What Size AC Do I Need?

A plain-English guide to BTU, tonnage, and sizing your system right.

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A real person answers. Free estimate, no obligation.

Picking the right size air conditioner is the single most important decision in a new install. Too small and it runs constantly without keeping up; too large and it short-cycles, wastes energy, and never properly dehumidifies — a real problem in humid climates like South Carolina's.

BTU & tonnage basics

Cooling capacity is measured in BTUs per hour. One "ton" of cooling equals 12,000 BTU/hr. As a rough starting point, homes need roughly 20–30 BTU per square foot depending on climate, insulation, and sun exposure.

Rule of thumb: a 1,500 sq ft home in a hot, humid zone often lands around 3 tons — but only a load calculation confirms it.

Estimate your size

Use the calculator at the top of this page, then call a local pro for an exact Manual J load calculation.

What changes the number

Ceiling height and home layout
Insulation quality and window count
Sun exposure and shading
Local climate and humidity

Because these vary house to house, the estimate above is a starting point — not a final spec.

Next steps

Once you have a ballpark, a local technician can confirm sizing, recommend an efficiency level (SEER2), and handle the install. It's free to call and talk it through.

Talk it through with a local pro

Free to call — get connected with a local pro in your area.

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We connect homeowners with independent local HVAC technicians. Calls may be routed to a partner network; the number shown is a connection line, not a specific business.

Disclaimer: 781 Heating is a free service to assist homeowners in connecting with local service providers. All contractors/providers are independent and 781 Heating does not warrant or guarantee any work performed. It is the responsibility of the homeowner to verify that the hired contractor furnishes the necessary license and insurance required for the work being performed. All persons depicted in a photo or video are actors or models and not contractors listed on 781 Heating.

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