Free HVAC Load Calculator
Find the right AC size for any space. Adjusts for climate, insulation, windows, and sun exposure.
How to size an air conditioner (BTU and tonnage)
Air conditioners are sized in BTU (British Thermal Units) and tons. One ton equals 12,000 BTU per hour. The goal is to match the unit to the home's cooling load — an AC that's too small never keeps up, and one that's too big short-cycles and leaves the air humid.
The formula
Base BTU = square footage × 20. Then adjust for the factors that change the load — a hot climate and heavy sun raise it, good insulation lowers it. Divide the adjusted BTU by 12,000 to get tonnage, and round up to the nearest half-ton (units are sold in 0.5-ton increments).
Worked example
A 1,500 sq ft home: 1,500 × 20 = 30,000 BTU, or 2.5 tons. In a hot climate with average insulation, the load rises to about 33,000 BTU → round up to a 3-ton unit. The calculator above runs these adjustments for your specific climate, insulation, windows, and sun exposure.
Right-sizing matters
Bigger is not better. An oversized AC cools fast but stops before it removes humidity, wasting energy and leaving rooms clammy. For an actual install, a contractor's Manual J load calculation is the definitive sizing — this tool gives a fast, close estimate.
HVAC calculator FAQ
What size air conditioner do I need?
As a rule of thumb, a home needs about 20 BTU of cooling per square foot, so a 1,500 sq ft home needs roughly 30,000 BTU (2.5 tons). The calculator above refines that baseline for your climate, insulation, windows, and sun exposure.
How many BTU per square foot for AC?
About 20 BTU per square foot is the standard baseline. Hot climates, poor insulation, lots of windows, or heavy sun exposure push it higher; cold climates and good insulation pull it lower.
What is a ton of air conditioning?
One ton of cooling equals 12,000 BTU per hour — a measure of how much heat the unit removes, not its weight. Residential AC units typically range from 1.5 to 5 tons.
How do I convert BTU to tons?
Divide BTU by 12,000. For example, 36,000 BTU ÷ 12,000 = 3 tons. Units are sold in half-ton increments, so round up to the nearest 0.5 ton.
Can an air conditioner be too big?
Yes. An oversized AC short-cycles — cooling the air quickly but not running long enough to remove humidity — which wastes energy and leaves the home clammy. Right-sizing matters more than bigger-is-better.
What is a Manual J load calculation?
Manual J is the industry-standard method for precisely sizing HVAC equipment based on a home's construction, orientation, and local climate. This tool gives a fast estimate; a contractor's Manual J is the definitive sizing for an install.
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