HVAC calculator

Furnace BTU calculator

Default example (40 ft floor length, 30 ft floor width, 8 ft ceiling height, 30 BTU / ft² btu per square foot): Planning load is 36000 BTU/h. In metric, floor area is 111.48 m².

Formula. BTU/h = area × BTU per ft² × (ceiling height ÷ 8)

Planning heating load from floor area, ceiling height and a stated BTU per square foot. Not Manual J.

By Feroz Sheikh · Formula HVAC-BTU-001 v1.0.0 · Tested formula

FormulaHVAC-BTU-001v1.0.0

Project inputs

Enter your measurements. Switching units converts the same project, including coverage and unit rates. Package yields keep the unit printed beside each field.

Furnace BTU: measurement diagram. Measure the space and enter the stated planning factor. This sketch is not a room-by-room load design. — Measure the space and enter the stated planning factor. This sketch is not a room-by-room load design. View image

Floor lengthft + inConditioned floor lengthftinFloor widthft + inConditioned floor widthftinCeiling heightft + inAverage finished ceiling height. 8 ft is the rule-of-thumb basisftinBTU per square footBTU / ft²Planning factor. 30 is a mild-climate start; cold rooms need more. Not a Manual J load

Refine measurements and rates at any time. Your current values are included in the material-list PDF.

Calculated result

36000BTU/h

Planning load

FormulaHVAC-BTU-001

BTU/h = area × BTU per ft² × (ceiling height ÷ 8)

1200 ft²

Floor area

1 ×

Height factor vs 8 ft

3 tons

Tons (approx)

Planning estimate, not engineering advice. Confirm site measurements and product specifications. Calculation limits.

Planning load is floor area times a stated BTU per square foot, scaled by ceiling height over 8 ft. Not Manual J and not a furnace size.

Measure conditioned floor area. Garage and porch stay off this line unless they are heated.
30 BTU/ft² is a mild-climate start. Replace it with the factor you were given.
Carry the number into a conversation with the HVAC designer. Do not buy a furnace from this page.

Add your project costs

Add rates if you are pricing a related project line. Pick a currency — badges, inputs, and totals follow.

This page is geometry, not a material takeoff. Leave rates at zero unless you are pricing a related line. Construclytic does not fetch supplier prices.

View estimate Quote / invoice Add another item

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How this calculator works

A transparent furnace btu takeoff.

Feroz SheikhWritten by Feroz Sheikh · founderFormula HVAC-BTU-001 v1.0.0 automated checks Oct 3, 2026.

Planning load is floor area times a stated BTU per square foot, scaled by ceiling height over 8 ft. Not Manual J and not a furnace size.

HVAC-BTU-001 is a planning load: floor area × a stated BTU per square foot, scaled by ceiling height over 8 ft. Default 30 BTU/ft² on a 40 × 30 × 8 ft floor is 36,000 BTU/h. Not Manual J, not a furnace size, not a code path.

Worked example. Example: a 40 × 30 ft floor at 8 ft and 30 BTU/ft² returns 36,000 BTU/h of planning load.

See the example inputs and calculated result

This separate example uses the calculator's starting values in imperial units. It stays unchanged when you edit the tool above. Example rates, yields and dimensions are assumptions, not recommendations for your project.

Furnace BTU: example inputs
Input	Example value
Floor length	40 ft
Floor width	30 ft
Ceiling height	8 ft
BTU per square foot	30 BTU / ft²

Example result: Planning load — 36,000 BTU/h.

BTU/h = area × BTU per ft² × (ceiling height ÷ 8)

Use the labelled units above when reproducing this example. To work in metric, switch the calculator to Metric; applicable measurements are converted, while product-specific units remain labelled.

1. Measure conditioned floor area. Garage and porch stay off this line unless they are heated.
2. 30 BTU/ft² is a mild-climate start. Replace it with the factor you were given.
3. Carry the number into a conversation with the HVAC designer. Do not buy a furnace from this page.

Field notes that change the number

Measure conditioned floor area. Garage and porch stay off this line unless they are heated.
30 BTU/ft² is a mild-climate start. Replace it with the factor you were given.
Carry the number into a conversation with the HVAC designer. Do not buy a furnace from this page.

Common mistakes

Treating a rule-of-thumb BTU as Manual J.
Buying a furnace from a square-footage factor.
Ignoring ceiling height on a volume that is not 8 ft.

Need the broader workflow? Read the hvac planning guide.

How-to: How to plan a furnace BTU load

Questions

Questions about furnace btu

Is this Manual J?

No. Insulation, infiltration, glass and design temperatures are not in this arithmetic.

Can I size a furnace from this?

No. Oversizing and undersizing are field failures. This is a planning conversation starter.

What is the formula ID?

This furnace btu calculator publishes formula HVAC-BTU-001, version 1.0.0. The ID stays on the result and on the estimate line.