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Foot-Poundal to Joule Converter

Enter an energy in foot-poundals to convert it to joules.

How it is calculated

The conversion is one multiplication: joules = foot-poundals × 0.0421401100938048. The factor comes straight from the unit definitions. A poundal is the force that gives one pound of mass an acceleration of one foot per second squared, so 1 pdl = 0.45359237 kg × 0.3048 m/s² ≈ 0.138255 N. A foot-poundal is that force acting over one foot, so 1 ft·pdl = 0.138255 N × 0.3048 m ≈ 0.0421401 J.

The result is shown with six decimals and the unit J. Zero and negative values are accepted, and negatives keep their sign. Very small values are not collapsed to zero: the tool widens the decimals to keep about three significant digits, so 0.001 ft·pdl gives 0.000042 J. The second line repeats your input as ft·pdl; it is not a converted value. A decimal comma such as 12,5 is rejected and shows no result. A foot-poundal is not a foot-pound-force, which is about 32.17 times larger.

Worked example

Convert an energy of 250 foot-poundals. Using the factor, 250 × 0.0421401100938048 = 10.535028 J after rounding to six decimals. Checking the size: one ft·pdl is roughly 4.2 hundredths of a joule, so 250 of them should be around 250 × 0.042 = 10.5 J, which agrees. The second line shows 250.00 ft·pdl, confirming the value that was converted.

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Common mistakes

  • Confusing the foot-poundal with the foot-pound (ft·lbf). A foot-pound-force is about 1.3558 J, roughly 32 times more than a foot-poundal, so mixing them gives answers that are far off.
  • Using this for torque in lb·ft. The two have the same dimensions but torque is not energy, and torque in foot-pounds uses pounds-force.
  • Entering a decimal comma such as 12,5. The field needs a decimal point and otherwise shows no result.
  • Reading the second line as a converted-back value. It only repeats your input.
  • Rounding the factor to 0.04 before multiplying on large values. Use the six-decimal result instead.

About this calculator

The foot-poundal (ft·pdl) is a unit of energy from the little-used foot-pound-second (FPS) absolute system, where force is measured in poundals rather than pounds-force — distinguishing it from the more familiar foot-pound (ft·lbf) used in engineering. One foot-poundal equals exactly 0.0421401 joules, derived from the definitions of the foot, pound mass, and poundal (the force that accelerates one pound mass at one foot per second squared).

The unit rarely appears outside of older British physics and engineering textbooks, historical technical literature, and a handful of legacy mechanical engineering problems that specify torque or energy in poundal-based units, since the poundal system was largely abandoned in favor of the gravitational (pound-force) system and, eventually, SI.

This calculator converts a foot-poundal value to joules using the exact conversion factor, saving students working through older textbook problems, translators of historical engineering documents, and physics hobbyists from tracking down the poundal's precise definition themselves.

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Comments (1)

  • Editorial team

    Before trusting a historical text, check which unit it really uses. Under this tool's factor, 32.174 foot-poundals come to 1.355816 J, which is about one foot-pound-force, so the two units differ by a factor of roughly 32. Also remember that a decimal point is required, so 12,5 is rejected. If you come across a poundal-based figure in an old textbook, tell us what kind of problem it is from.

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