Newtons to poundals: the absolute unit that lost the argument
What this conversion does
The poundal is what happens when an imperial force unit is built the way the newton is built: one pound of mass, accelerated at one foot per second squared, with gravity left out of the definition entirely. It is a principled unit, and it is small — one poundal is about 0.138 newtons, so a force of ten newtons is seventy-two poundals. Converting means multiplying by 7.233, and the answer is what a foot-pound-second drawing would have printed.
Absolute against gravitational
The newton is an absolute unit: mass, length and time only, valid anywhere in the universe. The poundal was designed as the imperial equivalent, and by that standard it is the more modern of the two imperial forces, since the pound-force still leans on the gravity of one particular planet. Two absolute units in two systems, then, and one of them is used by every engineer alive while the other is a footnote.
Newton to poundal conversion table
Forces across the range where poundals were once the natural unit, from a hand press to a small engine. The middle rows show why the numbers grow so quickly: the poundal is a small unit by design.
| Newton (N) | Poundal (pdl) |
|---|---|
| 1 | 7.23301385121 |
| 10 | 72.3301385121 |
| 100 | 723.301385121 |
| 500 | 3616.5069256 |
| 1000 | 7233.01385121 |
| 9806.65 | 70931.635284 |
| 10000 | 72330.1385121 |
Why the pound-force won anyway
The poundal is coherent in the foot-pound-second system, which keeps the arithmetic clean: no factor of 32.174 appears when a mass is multiplied by an acceleration. That elegance turned out not to matter. Engineers already owned spring scales, pressure gauges and steam tables calibrated in pounds of force, and the pound-force let them keep every existing number and every existing instrument. The poundal asked them to rewrite a century of tables for a consistency they did not feel they were missing.
Spot a poundal figure in a conversion table
Poundal values run about seven times larger than the same force in newtons, which is the quickest way to recognise one. If a force of a few newtons has turned into a two-digit number, you are most likely looking at poundals rather than pound-force; pound-force figures, by contrast, are always smaller than the newton value they came from.
Frequently asked questions
What exactly is a poundal?
It is the force that gives a one pound mass an acceleration of one foot per second squared. The definition names mass, length and time and mentions gravity nowhere, which is the whole point of an absolute unit. One poundal works out at 0.138254954376 newtons.
Why did engineers keep pound-force instead?
Habit, instruments and existing paperwork. A pound-force is what a spring scale reads and what a pressure gauge was calibrated against, so switching to poundals would have invalidated every table, chart and standard already in the filing cabinet. The poundal was more consistent, and consistency was not enough to overcome a century of practice.
Where do poundals still appear today?
In textbooks on mechanics, in the foot-pound-second sections of older reference works, and in some aeronautical and textile-testing literature. They survive mainly as a reminder that the imperial system had a coherent absolute branch, and as an examination question about the difference between absolute and gravitational units.
How do I go back from poundals to newtons?
Divide the poundal figure by 7.233, or equivalently multiply it by 0.138255. A force of 100 poundals is therefore 13.83 newtons. The same pair of numbers serves both directions of this conversion, which is a useful check when a result looks suspiciously large.
Is the poundal an SI unit?
No. It belongs to the foot-pound-second family of units, while the SI system is built on the metre, the kilogram and the second. It is not even the imperial unit most people meet, since the pound-force took that role in trade and engineering long ago.