How this calculator works
The foot-pound-force is an energy unit equal to 1.3558179483314004 J. Use mass, not pounds-force, in the kinetic-energy equation. This page keeps the imperial readout beside the SI result. Solving for mass at zero speed is indeterminate.
Formula
E = m v^2/2; m = 2E/v^2; v = sqrt(2E/m). 1 ft lbf = 1.3558179483314004 J.
Worked example
A 1 lb mass at 10 ft/s has 2.1070055047 J, or 1.5540475086 ft lbf. At 20 ft/s its energy is four times larger.
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Frequently asked questions
Is this a torque calculator?
No. Foot-pounds here measure energy. Torque uses a similar unit but describes a turning moment.
Can I use grains?
Choose the grains input option. The displayed mass result also includes grains.
Can I enter feet per second?
Yes. Select feet per second for the speed entry; the calculation converts to metres per second.