How accurate is a rule-of-thumb cooling estimate?

It is a ballpark figure meant for phone quotes and feasibility, not equipment selection. Rule-of-thumb numbers can be off by 20 percent or more, so always run a full ACCA Manual J load calculation before buying or installing equipment.

How many BTU per square foot does a home need?

Residential cooling typically falls between about 20 and 35 BTU per hour per square foot depending on climate, insulation, and window area. Offices and retail run higher because of internal heat from people, lighting, and equipment.

Why does the window ratio matter so much?

Windows admit far more solar heat than insulated walls. A higher window-to-floor ratio adds significant cooling load, which is why the tool increases the estimate above a baseline ratio and decreases it below.

How is occupant heat gain figured?

Each person adds sensible and latent heat. The tool uses roughly 400 BTU per hour per person for homes and 600 for offices and retail, where activity levels and ventilation requirements are higher.

What is a ton of cooling?

One ton of cooling equals 12,000 BTU per hour, historically the rate needed to melt one ton of ice in 24 hours. The tool divides total BTU per hour by 12,000 to express the load in tons.

Cooling Load Quick Estimate (Rule of Thumb) Calculator

Before a formal load calculation, contractors often need a tonnage figure in under a minute to answer a customer on the phone. This tool applies published BTU-per-square-foot ranges by climate and building type, then layers on window, occupant, and lighting adjustments for a more honest quick estimate.

How it works

A base area load is set by building type and climate, then three corrections are added:

area load     = floor area × base BTU/ft²
window adj     = area load × (window% − 12) × 0.015
occupant load  = occupants × 400 (home) or 600 (office/retail) BTU/h
lighting extra = max(0, W/ft² − 0.8) × area × 3.41 BTU/h per watt
total          = area load + window adj + occupant load + lighting extra
tons           = total ÷ 12000

The base already assumes a moderate 12 percent window ratio and 0.8 W/ft² lighting, so the adjustments only add or subtract the difference from those baselines.

How it works in practice and tips

A 1,500 ft² warm-climate home with a 15 percent window ratio, four occupants, and 1 W/ft² lighting estimates near 4 tons. Treat the result as a starting point only: short-cycling from an oversized system hurts comfort, humidity control, and equipment life far more than slight undersizing. Always confirm with ACCA Manual J — and Manual S for equipment selection — before you specify a unit.