Energy & HVAC

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

Estimate room air-conditioner size in BTU/hr from room square footage, ENERGY STAR adjustments, 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

This is the nearest planning estimate based on room size and applicable ENERGY STAR adjustments, not a whole-home load calculation.

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.

What size air conditioner do I need?

For a room air conditioner, floor area establishes the ENERGY STAR base capacity. The AC size may then move up or down for strong shade, strong sun, more than two occupants, or kitchen use. Unusual heat gain, poor insulation, large windows, local climate, whole-home HVAC equipment, and ducted systems may require a professional load calculation.

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

AC size by room square footage

The table below is the same ENERGY STAR base-capacity table used by the calculator before adjustments.

Room areaBase capacity
100-150 sq ft5,000 BTU/hr
>150-250 sq ft6,000 BTU/hr
>250-300 sq ft7,000 BTU/hr
>300-350 sq ft8,000 BTU/hr
>350-400 sq ft9,000 BTU/hr
>400-450 sq ft10,000 BTU/hr
>450-550 sq ft12,000 BTU/hr
>550-700 sq ft14,000 BTU/hr
>700-1,000 sq ft18,000 BTU/hr
>1,000-1,200 sq ft21,000 BTU/hr
>1,200-1,400 sq ft23,000 BTU/hr
>1,400-1,500 sq ft24,000 BTU/hr
>1,500-2,000 sq ft30,000 BTU/hr
>2,000-2,500 sq ft34,000 BTU/hr

BTU, tons and cooling kW

BTU/hr, cooling tons, and cooling kW all describe cooling capacity. One cooling ton is approximately 12,000 BTU/hr. Cooling kW is another capacity unit, not the electrical input power shown on a utility bill. To estimate running cost after choosing a unit, enter the unit's rated wattage in the Electricity Cost Calculator.

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.

If the same 300 sq ft room is very sunny, the ENERGY STAR sun adjustment adds 10%, or 700 BTU/hr, for an estimated 7,700 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

What size AC do I need for my room?

Start with the room floor area. The calculator uses the ENERGY STAR room air conditioner table for base BTU/hr, then applies shade, sun, occupant, and kitchen adjustments where they apply.

How many BTUs do I need per square foot?

HomeQuantifi does not use one universal BTU-per-square-foot rule. It uses the ENERGY STAR room-size table because the recommended capacity changes by area range.

Is a bigger air conditioner always better?

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

What is 12,000 BTU in tons?

12,000 BTU/hr is approximately 1 cooling ton. The tons result is a cooling-capacity comparison, not the electrical power the unit consumes.

Is this the same as a Manual J calculation?

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

Reference

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