kilogram-force

The kilogram-force answers the question people naturally ask: how hard does a kilogram press down? Making that a unit lets an ordinary scale, which measures force, be read directly as mass, and for decades European machine builders specified everything in it: bolt torques in kgf·m, cable tensions in kgf, pressures in kgf/cm². The price is the same one the pound and pound-force pair charges. A spec sheet saying a bracket holds "150 kg" almost always means 150 kgf, a force, and feeding that number into $F = ma$ as if it were a mass is silently wrong by a factor of 9.80665.

The unit survives inside one definition still printed on every European car brochure: the metric horsepower, PS or CV, is 75 kgf·m/s, so the kilogram-force is baked into every PS rating even in countries that dropped the unit itself decades ago. Soviet and Russian rocketry took the gravitational logic to larger scale, quoting engine thrust in tonne-force, a thousand kgf, so that thrust and vehicle mass shared a number line and the thrust-to-weight ratio could be read straight off the datasheet.

Wherever the unit persists, in tyre and boiler gauges marked kg/cm² and in older torque tables, the conversion is exact by definition, 9.80665 N per kgf, because standard gravity is a defined number rather than a measurement. Local gravity differs from it by up to about 0.3 percent, which is the sense in which the kilogram-force says what a kilogram weighs by convention, not on your particular floor.