Mass & Force Converter
kg, lb, oz, slug — and their weights in N, kgf, lbf at standard gravity.
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The engineering
Mass and force share names in customary units, which is the whole hazard: 'pound' means lb (mass) on a scale and lbf (force) on a load cell, and kgf haunts older European gauges and Asian machine specs. This card bridges the two families through standard gravity g₀ = 9.80665 m/s² (fixed by convention in 1901) — the force rows are what each mass *weighs* at g₀, and the mass rows are what each force corresponds to there. On the Moon or in a centrifuge, the bridge factor changes; the masses don't. The slug (14.594 kg) is the customary unit that keeps F = ma honest: 1 lbf accelerates 1 slug at 1 ft/s².
This is the conversion that killed a spacecraft. In 1999 the Mars Climate Orbiter burned up because thruster impulse was delivered in pound-force-seconds and consumed as newton-seconds — a silent factor of 4.448 accumulated over months of trajectory corrections, and a $327M mission hit the atmosphere 170 km low. The everyday versions of the same mistake: quoting thrust in 'kg', mixing lb and lbf in F = ma without g_c, and calibrating a load cell in mass units.
Where this math comes from
The kilogram is revolutionary France again: defined from a liter of water, realized as the 1799 Kilogramme des Archives, replaced by the platinum-iridium International Prototype in 1889, and finally cut loose from artifacts in 2019 when the CGPM fixed the Planck constant. The pound joined the metric bedrock in 1959, when the international agreement set it at exactly 0.45359237 kg.
The mass/force divorce was formalized by the 3rd CGPM in 1901: the kilogram is mass, weight is a force, and standard gravity is 9.80665 m/s² by decree — an average sea-level value frozen so that kgf and lbf mean the same thing in Quito and Oslo. Engineering dialects kept regenerating the confusion (slugs, poundals, kgf, and g_c in US textbooks), and in 1999 the Mars Climate Orbiter demonstrated the stakes at planetary scale.
- 1799French AcademyKilogramme des Archives — the kilogram realized.
- 19013rd CGPMMass vs weight separated; g₀ = 9.80665 m/s² fixed.
- 1959Six-nation agreement1 lb = 0.45359237 kg exactly.
- 1999NASA / Lockheed MartinMars Climate Orbiter lost to a lbf·s vs N·s mix-up.
- 201926th CGPMKilogram redefined via the Planck constant.
See the full timeline of the math behind every calculator →
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