AC BTU Calculator: What Size Air Conditioner for Your Room

Choosing the right size room air conditioner matters more than most people think. Too small and it runs nonstop without catching up. Too big and it chills the room fast but leaves it damp and clammy. This AC BTU calculator uses the sizing chart and adjustments published by ENERGY STAR.

How the calculation works

The calculator multiplies length by width to get the floor area, looks up that area in the ENERGY STAR room air conditioner chart, then applies ENERGY STAR’s four adjustments.

Area (sq ft)        = length × width
Chart capacity      = BTU per hour for that area band (ENERGY STAR chart)
Heavily shaded      → chart capacity × 0.9   (reduce 10%)
Very sunny          → chart capacity × 1.1   (increase 10%)
More than 2 people  → add 600 BTU for each person over 2
Kitchen             → add 4,000 BTU
Recommended         = adjusted capacity, rounded to the nearest BTU

BTU here means British thermal units per hour, the cooling capacity printed on every unit. ENERGY STAR says its chart assumes an 8-foot ceiling.

The chart covers rooms from 100 to 2,500 square feet. Below 100 square feet the calculator points you to the smallest common unit, 5,000 BTU. Above 2,500 square feet it tells you to get a professional load calculation instead.

Worked example

The default room is 15 feet by 20 feet, with average sun, 2 regular occupants and no kitchen.

1. Area: 15 × 20 = 300 square feet.
2. Chart lookup: the calculator places 300 square feet in the ENERGY STAR band “300 up to 350,” which calls for 8,000 BTU.
3. Adjustments: average sun, 2 people and no kitchen mean no changes.
4. Recommended capacity: 8,000 BTU.

A second example

A sunny bedroom-office is 18 by 22 feet and four people often use it. The area is 396 square feet, so the chart says 9,000 BTU (the band “350 up to 400”). The calculator adds 10 percent for sun (9,900 BTU) and 600 BTU for each of the two extra people (1,200 BTU). The recommendation is 11,100 BTU. Most stores sell 10,000 and 12,000 BTU units, so a 12,000 BTU unit is the closer fit.

A kitchen shows how much cooking heat matters. A 12 by 14 foot kitchen is 168 square feet. The chart says 6,000 BTU, and the kitchen adjustment of 4,000 BTU brings the recommendation to 10,000 BTU.

What your result means

Chart capacity is the ENERGY STAR starting point for your floor area alone. Recommended capacity is that number after adjustments. Shop for the nearest size you can find. When you are between sizes, think about humidity: in a muggy climate, a slightly smaller unit that runs longer removes more moisture.

If the calculator says your space is over 2,500 square feet, a room unit is the wrong tool. Ask for a Manual J load calculation for a central system or a multi-zone heat pump. Our heat pump cost and savings guide explains what a proper sizing visit should include.

The ENERGY STAR sizing chart

These bands come straight from ENERGY STAR. The calculator uses them before applying adjustments.

Factors that change the right size

Climate and humidity. The chart is national. In very hot climates or rooms that bake in afternoon sun, the “very sunny” adjustment helps. In humid areas, avoid going bigger than you need, because ENERGY STAR notes an oversized unit leaves the room clammy.

Ceiling height and insulation. ENERGY STAR’s chart assumes 8-foot ceilings. Higher ceilings, an uninsulated attic above, or leaky windows all add heat. Sealing and insulating first can let a smaller unit do the job; see our air sealing guide.

Efficiency. ENERGY STAR says most certified room air conditioners now include variable speed compressors, which adjust output instead of switching on and off. They handle a range of loads better and run more quietly.

Running cost. Before you buy, it is worth checking that the room itself is not wasting cooling. Our guide on how to lower your energy bill covers shading, fans and thermostat settings. Electricity prices also vary widely. In July 2026, the US average residential price was 18.31 cents per kWh, ranging from about 12.72 cents in Louisiana to 48.00 cents in Hawaii (EIA). Use the appliance energy cost calculator with the unit’s watts to estimate its monthly cost.

Central systems. Central air conditioners and heat pumps must meet federal minimums set by the Department of Energy. Since January 1, 2023, split-system central air conditioners must reach at least 13.4 SEER2 in the North and 14.3 SEER2 in the Southeast and Southwest for units under 45,000 BTU per hour (DOE FAQ). To compare running costs of central systems, use the SEER2 savings calculator.

Common mistakes

• Buying the biggest unit you can afford. Oversizing wastes money and leaves the room damp.
• Measuring the whole floor instead of the room. Only count the space the unit will cool. Open doorways to other rooms add load, so close them or size up a little.
• Forgetting the kitchen adjustment. Cooking heat needs about 4,000 BTU extra.
• Ignoring ceiling height. The chart assumes 8 feet.
• Using a room chart for a whole house. Central systems need a Manual J load calculation, not a square-foot rule.
• Skipping the window seal. Gaps around a window unit let hot air in. Use the side panels and weatherstripping that come with it.

Limits of this calculator

This calculator follows the ENERGY STAR chart and its four adjustments. It does not account for ceiling height, insulation levels, window size and direction, local climate, heat from electronics, or open floor plans. It does not size central systems, ducts or multi-zone heat pumps.

The result is an educational estimate, not a load calculation, a quote or advice. For a whole-home system, ask a contractor for a written Manual J calculation and at least three written quotes.