Energy & HVAC

AC BTU Calculator – What Size Air Conditioner Do I Need?

Estimate a preliminary room air conditioner size using the ENERGY STAR room AC capacity table, sun exposure, occupants, kitchen use, and a clearly labeled ceiling-height approximation.

Calculate room AC size

Estimate room air conditioner capacity from the ENERGY STAR sizing table and documented adjustments.

This is a preliminary single-room sizing estimate. Whole-home HVAC equipment should be professionally sized with Manual J or an equivalent load calculation where required.

Results

Based on the ENERGY STAR room air conditioner table for an 8 ft ceiling.

Adjusted estimated capacity

7,000 BTU/hr

0.58 tons / 2.05 kW

ENERGY STAR base capacity

7,000 BTU/hr

Before listed adjustments

Room size

300 sq ft

2,400 cubic ft

Adjustments applied

Ceiling height0 BTU

ENERGY STAR table assumes an 8 ft ceiling

Sun exposure0 BTU

No ENERGY STAR sun adjustment

Extra occupants0 BTU

ENERGY STAR +600 BTU/h per person above two

Kitchen0 BTU

ENERGY STAR +4,000 BTU/h for kitchens

How to measure the room

Measure the room length and width, then enter the ceiling height. For connected rooms without doors, include the area that the unit must cool, not just the wall where the AC will sit.

Sizing method

The calculator starts with the EPA ENERGY STAR room air conditioner capacity table for an 8 ft ceiling. It then applies the ENERGY STAR sun, occupant, and kitchen adjustments. If the ceiling is above 8 ft, HomeQuantifi adds a simple planning approximation for the extra room volume.

Base BTU/hr = ENERGY STAR capacity table value for floor area
Heavily shaded room = base capacity - 10%
Very sunny room = base capacity + 10%
Add 600 BTU/hr for each occupant above two
Add 4,000 BTU/hr for a kitchen
Ceiling above 8 ft = HomeQuantifi planning approximation

Worked example

A 20 ft by 15 ft room with an 8 ft ceiling is 300 sq ft. The base ENERGY STAR table capacity is 7,000 BTU/hr. With average sun, two occupants, and a standard non-kitchen room, the adjusted estimate remains 7,000 BTU/hr.

Assumptions

  • Base sizing uses the ENERGY STAR room AC table for an 8 ft ceiling.
  • Sun, occupant, and kitchen adjustments follow ENERGY STAR guidance.
  • Ceiling height above 8 ft uses a HomeQuantifi planning approximation, not an ENERGY STAR formula.
  • Climate, insulation, large windows, air leakage, and other heat gains are not converted into arbitrary percentages.
  • Whole-home systems, ducts, humidity, air leakage, and local code are outside this room estimator.

Planning guidance

Use this result to compare room AC sizes, then read the unit's manufacturer coverage guidance. For central air, heat pumps, major renovations, or code-sensitive work, ask for a professional load calculation.

FAQs

Is this a Manual J calculation?

No. This is a transparent homeowner estimate for room AC planning. Whole-home HVAC sizing should use Manual J or another professional load calculation where appropriate.

Does this include climate and insulation?

Not as numeric modifiers. Unusual climate, poor insulation, large glazing, air leakage, and other heat gains can matter, but they should be handled with a professional load calculation instead of arbitrary percentages.

Can an oversized AC be a problem?

Yes. Oversizing can reduce humidity removal and comfort. This calculator is for preliminary shopping guidance, not final system design.

Reference

Base capacity ranges and the shade, sun, occupant, and kitchen adjustments are based on EPA ENERGY STAR room air conditioner sizing guidance.