Foot-Pounds of Energy Calculator — ft·lbf to Joules
Compute the work done (or energy stored) when a force in pound-force acts over a distance in feet — and instantly see the result in joules, BTU, calories and kilowatt-hours.
lbf
ft
Energy = Force × Distance (1 ft·lbf = 1.3558 J)
- 1
Work in foot-pounds
10 lbf × 5 ft = 50W = F × d — force times distance. - 2
Convert to joules
50 × 1.3558 = 67.7909
How does this calculator work?
Foot-pounds of energy = force (lbf) × distance (ft). One ft·lbf equals 1.355 817 948 J exactly. Enter force and distance to get the work done in ft·lbf and see conversions to joules, BTU, calories and kWh.
Formula
How this is calculated
Work (or energy) equals force times distance: W = F × d. In the imperial system, force is measured in pound-force (lbf) and distance in feet (ft), giving work in foot-pounds (ft·lbf). One foot-pound is the work done lifting a one-pound weight through one foot against standard gravity.
The conversion to SI is exact: 1 lbf = 0.453 592 37 kg × 9.806 65 m/s² and 1 ft = 0.304 8 m, so 1 ft·lbf = 1.355 817 948… J. The calculator uses this exact factor. The BTU (British Thermal Unit) conversion uses 1 BTU = 1055.055 852 62 J (IT definition). Calories use the International Table calorie, 1 cal = 4.186 8 J.
Foot-pounds are commonly used in US and UK engineering contexts for torque specifications, firearm ballistics (muzzle energy), and mechanical work calculations. Note: torque is also expressed in ft·lbf but is dimensionally a moment (force times lever arm), not energy — numerically identical but physically distinct.
Frequently asked questions
They use the same units (ft·lbf) and the same numerical calculation, but they are different physical quantities. Torque (or moment) is a rotational force at a distance — it only becomes energy when an angular displacement occurs (energy = torque × angle in radians). A bolt torqued to 50 ft·lbf is not "storing" 50 ft·lbf of energy.
Exactly 1 J = 0.737 562 149… ft·lbf (the reciprocal of 1.355 817 948… J/ft·lbf). For a rough mental check: 1 joule ≈ 0.74 ft·lbf.
US firearm and hunting standards have long used ft·lbf for muzzle energy (½mv²). A 9 mm handgun typically produces around 350–450 ft·lbf (475–610 J); a .308 rifle around 2 600 ft·lbf (3 500 J). The convention persists because hunting regulations and ballistics tables were written in imperial units.
Also known as
TG we-Calculate Editorial Team. (2026). Foot-Pounds of Energy Calculator — ft·lbf to Joules [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/foot-pounds-of-energy-calculator
TG we-Calculate Editorial Team. "Foot-Pounds of Energy Calculator — ft·lbf to Joules." TG we-Calculate. 2026. https://we-calculate.com/calculator/foot-pounds-of-energy-calculator.
TG we-Calculate Editorial Team, "Foot-Pounds of Energy Calculator — ft·lbf to Joules," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/foot-pounds-of-energy-calculator
@misc{wecalculate_foot_pounds_of_energy_calculator, title = {Foot-Pounds of Energy Calculator — ft·lbf to Joules}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/foot-pounds-of-energy-calculator}}, year = {2026}, note = {TG we-Calculate} }
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