AC Unit Size Calculator

What size air conditioner do you really need?

What is this? Air conditioner size is measured in BTUs (British Thermal Units). Undersized units run constantly and can't cool your space. Oversized units short-cycle, waste energy, and fail to dehumidify properly. This calculator finds the right BTU rating for your room.

The rule of thumb: 20 BTU per square foot of living space. But ceiling height, sun exposure, and room occupancy all push that number up or down.

Who it's for: Homeowners buying a new AC window unit or sizing a central air system.

Room Details

Room Area (sq ft)

Ceiling Height

Sun Exposure

Number of Occupants

Kitchen

Climate Zone

Recommended AC Size

BTUs Required

—

minimum cooling capacity

Recommended

—

with 10% safety margin

Room Tonnage

—

for central AC systems

Energy Star Range

—

models to consider

Est. Annual Cost

—

at $0.12/kWh

Room Type

—

typical use case

Estimates only. Consult an HVAC contractor for central air sizing. Actual BTU needs vary by insulation, windows, and appliance heat load.

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Frequently Asked Questions

How do I size an air conditioner?

The standard rule: 1 ton (12,000 BTU) per 400-600 square feet of living space. More precisely: calculate total cooling load based on square footage, insulation, sun exposure, number of occupants, and heat-generating appliances.

What's the downside of an oversized AC?

An oversized unit short-cycles — it turns on, cools quickly, then shuts off before completing a full dehumidification cycle. This leads to: uncomfortable humidity, higher energy bills, more wear and tear, and uneven temperatures.

What's a SEER rating?

SEER (Seasonal Energy Efficiency Ratio) measures cooling output divided by energy input over a season. Higher SEER = more efficient. Minimum is SEER 14 in most states. Investing in SEER 16-22 units pays back in energy savings over 5-10 years.