Foot pounds-force minute to Calories (th)/second
ft·lbf/min
cal(th)/s
Conversion History
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Quick Reference Table (Foot pounds-force minute to Calories (th)/second)
| Foot pounds-force minute (ft·lbf/min) | Calories (th)/second (cal(th)/s) |
|---|---|
| 100 | 0.54008044468414674952 |
| 1,000 | 5.40080444684146749522 |
| 5,000 | 27.0040222342073374761 |
| 10,000 | 54.0080444684146749522 |
| 33,000 | 178.22654674576842734226 |
| 100,000 | 540.08044468414674952199 |
| 330,000 | 1,782.26546745768427342256 |
About Foot pounds-force minute (ft·lbf/min)
Foot pounds-force per minute (ft·lbf/min) equals approximately 0.02260 watts. It is used in US mechanical engineering for low-power applications and in the historical definition of horsepower: one horsepower was defined by James Watt as 33,000 ft·lbf/min — the rate at which a horse could lift coal from a mine. This unit is now mostly encountered in legacy engineering references and historical machinery specifications.
One mechanical horsepower = 33,000 ft·lbf/min. A hand-cranked generator might produce 2,000–5,000 ft·lbf/min of mechanical power output.
About Calories (th)/second (cal(th)/s)
Calories (thermochemical) per second (cal(th)/s) equals 4.184 watts. It is a caloric power unit used in thermochemistry and laboratory heat-flow measurements where energy is expressed in thermochemical calories rather than joules. Reaction calorimeters and bomb calorimeters sometimes report heat release rates in this unit. It is closely related to the watt but retains the calorie convention of chemistry rather than physics.
A 60 W light bulb dissipates about 14.3 cal(th)/s as heat. A vigorous chemical reaction releasing 100 cal(th)/s generates 418 W of thermal power.
Foot pounds-force minute – Frequently Asked Questions
Where does the number 3,960 come from in US pump sizing formulas?
The pump horsepower formula HP = (GPM × Head in ft) / 3,960 hides a chain of unit conversions. Water weighs 8.33 lb per US gallon. Multiplying GPM × Head × 8.33 gives ft·lbf/min. Dividing by 33,000 ft·lbf/min per hp gives horsepower. So 33,000 ÷ 8.33 ≈ 3,960. The number is so ubiquitous in US mechanical engineering that pump designers recognize it on sight, yet few remember the derivation. It breaks down for fluids other than water — multiply by specific gravity for anything denser or lighter.
What real-world tasks produce 33,000 ft·lbf/min?
Lifting 330 lbs (150 kg) at 100 feet per minute — roughly the speed of a slow freight elevator. Or lifting 33 lbs at 1,000 ft/min (a fast dumbwaiter). A human on a bicycle sustainably produces about 5,000–10,000 ft·lbf/min (0.15–0.3 hp). A small outboard boat motor produces about 165,000 ft·lbf/min (5 hp). The unit makes intuitive sense for lifting and hoisting — the original application Watt cared about.
Why use ft·lbf/min instead of ft·lbf/s?
Historical convention and practical timescale. Mine hoists, waterwheels, and early steam engines operated at rates naturally measured per minute — the machinery completed one cycle every few seconds to minutes. Watt himself measured horses per minute because that's how mine work was timed. The per-minute unit also gives larger, more manageable numbers: "33,000 ft·lbf/min" is easier to work with than "550 ft·lbf/s" when you're doing longhand arithmetic in 1780.
How much ft·lbf/min can a human sustain?
A healthy adult can sustain about 4,000–6,000 ft·lbf/min (roughly 90–135 W or 0.12–0.18 hp) of useful mechanical work for hours — think steady cycling or rowing. Short bursts reach 15,000–25,000 ft·lbf/min (0.5–0.75 hp). Elite cyclists sustain 12,000+ ft·lbf/min (0.4 hp) for an hour. By Watt's definition, a horse sustains 33,000 ft·lbf/min, meaning one horse ≈ 5–8 sustained humans. The ancient rule of "ten slaves per horse" wasn't far off.
Is 33,000 ft·lbf/min still used in any modern calculations?
Yes — it's embedded in US pump and fan engineering. The formula for pump horsepower is: HP = (GPM × Head in ft × Specific Gravity) / 3,960, where 3,960 = 33,000 / (8.33 lb/gal). The number 33,000 ft·lbf/min lurks inside every US pump sizing calculation, even if the engineer never writes it explicitly. It also appears in ASME standards for hoists, cranes, and elevators — anywhere lifting power needs to be specified.
Calories (th)/second – Frequently Asked Questions
Why do chemists use calories per second instead of watts?
Tradition and unit consistency. When your energy measurements are in calories (specific heat of water = 1 cal/g/°C makes calculations beautifully clean), expressing rates in cal/s keeps everything in the same system. A chemist measuring how fast a reaction heats 500 mL of water doesn't want to convert to joules just to report a rate. The calorie makes water-based calorimetry arithmetic almost trivial.
What is the difference between a calorie and a Calorie in this context?
The thermochemical calorie (lowercase "c") used in cal/s equals 4.184 joules. The food Calorie (uppercase "C" or kilocalorie) is 1,000× larger at 4,184 joules. So 1 food Calorie/s = 4,184 watts — roughly the power of a space heater. Nutrition labels use kilocalories but write "Calories" with a capital C, creating one of the most persistent unit confusions in science. When you see cal/s in chemistry, it's always the small calorie.
How many cal/s does an exothermic chemical reaction typically release?
It varies enormously. Neutralizing a strong acid with a strong base might release 0.5–5 cal/s in a teaching lab. Combustion of magnesium ribbon produces 50–200 cal/s of intense white-hot heat. Thermite reactions can exceed 10,000 cal/s (42 kW). Explosive decomposition of TNT releases energy at roughly 250,000 cal/s (1 MW) during detonation. The rate depends on both the enthalpy change and how fast the reaction proceeds.
How do you measure heat output in calories per second experimentally?
A reaction calorimeter submerges the reaction vessel in a known mass of water and measures temperature rise over time. If 1,000 g of water rises 0.5°C in 10 seconds, the heat release is 500 cal in 10 seconds = 50 cal/s. Modern isothermal calorimeters use Peltier elements to maintain constant temperature, measuring the electrical power needed to compensate — giving cal/s readings with milliwatt precision.
Is the thermochemical calorie still used in modern research?
Increasingly rarely. IUPAC officially recommends joules, and most modern journals require SI units. However, the calorie persists in biochemistry (metabolic rates), nutrition (food energy), and some physical chemistry subfields where decades of reference data are in calories. Older researchers and textbooks still think in calories. The 4.184 conversion factor is burned into every chemist's brain, even if they wish it weren't.