BTU Calculator — What Size Air Conditioner Do I Need?

Oversizing is the expensive mistake

Almost everyone errs large, and it feels like the safe direction. It is not.

An air conditioner removes humidity only while it is running, because moisture condenses on a cold coil over time. An oversized unit reaches the thermostat setpoint quickly, shuts off, and never runs long enough for that to happen. The room ends up cold and damp — the specific complaint of a house that is "always clammy in summer" despite an air conditioner that works.

Short cycling also wears equipment. Compressor start is the hardest moment in its life, drawing several times running current and stressing the mechanical parts. A unit cycling six times an hour instead of twice ages roughly three times as fast at the compressor.

And efficiency falls. Both a compressor and a heat pump reach peak efficiency after several minutes of steady operation. A system that never gets there operates permanently in its worst region.

Undersizing has one symptom — the room does not get cool enough on the hottest days — and it is obvious, cheap to diagnose and rarely dangerous. Oversizing has four symptoms and none of them look like a sizing problem.

What a real load calculation models that this does not

ACCA Manual J is the recognised procedure, and the gap between it and any square-footage estimate is worth understanding before relying on one.

Manual J models each wall, window and door separately by orientation, area and construction. A west-facing window gains far more heat in the afternoon than a north-facing one of the same size, and a Manual J calculation captures that while a per-square-foot figure cannot.

It uses local design temperatures — the 1% dry bulb and the coincident wet bulb for your specific location — rather than a national average. The difference between Phoenix and Portland is not a correction factor, it is a different calculation.

It accounts for infiltration based on construction tightness, duct losses where ducts run outside the conditioned envelope, and internal gains from occupants, lighting and appliances by count rather than by rule of thumb.

Use this page to shortlist a window unit or check whether a quoted size is plausible. Use Manual J before installing central equipment, applying for a permit, or spending several thousand dollars.

Reading the adjustments

Each line in the result exists because it independently moves the answer, and seeing them separately tells you where your particular room is unusual.

Ceiling height is the largest single adjustment for most rooms because it is proportional to volume. Going from eight feet to twelve adds fifty percent, which is frequently the difference between two unit sizes and is the adjustment most rules of thumb omit entirely.

Sun exposure moves the answer about ten percent in each direction. It matters most for rooms with substantial west or south glazing, where the real figure can exceed the ten percent this uses.

Occupants add roughly 600 BTU per hour each beyond the first two. This is small for a bedroom and significant for a room that regularly holds eight people.

The kitchen allowance covers cooking appliances, which release a large amount of sensible heat directly into the room. It is a flat figure here because the range of real kitchen loads is too wide for a multiplier to be meaningful.