AC Size Calculator

Choosing an air conditioner by square footage alone often gets the size wrong. This calculator starts from the widely used rule of about 20 BTU per square foot and adjusts for ceiling height, sun, people, kitchens, insulation and climate, then suggests the nearest standard window, portable or central AC size — with an honest note on when you need a professional load calculation.

The 20 BTU per square foot rule

This chart follows the familiar rule of about 20 BTU per hour for each square foot of living space, with standard window-unit sizes. It assumes 8-foot ceilings, average sun and two occupants. The US ENERGY STAR programme publishes a similar room-AC sizing guide with adjustments of about 10% for shade or sun, 600 BTU for each additional person, and 4,000 BTU for a kitchen.

Adjustments

The ceiling, insulation and climate factors are rough allowances for this calculator, not a formal standard. They are there to show how much these factors matter — a sunny, poorly insulated room in a hot climate can need 40–50% more cooling than the base rule suggests.

BTUs, tons and kilowatts

A “ton” of cooling is 12,000 BTU per hour — originally the cooling from melting one ton of ice in 24 hours. One kilowatt of cooling is about 3,412 BTU/h. So a 3-ton central system provides about 36,000 BTU/h, or 10.5 kW. Note that this is cooling output, not the electricity the unit uses; efficiency ratings (EER, SEER2 or, outside the US, SEER and COP) describe how much cooling you get per unit of electricity.

Why oversizing is a problem

• An oversized AC cools the air quickly and switches off before it has removed much moisture, leaving the room cool but clammy.
• Frequent short cycles wear out compressors and waste energy.
• Rooms far from the thermostat may swing between too hot and too cold.
• Bigger units cost more to buy.

Undersizing has the opposite problem: the unit runs constantly and may not keep up on the hottest days. Aim for a size close to the calculated load.

Worked example

A 350 sq ft bedroom with 8-foot ceilings, average sun, two occupants, average insulation and warm summers needs 350 × 20 = 7,000 BTU/h — an 8,000 BTU window unit. If the same room is very sunny, has single glazing and is in a hot climate, the estimate becomes 7,000 × 1.1 × 1.15 × 1.1 ≈ 9,740 BTU/h, and a 10,000 BTU unit is the better match. For a 1,800 sq ft home with average conditions, 36,000 BTU/h suggests about 3 tons — a figure a contractor’s Manual J should confirm or refine.

Window, portable, mini split or central?

When to get a Manual J

In the US, ACCA Manual J is the standard method for calculating residential heating and cooling loads, room by room. It accounts for wall and window areas and orientation, insulation levels, air leakage, duct location and your local design temperatures. It is strongly recommended — and in many places required — when installing or replacing central AC or a heat pump. Rules of thumb like the one on this page are for single rooms and sanity checks.

Energy efficiency ratings

In the US, room air conditioners are rated by CEER and central systems by SEER2 and EER2; higher numbers use less electricity for the same cooling. Choosing the right size matters as much as the rating — an efficient unit that is too big still short-cycles.

The AC sizing pack

The optional pack includes a room-by-room load estimating workbook, a home survey sheet for windows and orientation, a questions-for-your-contractor checklist and an AC maintenance log. The calculator on this page is free.

AI-assisted content

This page was drafted with AI assistance and reviewed by Kedop. It is an estimate, not an engineering calculation.