The unit of absolute viscosity in the British gravitational system of units, 1 pound-force second per square inch, approximately 6.89476 × 106 centipoise.

It is named for Sir Osborne Reynolds (1842–1912).



The English equivalent of the poise is the unit of absolute viscosity having the dimensions pound second per square inch, which is named the reyn in honor of Sir Osbourne Reynolds. At the 1957 London Conference on Lubrication and Wear, arranged by the Institution of Mechanical Engineers with the collaboration of the American Society of Mechanical Engineers, the two units of viscosity recommended were the centipoise and the reyn. However, the reyn like the poise is a very large unit. Recently a unit, one-millionth the size of the reyn, has been suggested and appropriately called the “microreyn.” Thus the viscosity of water, which is 1 centipoise at room remperature becomes 1.45 × 10⁻⁷ reyn or 0.145 microreyn. The microreyn is about the same order of size as the centipoise.

Other units of absolute viscosity are commonly used in the English-speaking world. These are: pound second per square foot, pound minute per square inch, and pound mass per foot hour.

Dudley D. Fuller.
Unit systems in bearing lubrication.
Carl F. Kayan, editor.
Systems of Units. National and International Aspects. A Symposium Organized by Section M on Engineering and Presented December 29-30, 1958, at the Washington Meeting.
Publication No. 57.
Washington: American Assoc. for the Advancement of Science, 1959.


As indicated, in most calculations the relevant viscosity is the absolute or dynamic viscosity. ... in engineering in the USA, UK and other English-speaking countries, the basic unit of absolute viscosity is the pound (force) — second per square foot. A derived unit, one pound (force) — second per square inch, is widely used. (This is sometimes called the ‘reyn’.)

A. C. Smith.
Some notes on the data on hydraulic oil properties required by systems designers.
Industrial Lubrication and Tribology, vol. 17, no. 2, pages 63 - 69, (1965)


In the English system of units absolute viscosity has units of lb-sec/in.² and are called reyns; that is,

1 lb-sec/in.² = 1 reyn

A reyn is a very large unit and a smaller unit, or microreyn, is often used.

1 microreyn = 10⁻⁶ reyn

…Without difficulty it can be shown that conversion factors between the two systems of units [English and cgs] are 1.45 × 10⁻⁷ reyn/cp and 1.45 × 10⁻⁵ reyn/poise.

Herbert E. Merritt.
Hydraulic control systems.
New York: Wiley, 1967.


... This Viscosity is Absolute Viscosity and has the units of— Poise in the CSG System; BVU in the British Engineers Units; Reyn in the American Code of Units. Of these the Reyn is the biggest and the Poise the smallest. 1 Reyn= 144 BVU = 69,003 poise.

S. W. Willmer.
A rational exposition of viscosity in engineers terms.
Industrial Lubrication and Tribology, 1969.

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