Energy & Workshop Math

AC Size Calculator

Estimate room air-conditioner capacity from floor area and conditions.

Units

Your measurements

Results update as you type.

Example shown — use your own measurements.

The cited room chart covers 100–2,500 ft² (about 9.29–232.26 m²). Outside that range, obtain a suitable equipment/load assessment instead of extrapolating.

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

Choose room air-conditioner capacity using ENERGY STAR's room-size bands, then adjust for ceiling height, sun exposure, occupants and a kitchen. The result is a planning estimate for a single room or zone.

How to measure

  • Measure the floor area of the room to be conditioned.
  • Note ceiling height, window sun exposure, occupants and any kitchen.
  • Match to a nominal equipment size at or above the estimate.

How to use this calculator

  1. Measure the room's floor area.
  2. Confirm the ceiling height and sun exposure.
  3. Add occupants and a kitchen if they apply.
  4. Compare available capacities with an installer; an oversized unit can dehumidify poorly.

Formula

Base BTU/h from ENERGY STAR room bands: 100–150 ft² 5,000; 250 6,000; 300 7,000; 350 8,000; 400 9,000; 450 10,000; 550 12,000; 700 14,000; 1,000 18,000; 1,200 21,000; 1,400 23,000; 1,500 24,000; 2,000 30,000; 2,500 34,000. Areas below 100 ft² or above 2,500 ft² are outside this chart.

Adjusted BTU/h = base × (ceiling height ÷ 8) × exposure factor

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

Tons = BTU/h ÷ 12,000

Assumptions

  • The base bands follow ENERGY STAR room air-conditioner sizing guidance for typical spaces.
  • Sun, occupant and kitchen adjustments follow the cited room guidance. Ceiling-height scaling is an additional simple volume assumption.
  • The space is a single room or zone, not a whole house.

Selected ENERGY STAR room chart bands

Selected ENERGY STAR room chart bands
Room area, upper band limitBase capacity
400 ft²9,000 BTU/h
700 ft²14,000 BTU/h
1,000 ft²18,000 BTU/h
1,200 ft²21,000 BTU/h

Selected entries from the cited 100–2,500 ft² room chart, before adjustments. No estimate is supplied outside that area range. Not a whole-house load calculation.

Common mistakes

  • Using the room chart above its supported area range.
  • Treating the optional ceiling-height rule as part of the published chart.
  • Oversizing an air conditioner without checking humidity control.

Worked example

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

A 400 ft² room at 8 ft with mixed exposure estimates 9,000 BTU/h, equal to 0.75 refrigeration ton.

Limitations

Not a Manual J load calculation. Room shape, wall construction, glazing, floor level and climate affect the real load; seer2/efficiency and noise are separate choices.

Frequently asked questions

How many BTU/h does a 400 ft² room need?

The cited ENERGY STAR base chart gives 9,000 BTU/h for a room up to 400 ft². That is 0.75 refrigeration ton. Shade, occupants and a kitchen can change the comparison.

Should I always buy the next larger air conditioner?

No. Oversizing can reduce dehumidification and comfort. Use the room chart as a starting point, then confirm the actual equipment selection with a qualified installer.

What does the worked example show?

A 400 ft² room at 8 ft with mixed exposure estimates 9,000 BTU/h, equal to 0.75 refrigeration ton.

Which assumptions should I check before using the result?

Not a Manual J load calculation. Room shape, wall construction, glazing, floor level and climate affect the real load; seer2/efficiency and noise are separate choices.

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 2026-11-11 · 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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