AC / BTU Size Calculator

Size a room air conditioner in BTU and tons from area, sun exposure and climate.

How the AC size works

Start with 20 BTU per square foot of floor area, then scale by sun exposure and climate. Add 600 BTU for each person beyond two, and 4,000 BTU if the space includes a kitchen. Finally round up to the nearest standard air conditioner size.

BTU = area x 20 x sun x climate + people x 600 + kitchen
Tons = BTU / 12,000

Why oversizing is a mistake

A unit that is too big cools the air quickly but does not run long enough to remove humidity, leaving the room clammy. Slight undersizing runs longer and dehumidifies better. That is why we round to the next standard size, not the next size up.

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Use cases

Window unit shopping - match the BTU number to a labeled unit.
Server or equipment rooms - heat load is higher; add occupants generously.
Sunrooms - pick "very sunny" so the unit is not starved in afternoon heat.
Open plans - size the whole open area as one zone, not room by room.

Standard unit sizes

Window and portable units come in 5,000, 6,000, 8,000, 10,000 and 12,000 BTU steps. Central and mini-split systems are sized in tons (12,000 BTU each). A 12,000 BTU mini-split is "one ton" and handles about 500-600 sq ft of typical space.

Frequently asked questions

How many BTU per square foot do I need?

A common rule is about 20 BTU per square foot of conditioned space for an average room. Sun exposure, climate, ceiling height and occupancy then adjust that base up or down.

How many square feet does one AC ton cool?

One ton is 12,000 BTU. At 20 BTU per sq ft, one ton cools roughly 500-600 square feet of typical space, though sunny or poorly insulated rooms need more.

Sizing estimate only. Insulation, windows and duct losses change real load. For whole-home systems consult an HVAC pro.

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Sizing an air conditioner the right way

How to use this calculator

Measure the room's length and width, then multiply to get the square footage. For L-shaped rooms, split the space into rectangles and add the areas.
Pick the sun exposure that matches the room: heavily shaded rooms need about 10% fewer BTU, very sunny rooms about 10% more.
Enter your climate zone. Hot-humid regions push the recommendation upward because the unit runs longer against higher outdoor moisture loads.
Account for the people count: add roughly 600 BTU for each person beyond the first two who regularly occupy the room.
Read the result in BTU/hr and tons (1 ton of cooling = 12,000 BTU/hr) and match it to the nearest commonly sold unit size.

Quick reference

Room size	Basic capacity	Hot climate / sunny
100–150 sq ft	5,000 BTU	6,000 BTU
150–250 sq ft	6,000–7,000 BTU	8,000 BTU
250–350 sq ft	8,000 BTU	9,000–10,000 BTU
350–450 sq ft	10,000 BTU	12,000 BTU
450–550 sq ft	12,000 BTU	14,000 BTU

Pro tips

Kitchens need about 4,000 extra BTU because stoves and ovens add heat.
Ceilings higher than 8 feet increase the air volume — step up one size rather than calculating exact volume.
An oversized unit cools fast but leaves the room clammy because it shuts off before dehumidifying; slightly undersized is usually more comfortable.

Frequently asked questions

Does an open floor plan change the sizing?

Yes. Treat connected open spaces as one large room instead of sizing each area separately, and include the kitchen if it is part of the same open volume.

Do ceiling fans reduce the BTU I need?

Fans make people feel cooler at no cooling cost, so they can let you run a right-sized unit on higher setpoints, but they do not remove heat or humidity — keep the calculated capacity.

What does 'ton' mean in air conditioning?

One ton equals 12,000 BTU per hour, a figure carried over from the era when cooling was measured by tons of ice melted per day. Residential units typically range from 0.5 to 5 tons.