A unit of thermal resistance used in describing the insulating value of clothing, defined in 1941¹ with the goal of providing a unit that would be more descriptive than measurements expressed directly in calories, square meters, etc.

One clo is the amount of thermal resistance which “is necessary to maintain in comfort such a sitting-resting subject in a normally ventilated room (air movement 20 ft/min or 10 cm/sec) at a temperature of 70°F (20°C) and a humidity of the air which is less than 50 per cent.” Taking into account the above conditions, the insulating power of air, and heat loss through perspiration, one clo of thermal resistance is

A fraction, 0.18 times the temperature difference in degrees Celsius divided by calories per hour per square meter


A fraction, 0.88 times the temperature difference in degrees Fahrenheit divided by BTU per hour per square foot

The value of 0.18 was chosen so that 1 clo would be roughly the insulating value afforded by a man’s underwear and a lightweight suit, or “a heavy top coat alone.”

One clo is approximately 0.155 square meter kelvin per watt.

The following table gives the ideal insulating value, in clo, for clothing under various conditions.

Environmental temp. Resting sitting Slow level walking Normal level walking Fast level walking
70°F – Normal outdoors 1.5 0.7 0.4 0.3
50°F – Normal outdoors 3.1 1.5 0.9 0.7
30°F – Normal outdoors 4.7 2.3 1.5 1.1
0°F – Normal outdoors 7.2 3.5 2.3 1.7

1. A. Pharo Gagge, A. C. Burton, and H. C. Bazett.
A practical system of units for the description of the heat exchange of man with his environment.
Science, vol 94, number 2445 (November 7, 1941). Page 429.  [back]

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