3,000 sq ft
Moderate climate: 2,000–2,250 sq ft
Cold climate: 1,500–1,800 sq ft

How many BTUs are needed per square foot for heating?

A rough planning range is 30 to 60 output BTU per square foot.

Use the lower end for warm climates and efficient buildings. Use the higher end for cold climates and higher heat-loss conditions.

Final sizing should use Manual J rather than a fixed national multiplier.

Final Furnace Sizing Checklist

Before purchasing a furnace, confirm:

Heated square footage
Local winter design temperature
Insulation levels
Window and door performance
Air leakage
Ceiling height
Finished basement area
Duct condition
Furnace input capacity
Furnace output capacity
AFUE rating
Available airflow
Fuel type
Electrical requirements
Manual J heating load
Manual S equipment selection

The furnace BTU calculator provides a useful preliminary estimate. It should be used to understand the likely capacity range, not as the sole basis for ordering or installing HVAC equipment.