Dynamic viscosity, or absolute viscosity, measures a fluid's internal resistance to flow under an applied shear stress. In engineering, converting between the Imperial system and the CGS (centimeter-gram-second) system is a frequent requirement for fluid mechanics calculations.
Pound per foot-second (lb/(ft·s)): This is the English Engineering unit for dynamic viscosity. It represents the force in pounds-force required to move a plane surface of one square foot at a velocity of one foot per second relative to another plane surface separated by a distance of one foot.
Poise (P): Named after Jean Léonard Marie Poiseuille, the Poise is the CGS unit of dynamic viscosity, defined as 1 dyne-second per square centimeter. It is commonly used in chemistry and rheology.
The relationship between these units is derived from the conversion of mass, length, and time units. The conversion factor is defined as \(1 \text{ lb/(ft·s)} \approx 14.88164 \text{ P}\).
Pound per foot-second to Poise Conversion Reference Table
Pound per foot-second (lb/(ft·s))
Poise (P)
0.1
1.4882
0.5
7.4408
1.0
14.8816
2.0
29.7633
5.0
74.4082
10.0
148.8164
20.0
297.6328
50.0
744.082
100.0
1488.164
500.0
7440.82
1000.0
14881.64
Conversion Example
To convert a dynamic viscosity value from lb/(ft·s) to Poise, multiply the given value by the conversion factor of 14.881639999999999.
The Poise is a relatively large unit for many common fluids like water, which has a viscosity of approximately 0.01 Poise at room temperature. Therefore, engineers typically use the centipoise (1 cP = 0.01 P) to provide more manageable numerical values.
No. The lb/(ft·s) unit is based on the pound-force, whereas the slug/(ft·s) is based on the pound-mass. Using the wrong unit can lead to significant errors in Reynolds number calculations; always verify if your source data uses force or mass-based units.
"On fait la science avec des faits, comme on fait une maison avec des pierres ; mais une accumulation de faits n'est pas plus une science qu'un tas de pierres n'est une maison." "Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house." — Henri Poincaré (French Mathematician, Theoretical Physicist & Mining Engineer)