Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Recommended cooling capacity (BTU)
8,000
Tons of cooling
0.67
Heating BTU estimate
14,000
Equivalent electric watts
2345
Room volume (cu ft)
3,200
Raw calculated BTU
8000
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter room area (sq ft).
- 2Enter ceiling height (ft).
- 3Enter sun exposure.
- 4Enter insulation.
- 5Enter regular occupants.
- 6Enter is it a kitchen?.
- 7Read your recommended cooling capacity (btu) on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
The 20 BTU per square foot baseline shifts with ceiling height, sun exposure, insulation, and occupancy. Oversizing short-cycles the unit and leaves the room humid.
Worked example
Using the values the calculator loads with:
Inputs
- Room area: 400 sq ft
- Ceiling height: 8 ft
- Sun exposure: Average
- Insulation: Average
- Regular occupants: 2
- Is it a kitchen?: No
Results
- Recommended cooling capacity (BTU): 8,000
- Tons of cooling: 0.67
- Heating BTU estimate: 14,000
- Equivalent electric watts: 2,345
- Room volume (cu ft): 3,200
- Raw calculated BTU: 8,000
What each field means
Inputs
- Room area (sq ft)
- The room area used in the calculation, measured in sq ft. Starts at 400 sq ft so you have a working example on load.
- Ceiling height (ft)
- The ceiling height used in the calculation, measured in ft. Starts at 8 ft so you have a working example on load. Accepted range: 6–20 ft.
- Sun exposure
- Pick the option that matches your situation — the maths changes per option. Choices: Heavily shaded, Average, Very sunny.
- Insulation
- Pick the option that matches your situation — the maths changes per option. Choices: Poor, Average, Well insulated.
- Regular occupants
- The regular occupants used in the calculation. Starts at 2 so you have a working example on load. Accepted range: 1–20.
- Is it a kitchen?
- Pick the option that matches your situation — the maths changes per option. Choices: No, Yes (+4,000 BTU).
Results
- Recommended cooling capacity (BTU)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Tons of cooling
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Heating BTU estimate
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Equivalent electric watts
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Room volume (cu ft)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Raw calculated BTU
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the air conditioner & heater btu calculator work?
The 20 BTU per square foot baseline shifts with ceiling height, sun exposure, insulation, and occupancy. Oversizing short-cycles the unit and leaves the room humid. The underlying maths is: BTU = Square feet × 20, adjusted for sun, ceiling height, insulation, and occupants.
What do I need to enter?
6 values: room area, ceiling height, sun exposure, insulation, regular occupants, and is it a kitchen?. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the recommended cooling capacity (btu) result mean?
It is returned as a whole number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). BTU Sizing Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/btu-sizing
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/btu-sizing" target="_blank" rel="noopener">BTU Sizing Calculator — RevenueLab</a> (2026).</p>
Source: [BTU Sizing Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/btu-sizing) (2026).