Energy & Workshop Math

BTU Calculator

Estimate room cooling BTU per hour from area and simple adjustments.

Units

Your measurements

Results update as you type.

Example shown — use your own measurements.

Fractions and number formats

Use decimals (12.5), fractions (12 1/2 or 12-1/2), or explicit lengths (12ft 6in, 12′ 6″, 25 mm). A comma followed by one or two digits is a decimal comma: 12,5 = 12.5. Grouped commas mean thousands: 1,200 = 1200. Use a decimal point for ambiguous values. Scientific notation is not accepted.

Enter your measurements

Results update as you type.

About this calculator

Compare a room cooling estimate using an explicit 20 BTU/h per ft² planning assumption. This method does not size heating equipment. For a published room-size chart use the AC Size Calculator; for equipment selection obtain a load calculation.

How to measure

  • Measure the conditioned floor area and ceiling height.
  • Note window orientation and shade, occupant count and whether a kitchen is in the zone.
  • Treat the result as a sizing comparison, not a load calculation.

How to use this calculator

  1. Measure the room's floor area.
  2. Confirm the ceiling height and how much sun the room gets.
  3. Enter occupants and whether a kitchen is in the same space.
  4. Use the estimate for comparison, then check equipment size with an installer.

Formula

BTU/h = 20 × floor area ft² × (ceiling height ÷ 8) × exposure factor

Exposure factor: shaded 0.9, mixed 1.0, sunny 1.1

Add 4,000 BTU/h for a kitchen and 600 BTU/h for each occupant above two

Tons = BTU/h ÷ 12,000

Assumptions

  • 20 BTU/h per ft² is an illustrative cooling rule, not an ENERGY STAR sizing chart or a heating-load model.
  • Ceiling-height scaling assumes the same floor area and similar envelope losses.
  • Kitchen and occupant allowances are flat planning adders.

Cooling rule comparison at 8 ft ceiling

Cooling rule comparison at 8 ft ceiling
Floor area20 BTU/h per ft² assumption
100 ft²2,000 BTU/h
400 ft²8,000 BTU/h
1,000 ft²20,000 BTU/h

Explicit area rule only, before adjustments. For the published room chart, use AC Size. Not a heating model.

Common mistakes

  • Using the cooling area rule to size heating equipment.
  • Treating refrigeration tons as a mass unit.
  • Selecting equipment without considering the envelope and local climate.

Worked example

Try it: press “Load example” above the form.

1,000 ft² at 8 ft with mixed exposure and two occupants estimates 20,000 BTU/h — about 1.67 tons.

Limitations

Not a Manual J or ASHRAE load calculation. Insulation, glazing, air leakage, duct losses and local design temperature dominate real loads.

Frequently asked questions

Does this size a heating system?

No. It is a simple cooling-capacity comparison based on an explicit area rule. Heating loads need indoor and outdoor design temperatures, envelope losses, leakage and suitable design methods.

Why use the AC Size Calculator too?

The AC tool uses the cited ENERGY STAR room-area bands. This tool exposes a simpler rule so you can see its assumptions. Neither replaces a professional cooling load calculation.

What does the worked example show?

1,000 ft² at 8 ft with mixed exposure and two occupants estimates 20,000 BTU/h — about 1.67 tons.

Which assumptions should I check before using the result?

Not a Manual J or ASHRAE load calculation. Insulation, glazing, air leakage, duct losses and local design temperature dominate real loads.

Reviewed by Codex AI editorial agent (Editorial, 2026-11-12).

Sources

Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.

Calculation documentation · Updated 2026-11-08 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.

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