Dyne and poundal: the absolute units that lost the argument
What this conversion does
These two units come from systems that were trying to do the same thing — define force from mass and acceleration alone, without borrowing the strength of gravity. The dyne uses the centimetre and the gram, the poundal the foot and the pound. Each was eventually displaced, one from above by the newton and the other from below by the pound-force, and this page is where the two of them meet.
Two attempts at an absolute unit
A poundal is the force that gives a pound mass an acceleration of one foot per second squared, exactly as a dyne gives a gram one centimetre per second squared. Neither refers to a weighing scale, and that was the point: the pound-force depends on the local value of gravity and therefore shifts with latitude, while an absolute unit does not. The trade was coherence in the definition for convenience in the workshop.
Dyne to poundal conversion table
Both units are built from a mass and a length alone, so every row here is a fixed ratio: a pound is 453.59237 grams and a foot is 30.48 centimetres, which puts 13825.5 dynes in a poundal in every laboratory on the planet.
| Dyne (dyn) | Poundal (pdl) |
|---|---|
| 1 | 0.0000723301385121 |
| 100 | 0.00723301385121 |
| 1000 | 0.0723301385121 |
| 10000 | 0.723301385121 |
| 100000 | 7.23301385121 |
| 1000000 | 72.3301385121 |
Why one of them lasted longer
The dyne had a soft landing, because the metre and the kilogram are whole-number multiples of the centimetre and the gram — the newton is a power of ten away, so no table of factors was ever needed. The poundal had no such luck. The pound-force is what a pound actually reads on a scale, it was more convenient for working engineers, and it won in practice long before SI settled the argument.
A little under a seventh, and one big factor
A poundal is a bit less than a seventh of a newton, at 0.138255 N, so a poundal figure is always smaller than the newton spelling of the same force. Going straight to dynes, hold on to 13825.5: a dyne figure divided by roughly fourteen thousand lands on poundals.
Frequently asked questions
What is a poundal in modern units?
It is the force that accelerates a pound mass at one foot per second squared, equal to 0.138255 newtons or 13825.5 dynes. It has no place in the International System and has largely gone from engineering practice, though it still appears in older textbooks and in mechanics problems set in imperial units.
Why did the poundal not catch on?
Engineers weighed things. A pound-force is what a one-pound object registers on a scale, so it needed no calculation at all, while the poundal asked the reader to separate mass from weight before anything could be written down. Convenience beat coherence and the pound-force stayed in the workshop.
Are the dyne and the poundal both absolute units?
Yes, and that is the connection between them. Each is defined purely from a mass, a length and a time, with no reference to gravity anywhere in the definition. The two systems picked different masses and different lengths, which is the only reason the conversion factor is not a round number.
Do any fields still use poundals?
Very few. Some imperial-unit mechanics teaching still introduces it, and a handful of older texts on ballistics and on textile machinery use it, but working engineers abandoned it decades ago. The dyne, by contrast, is still published today in surface science and biophysics.
How accurate is the factor quoted here?
It is exact to the precision of the definitions. A pound is 453.59237 grams and a foot is 30.48 centimetres, both fixed by international agreement, which puts a poundal at 13825.4954 dynes; the six-figure value used on this page is rounded only in the last place.