Foot pounds-force second to Donkeypower
ft·lbf/s
dp
Conversion History
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Quick Reference Table (Foot pounds-force second to Donkeypower)
| Foot pounds-force second (ft·lbf/s) | Donkeypower (dp) |
|---|---|
| 1 | 0.00542327179334032 |
| 10 | 0.0542327179334032 |
| 100 | 0.542327179334032 |
| 200 | 1.084654358668064 |
| 550 | 2.982799486337176 |
| 1,000 | 5.42327179334032 |
| 5,000 | 27.1163589667016 |
About Foot pounds-force second (ft·lbf/s)
Foot pounds-force per second (ft·lbf/s) is the fundamental mechanical power unit in the US customary system, equal to approximately 1.356 watts. It is the rate of doing work when a force of one pound-force moves through one foot per second. The unit is directly related to the mechanical horsepower: one horsepower equals exactly 550 ft·lbf/s. It appears in US mechanical engineering, ballistics, and machinery design texts.
One mechanical horsepower equals 550 ft·lbf/s (~746 W). A person climbing stairs moderately expends about 200–300 ft·lbf/s of mechanical power.
About Donkeypower (dp)
Donkeypower (dp) is a humorous but technically defined unit equal to 250 watts — approximately one third of a mechanical horsepower. The unit was proposed as a more modest alternative to horsepower for rating small engines and motors, since most small machines operate at power levels far below one horsepower. Despite its informal origin, it is occasionally cited in engineering education to illustrate the arbitrariness of unit naming and the scale between common power sources.
A strong human cyclist at sustained effort produces about 0.8–1 donkeypower (200–250 W). A typical hand drill draws about 0.3 donkeypower (75 W).
Etymology: The name is a playful extension of "horsepower" — a donkey being considerably less powerful than a horse. Proposed in various engineering contexts as a unit for the 250 W level, roughly matching the sustained output of a strong human athlete.
Foot pounds-force second – Frequently Asked Questions
Why is horsepower defined as exactly 550 ft·lbf/s?
James Watt calculated that a mill horse could turn a mill wheel 144 times per hour, doing 32,572 ft·lbf of work per minute — he rounded up to 33,000 ft·lbf/min (550 ft·lbf/s) for marketing purposes. He wanted to sell steam engines by comparing them to horses, so he likely overestimated the horse to make his engines look like better value. A real horse sustains closer to 350–500 ft·lbf/s, so Watt's "1 HP" is actually more than one horse.
How do you calculate ft·lbf/s from torque and RPM?
Power (ft·lbf/s) = Torque (ft·lbf) × RPM × 2π / 60. This is the workhorse formula (pun intended) of US mechanical engineering. For example, an engine producing 200 ft·lbf of torque at 3,000 RPM delivers 200 × 3,000 × 6.2832 / 60 = 62,832 ft·lbf/s ≈ 114 hp. The formula works because angular velocity in rad/s times torque in ft·lbf gives power directly in ft·lbf/s.
What everyday forces produce 1 ft·lbf/s of power?
Pushing with 1 pound of force at 1 foot per second — roughly the effort of slowly sliding a light book across a table against friction. Lifting a 1-pound weight 1 foot in 1 second. Turning a doorknob with a very light touch. It's about 1.36 watts — enough to dimly light an LED. In human terms, it's almost effortless: casual walking produces about 50–80 ft·lbf/s of mechanical power, and you don't even notice.
Is ft·lbf/s still used in modern American engineering?
Yes, particularly in ballistics (muzzle energy rates), mechanical testing (dynamometer output), agricultural machinery specs, and industrial equipment designed for the US market. However, even in the US, many engineering firms are switching to SI units for international compatibility. The automotive industry increasingly quotes power in both hp and kW. Aerospace has been mostly metric since the 1990s. Ft·lbf/s survives mainly in traditional mechanical and manufacturing industries.
How does ft·lbf/s relate to bullet kinetic energy?
Bullets are rated in ft·lbf of muzzle energy (not per second), but the power of a firearm is the muzzle energy divided by barrel time. A .308 rifle bullet exits with about 2,600 ft·lbf of energy over a barrel transit time of ~0.001 seconds, meaning the instantaneous power is roughly 2,600,000 ft·lbf/s (about 3,500 hp). That's why rifle recoil feels punchy — for a millisecond, you're absorbing the reaction force of a truck engine.
Donkeypower – Frequently Asked Questions
Is donkeypower a real unit or just a joke?
It's both. The value of 250 watts is well-defined and occasionally referenced in engineering education and humorous technical papers. It never achieved official recognition from any standards body, but it has appeared in legitimate engineering textbooks as a pedagogical tool. The unit highlights a genuine gap in the power scale — many household devices and human activities fall in the 50–500 W range where fractional horsepower feels awkward. "Two donkeypower" sounds better than "0.67 horsepower."
How powerful is an actual donkey compared to donkeypower?
A real donkey can sustain about 125–200 watts (0.5–0.8 dp) of useful mechanical work over a full day, and briefly peak at 750–1,500 W (3–6 dp) during a short burst. So the unit slightly overestimates a donkey's sustained output — much like horsepower overestimates a horse. A working donkey in a developing country might turn a water pump or grain mill for 6–8 hours, delivering roughly 0.5–0.7 dp of sustained useful work.
What common devices operate at about 1 donkeypower?
A desktop computer (200–300 W), a bread toaster on low setting (250 W), a box fan on high (200–250 W), a sewing machine motor (250 W), and a human cycling at a moderate sustainable pace (200–250 W). It's a delightfully human-scale unit — roughly the sustained mechanical output of one fit person, or the electrical draw of one modest appliance. Your entire body at rest produces about 0.35 donkeypower of heat.
Are there other animal-based power units besides horsepower and donkeypower?
There's "manpower" (about 75 W sustained, or 0.3 dp), which was used in ancient and medieval engineering for human-powered machines like treadwheels and capstans. "Oxpower" appears in some agricultural texts at roughly 500 W (2 dp). None are standardized. Some engineers have jokingly proposed "hamster power" (~0.5 W, so 0.002 dp) and "ant power" (~10⁻⁵ W). The zoo of animal power units perfectly illustrates why SI standardisation was necessary.
Could donkeypower replace horsepower for small appliances?
It would actually make more sense for many applications. A blender at "3/4 horsepower" sounds industrial; "2 donkeypower" is more honest and relatable. A hand mixer at "1/8 hp" is awkwardly fractional; "1/3 donkeypower" is cleaner. But the ship has sailed — horsepower (and watts) are too entrenched. Donkeypower's true legacy is as a beloved teaching tool and pub quiz answer. It reminds engineers that units are human inventions, not laws of nature.