Generated on: June 17, 2026 at 05:24 PM

Air Conditioner BTU Sizing Calculator

Inputs

Room floor area25 m²
Ceiling height2.4 m
Sun exposure adjustment0 %
Regular occupants2
Is this a kitchen?No

This is a rule-of-thumb estimate. For an equipment purchase, a professional Manual J heat-load calculation is more accurate because it accounts for insulation, climate zone, window area and orientation, and air infiltration.

Generated with OneCalc · https://onecalc.app/en/lifestyle/ac-btu-sizing

Lifestyle

Air Conditioner BTU Sizing Calculator

Estimate the cooling capacity in BTU/hr and kW needed for a room from its floor area, with adjustments for ceiling height, sun exposure, occupancy, and kitchen heat.

Inputs

≥ 0 m²
≥ 0 m
%
-10 – 10 %
1 – 20

Results

Enter a value to see results.

A 25 m² room needs about ... BTU/hr of cooling capacity, or roughly ....

Embed this calculator

Preview

Paste this code into your page to show the calculator.

Share this calculation

Anyone who opens this link sees your values already filled in.

Cooling Capacity and the BTU Rule

A room air conditioner is rated by its cooling capacity, usually given in BTU per hour (BTU/hr): the amount of heat the unit can remove from a space each hour. Matching that capacity to the room is the goal of sizing. The ENERGY STAR and U.S. Department of Energy rule of thumb is about 20 BTU/hr for every square foot of floor area, assuming a standard 8 ft (2.4 m) ceiling.

Adjusting the Base Estimate

Floor area sets the starting point, but four factors shift it:

  • Ceiling height. A taller room holds more air to cool, so the estimate scales with ceiling height relative to the 2.4 m reference.
  • Sun exposure. A sunny, south-facing room gains heat through its windows; add up to 10%. A heavily shaded room can take 10% less.
  • Occupancy. Each person beyond the first two adds roughly 600 BTU/hr of body heat.
  • Kitchens. Cooking appliances add about 4,000 BTU/hr, so a kitchen carries a fixed surcharge.

Worked Example

Consider a shaded-neutral 25 m² living room with a 2.4 m ceiling and two occupants. Converting the area to square feet and applying the rule:

ft2=250.092903≈269.1Q=269.1×20×2.42.4384≈5297 BTU/hr\begin{aligned} \text{ft}^2 &= \frac{25}{0.092903} \approx 269.1 \\ Q &= 269.1 \times 20 \times \frac{2.4}{2.4384} \approx 5297 \text{ BTU/hr} \end{aligned}

That is about 1.55 kW of cooling power, since 1 kW equals roughly 3,412 BTU/hr. A 5,000–6,000 BTU/hr unit would suit this room.

Why Oversizing Backfires

A unit that is too large cools the air quickly and then shuts off before removing much moisture, restarting minutes later. This short cycling leaves the room cold but humid, strains the compressor, and wastes energy. A right-sized unit runs in longer, steadier cycles that dehumidify as they cool. The figures here are a sizing estimate; for an equipment purchase a Manual J load calculation is more precise because it weighs insulation, climate, and window area.

For related running-cost estimates, see Electricity Cost Calculator and Solar Panel Sizing Calculator.

Frequently Asked Questions (FAQ)

How many BTU per square foot does a room need?

The ENERGY STAR rule of thumb is about 20 BTU/hr per square foot for a room with an 8 ft (2.4 m) ceiling. A 250 sq ft room therefore needs roughly 5,000 BTU/hr before adjustments for sun, occupancy, or a kitchen. The figure rises for sun-facing or high-ceilinged rooms and falls for heavily shaded ones.

What happens if the air conditioner is too big?

An oversized unit cools the air to the set temperature so quickly that it shuts off before it has removed much humidity, then restarts a few minutes later. This short cycling leaves the room cold but clammy, wears the compressor, and uses more energy than a correctly sized unit running in longer, steadier cycles.

How do I convert BTU/hr to kilowatts?

Divide BTU/hr by 3412 to get kilowatts of cooling power: 1 kW equals about 3,412 BTU/hr. For example, a 9,000 BTU/hr unit is about 2.64 kW. Many regions rate air conditioners in kW of cooling rather than BTU/hr, so the conversion is useful when comparing models.

Disclaimer

This is a rule-of-thumb estimate. For an equipment purchase, a professional Manual J heat-load calculation is more accurate because it accounts for insulation, climate zone, window area and orientation, and air infiltration.

Recommended Next

Electricity Cost Calculator

Find out what any appliance costs to run — per day, month, and year — from its wattage, daily hours, and your electricity rate.

 Learn more
Solar Panel Sizing Calculator

Enter monthly kWh usage, peak sun hours, and system losses to find the right rooftop solar system size, panel count, annual output, and estimated cost.

 Learn more
Annual Carbon Footprint Calculator

Estimate your yearly CO₂-equivalent emissions from transport, flights, home energy, and diet — and see which activities contribute most.

 Learn more

200+ calculators · 10 languages · 100% free

Was this calculator helpful?

No

How can we improve this calculator?
Powered by OneCalc ↗