WeekendCalc

AC Size Calculator (BTU)

Enter the size of your room and a few conditions. You get the cooling capacity in BTU per hour, and the standard window or portable air conditioner size to shop for.

Your project

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Results

Cooling capacity needed
7,000 BTU/hr
Standard unit size to shop for
8,000 BTU
In tons of cooling
0.58 tons

How we got there

Room area
300 ft²
Base capacity for this area
7,000 BTU/hr
Adjustments (sun, ceiling, people, kitchen)
0 BTU/hr
  • Based on the Energy Star room-size chart, with typical adjustments for sun, ceiling height, occupancy and kitchens. If the room opens onto another space, size for both. Do not oversize: a unit that is too big cools quickly, shuts off, and leaves the air clammy.

How this calculator works

The starting point is the Energy Star room-size chart: a 300 ft² room needs about 7,000 BTU per hour, a 450 ft² room about 10,000, and so on. We then adjust for the things that change the heat load: 10% less for a heavily shaded room and 10% more for a very sunny one, scaled up for ceilings taller than 8 ft, 600 BTU for each person beyond two, and 4,000 BTU if it is a kitchen.

BTU = base for area × (1 ± sun %) × (ceiling ÷ 8) + 600 × (people − 2) + 4,000 if kitchen

Worked example: a 20 × 15 ft living room (300 ft²) with 8 ft ceilings, average sun and two people needs 7,000 BTU, so you would shop for an 8,000 BTU unit. If it were a kitchen, 11,000 BTU, so a 12,000 BTU unit.

Tips before you buy

  • Bigger is not better. An oversized AC short-cycles, cools unevenly, and removes less humidity, which leaves the room feeling cold and clammy.
  • Check the unit's voltage and plug. Units over about 15,000 BTU often need a dedicated 230 V circuit.
  • Seal gaps around window units and shade west-facing windows. They can make more difference than going up a size.
  • Modern ratings: look at the SACC rating on newer units, which is a more realistic measure of cooling than the older BTU figure.
  • For a whole floor or several rooms, ductless mini-splits are more efficient than multiple window units.

Quick answers by size

150 sq ft room5,000 BTU200 sq ft room6,000 BTU250 sq ft room6,000 BTU300 sq ft room8,000 BTU350 sq ft room8,000 BTU400 sq ft room10,000 BTU500 sq ft room12,000 BTU600 sq ft room14,000 BTU800 sq ft room18,000 BTU1000 sq ft room18,000 BTU

Tools & materials for this project

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Window air conditionerPortable air conditionerWindow AC insulation kitBlackout curtainsSmart thermostat plug

Frequently asked questions

What size AC do I need for a 300 sq ft room?

About 7,000 BTU, so shop for an 8,000 BTU unit. For 150 to 250 ft² you need 6,000 BTU, and 450 ft² needs 10,000 BTU.

How many BTUs per square foot do I need?

A rough rule is 20 BTU per square foot, though the Energy Star chart is more accurate for different sizes. Add for sun, tall ceilings, extra people and kitchens.

What happens if my air conditioner is too big?

It cools the air quickly but shuts off before it has time to remove humidity, so the room feels damp. It also cycles more, wears out sooner and uses more power.

How many tons is 12,000 BTU?

One ton of cooling is 12,000 BTU per hour.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.