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AC Sizing Calculator

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Enter your room's size and a few details to find the air conditioner capacity (BTU) you need.

AC Sizing Calculator

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Recommended capacity
8,000 BTU
Closest standard unit
8,000 BTU
A general sizing estimate - ceiling height above 8ft, poor insulation, or an open floor plan can meaningfully increase the capacity needed. For a whole-home system, consult an HVAC professional for a proper load calculation.

The formula

Base BTU = Room size (sq ft) x 20 Adjust for sun exposure, add ~600 BTU per person beyond 2 occupants, add 4,000 BTU if it includes a kitchen
Example

A 400 sq ft room, normal sun, 2 occupants, no kitchen: 400 x 20 = 8,000 BTU, rounding up to a standard 8,000-9,000 BTU unit.

Step-by-step guide

  1. Enter your room's square footage.
  2. Choose the sun exposure that best matches the room.
  3. Enter typical occupancy and whether the space includes a kitchen.

Why bigger isn't simply better when sizing an AC

It's tempting to assume a more powerful unit is always the safer choice, but an oversized air conditioner actually performs worse in an important way: it cools the room to the target temperature quickly, then shuts off before it has run long enough to properly dehumidify the air. This "short cycling" leaves a room feeling cool but clammy, and also wears out the compressor faster from frequent starts and stops. Getting the size right - not just "big enough," but appropriately matched to the space - genuinely matters for comfort and equipment life, not just energy cost.

Common mistakes

Rounding up "just to be safe" - oversizing causes short cycling and poor humidity control, not better cooling.
Ignoring ceiling height - this estimate assumes a standard 8ft ceiling; a notably taller or vaulted ceiling increases the air volume that needs cooling.

Frequently asked questions

Is this accurate enough for buying a whole-home HVAC system?

This is a solid estimate for a single room or a portable/window/mini-split unit. A whole-home system is a bigger investment and depends on additional factors (insulation quality, window count and orientation, local climate zone, ductwork) - a professional Manual J load calculation is worth it for that decision.

Why does a kitchen need extra capacity?

Cooking appliances generate significant additional heat that the AC also needs to remove, which is why open-plan spaces that include a kitchen typically need extra capacity beyond the base square-footage estimate.

Does insulation quality change this estimate?

Yes, meaningfully - this estimate assumes reasonably typical insulation. A poorly insulated older space may need more capacity, while a newer, well-insulated and well-sealed room may need somewhat less.

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