Type "kg to lb" into Google and the answer is one click away. Type "cP to SSU" and you get a textbook chapter. Engineering unit conversion is a different problem from kitchen unit conversion. Most online tools only handle the kitchen problem.
There are three horsepowers
Mechanical horsepower (HP, US) = 550 ft·lbf/s = 745.7 W. Metric horsepower (PS, Pferdestärke) = 75 kgf·m/s = 735.5 W. Boiler horsepower (BHP) = 33,475 BTU/h = 9,810 W — used for steam systems.
A car spec sheet that lists "200 HP" might mean any of these. The European convention is metric (PS). US convention is mechanical. Boiler horsepower shows up only in HVAC and industrial steam contexts.
Converting between them isn't the conversion most people expect. 200 HP (US) ≠ 200 PS — it's 197 PS, a 1.5% difference that compounds in engineering specs.
Viscosity is three different things
Dynamic viscosity (units: cP = centipoise = mPa·s). Kinematic viscosity (units: cSt = centistokes = mm²/s). Saybolt Universal Seconds (SSU) — an empirical measurement based on how long a fluid takes to drain through an orifice.
They aren't freely convertible. cP to cSt requires knowing density. SSU to cSt has lookup tables (it's nonlinear above 70 SSU). A spec that says "100 cP" and another that says "100 SSU" are not the same fluid.
Real engineering work — tribology, lubricant selection, oil specs — needs all three units side by side. Most consumer unit converters list "cP" alone or skip viscosity entirely.
The "weight" rating on motor oil (5W-30, 10W-40) is a viscosity grade defined by SAE J300, not a single number. Real spec sheets cite kinematic viscosity at 40°C and 100°C separately.
Radiation has three quantities that look interchangeable
Absorbed dose: Gray (Gy) or rad — energy per kg of tissue. Equivalent dose: Sievert (Sv) or rem — absorbed dose × radiation weighting factor (alpha radiation hits tissue ~20× harder than beta). Activity: Becquerel (Bq) or Curie (Ci) — disintegrations per second.
1 Gy ≠ 1 Sv. They have the same SI base units (J/kg) but mean different things. A nuclear medicine context uses Sv (biological effect). A material physics context uses Gy (energy). News reports often confuse them.
Torque conventions are direction-sensitive
Newton-meter (N·m) is the SI unit. The US customary unit is pound-foot (lbf·ft) — and pound-INCH (lbf·in), which is 1/12 of pound-foot but commonly written as "in-lb" or "in lbs" in US specs. Mixing them up is a 12× error.
Aerospace and high-precision automotive specs frequently use lbf·in. Standard automotive uses lbf·ft. The unit isn't always explicit on a torque wrench.
Our unit converter handles torque (N·m, lbf·ft, lbf·in, kgf·m), all viscosity systems (cP, Pa·s, cSt, SSU), radiation absorbed/equivalent/activity (Gy, Sv, Bq, plus US units), magnetic flux density, electrical resistivity, semiconductor wafer specs, and the three definitions of horsepower. All units in a category show side-by-side; type any field and the rest update.