Two masses, two multipliers: kilogram-force to poundals
What separates the two units
Both units begin with a mass and then ask a different question about it. The kilogram-force multiplies one kilogram by the acceleration gravity gives it; the poundal multiplies one pound by one foot per second squared, a number chosen for arithmetic rather than observed anywhere in nature. The two therefore sit on opposite sides of a split that ran through nineteenth-century mechanics, and the factor between them, 70.9316, is a record of that disagreement rather than a property of any particular object.
Gravitational against absolute
A gravitational unit is convenient on the surface of the planet where it was defined, because a load can be described by the mass that produces it: a twenty kilogram crate, a two tonne pallet. An absolute unit is defined from mass, length and time alone, so it means the same thing on a ship, in a laboratory or in orbit. Victorian engineers preferred the first for its familiarity and physicists the second for its coherence, and the poundal is what the physicists produced for imperial measure.
Kilogram-force to poundals conversion table
Loads in both units, with the poundal column showing what a unit built on one foot per second squared looks like beside one built on standard gravity.
| Kilogram-force (kgf) | Poundal (pdl) |
|---|---|
| 1 | 70.931635284 |
| 10 | 709.31635284 |
| 25 | 1773.2908821 |
| 50 | 3546.5817642 |
| 100 | 7093.1635284 |
| 1000 | 70931.635284 |
Two masses, two accelerations
The poundal is what you get when gravity is kept out of the definition. Take one pound, apply an acceleration of one foot per second squared, and the force is one poundal — no standard gravity, no reference to a particular place on the surface of the Earth. The kilogram-force takes the opposite route and folds the standard acceleration into the unit itself. Divide one by the other and the two masses contribute their ratio while the two accelerations contribute theirs.
A factor near seventy, not thirty-two
Two familiar numbers combine here. The kilogram-to-pound ratio is 2.20462 and the ratio of standard gravity to one foot per second squared is 32.174; multiply them and the answer is 70.9316, which is the whole conversion. Knowing that split is a good way to remember which constant belongs to which pair of units, and to keep 32.174 from being applied where it has no business.
Frequently asked questions
What makes the poundal different from the pound-force?
The pound-force is the weight of a pound under standard gravity; the poundal is the force that accelerates a pound by one foot per second squared. Same mass, two accelerations, and a factor of 32.174 between the results. Only one of the two is defined without borrowing a number from the planet.
Why would anyone use a unit built on one foot per second squared?
Because it keeps force, mass and acceleration in one coherent set with no hidden constant. In a poundal-based calculation, force equals mass times acceleration and nothing else appears in the formula, which is exactly what SI does with the newton and the kilogram. The pound-force version needs gravity inserted at every step.
Is the poundal still used anywhere?
Rarely. It survives in older textbooks, in some aeronautical and civil engineering literature written before metrication, and in problems set to demonstrate the coherent-unit idea. Modern imperial practice uses the pound-force and accepts the gravitational constant sitting in the equations.
How does the metric side of the family compare?
The kilogram-force is the metric gravitational unit and the newton is its absolute counterpart, occupying exactly the relationship that the pound-force and the poundal occupy in imperial measure. The newton won that argument outright, so metric practice carries no live split, while imperial practice kept both units in circulation.
Which number do I use for a load given in poundals?
Divide by 70.9316 to reach kilogram-force. A thousand poundals is 14.0981 kilogram-force, and fourteen kilogram-force is a sack of potatoes, which is the quickest way to see that the poundal is a small unit and the kilogram-force a large one.