Cooling Load Calculator

How Cooling Load Is Calculated

A proper cooling load calculation adds up every source of heat entering the house on a hot design day. Conduction through walls, roof, and windows follows Q = U × A × ΔT, using the outdoor design temperature minus your indoor setpoint (typically 75 °F). Solar gain through windows is A × SHGC × peak solar irradiance — west and east windows see the most heat during the day because the sun hits them at a low angle. People contribute about 300 BTU/hr each (sensible) plus another 300 latent. Lights and appliances add heat at 3.412 BTU/hr per watt. Air that leaks in from outside carries heat too: 1.08 × CFM × ΔT for the sensible portion.

Tons, BTU/hr, and kW

Air conditioner capacity is measured in tons in North America. One ton = 12,000 BTU/hr, the amount of heat needed to melt one ton of ice in 24 hours. So a 3-ton AC removes 36,000 BTU/hr of heat at rated conditions. Divide your total calculated load by 12,000 to get the required tonnage. In metric, use kilowatts: 3.516 kW per ton.

Don't Oversize

Sizing based on "500 sq ft per ton" is a shortcut that oversizes most modern houses and leaves older poorly-insulated ones undersized. An oversized AC runs short cycles, removes little moisture, leaves the air clammy, and wears itself out. Match the equipment to the calculated load with no more than 15% headroom. Heat pumps are often sized to the heating load and supplemented with backup heat or electric strips for peak cold, rather than oversized for cooling.

Window Orientation and SHGC

Single-pane clear glass passes about 86% of incoming solar heat. Double-pane argon Low-E around 30-40%. Solar-control glass (spectrally selective) can drop to 20%. For a 150 sq ft west-facing window, reducing SHGC from 0.60 to 0.30 cuts solar gain from 18,000 to 9,000 BTU/hr at design — nearly a ton of capacity.

Typical Cooling Load by Home Size and Climate

These are rough benchmark ranges for single-family homes at design conditions (95 °F outdoor, 75 °F indoor, west-facing windows). Use them to sanity-check your calculator result — a well-sized system should land near the middle of the range for your house quality:

• 1,000 sq ft, tight & well-insulated (R-21 walls, R-49 roof, Low-E windows): 12,000-18,000 BTU/hr (1.0-1.5 tons)
• 1,500 sq ft, average construction (R-13 walls, R-38 roof, double-pane): 24,000-30,000 BTU/hr (2.0-2.5 tons)
• 2,000 sq ft, average: 30,000-42,000 BTU/hr (2.5-3.5 tons)
• 2,500 sq ft, older leaky (R-11 walls, R-19 roof, single-pane): 42,000-54,000 BTU/hr (3.5-4.5 tons)
• 3,000 sq ft, older leaky: 54,000-72,000 BTU/hr (4.5-6 tons)

The old "500 sq ft per ton" rule consistently oversizes modern tight houses by 30-50% and undersizes 1970s homes in hot climates. Actual loads vary 2-3× between a well-sealed new build and an identical footprint from 1975.

Tons by Climate Zone

For the same 2,000 sq ft average home at design conditions: cool-dry zones (Seattle, Portland) need 1.5-2 tons. Mixed-humid (Chicago, DC, Atlanta) need 2.5-3 tons. Hot-humid (Miami, Houston, New Orleans) need 3-4 tons. Hot-dry (Phoenix, Las Vegas) need 3-5 tons at lower latent load. The difference is primarily outdoor design temperature (85 °F in Seattle vs 108 °F in Phoenix) and humidity.

Frequently Asked Questions

Is this Manual J?

No. ACCA Manual J is room-by-room and accounts for latent load, infiltration rate variation, and shading in detail. Use this for planning; hire a pro for code compliance.

Should the AC match the furnace size?

Not necessarily. Heating and cooling loads are usually different — heat loss often exceeds cooling load in cold climates, and vice versa in hot-humid climates.

What is latent cooling load?

The BTU required to remove moisture from the air. This calculator estimates sensible only; add 15-25% for latent in humid climates.

How does infiltration affect the load?

Air leaking through gaps carries its own heat. A leaky house at 1.0 ACH can add 20% to total cooling load; a tight house at 0.25 ACH adds almost nothing.

Can I use this for a heat pump?

Yes — heat pumps use cooling load for capacity selection. Also check heat loss to make sure the unit can heat the house.