Reference

BTU per Square Foot Table by Climate Zone

The rule-of-thumb loads behind every sizing calculator on this site, by climate zone, with the factors that adjust them and the room air conditioner chart.

Last updated

How to use the table

Find your DOE climate zone, multiply the floor area by the figure in the cooling or heating column, and apply the adjustment factors below. The cooling figures are before occupants and the kitchen; the heating figures are the heat the house loses on the design night, which a furnace’s output, not its input, must match. These are first estimates for an average-insulated house with 8 ft ceilings, compiled from published climate-zone sizing charts. An ACCA Manual J load calculation replaces them for a final equipment choice.

cooling load = floor area × cooling figure × insulation × ceiling × sun × windows + people + kitchen heating load = floor area × heating figure × insulation × ceiling × windows

Loads, design temperatures and degree days by zone

Zone Example cities Cooling, BTU/hr per sq ft Heating, BTU/hr per sq ft Winter design temperature Heating degree days Cooling degree days
1 hot-humid Miami, Honolulu 24 20 50 °F 300 4,300
2 hot Houston, Phoenix, Orlando 22 25 35 °F 1,500 2,900
3 warm Atlanta, Dallas, Los Angeles 20 30 25 °F 2,800 1,800
4 mixed Washington DC, St. Louis, Seattle 18 38 15 °F 4,500 1,200
5 cool Chicago, Denver, Boston 15 45 0 °F 6,300 800
6 cold Minneapolis, Burlington 13 52 −10 °F 7,800 500
7 very cold Duluth, Fargo 12 60 −20 °F 9,500 250

Design temperatures are typical 99% winter values; degree days are base 65 °F, rounded from NOAA normals for representative cities. For a specific address, use the nearest weather station’s figures.

Adjustment factors

Factor Values Source
Insulation 1.2 poor, 1.0 average, 0.85 good Common sizing-chart practice
Ceiling height +10% for each foot above 8 ft Common sizing-chart practice
Sun 0.9 shaded, 1.0 average, 1.1 very sunny ENERGY STAR room AC guidance
Windows 0.95 few, 1.0 average, 1.1 many Common sizing-chart practice
Occupants +600 BTU/hr per regular occupant beyond two (cooling) ENERGY STAR room AC guidance
Kitchen +4,000 BTU/hr (cooling) ENERGY STAR room AC guidance

Cooling sizes are rounded up to the next half ton, with a minimum of 1.5 tons, and design airflow is 400 CFM per ton (350 in humid climates, 450 in dry ones).

A 1,500 sq ft house in each zone

Average insulation, 8 ft ceilings, three occupants and a kitchen, as the calculators give it:

Zone Cooling load AC size Heating load (output) Furnace input at 95% AFUE
1 40,600 BTU/hr 3.5 tons 30,000 BTU/hr 40,000
2 37,600 BTU/hr 3.5 tons 37,500 BTU/hr 40,000
3 34,600 BTU/hr 3 tons 45,000 BTU/hr 60,000
4 31,600 BTU/hr 3 tons 57,000 BTU/hr 60,000
5 27,100 BTU/hr 2.5 tons 67,500 BTU/hr 80,000
6 24,100 BTU/hr 2.5 tons 78,000 BTU/hr 100,000
7 22,600 BTU/hr 2 tons 90,000 BTU/hr 100,000

ENERGY STAR room air conditioner chart

For window and portable units, sized room by room, before the sun, occupant and kitchen adjustments:

Room area up to Capacity Room area up to Capacity
150 sq ft 5,000 BTU/hr 700 sq ft 14,000 BTU/hr
250 sq ft 6,000 BTU/hr 1,000 sq ft 18,000 BTU/hr
300 sq ft 7,000 BTU/hr 1,200 sq ft 21,000 BTU/hr
350 sq ft 8,000 BTU/hr 1,400 sq ft 23,000 BTU/hr
400 sq ft 9,000 BTU/hr 1,500 sq ft 24,000 BTU/hr
450 sq ft 10,000 BTU/hr 2,000 sq ft 30,000 BTU/hr
550 sq ft 12,000 BTU/hr 2,500 sq ft 34,000 BTU/hr

Portable units deliver less than their ASHRAE label: the real cooling (SACC) is about 55% of it for a single-hose unit and 65% for a dual-hose unit.

Calculators that use these figures

The guide to BTU per square foot by climate zone explains the figures and their limits.