Kilogram

The kilogram is the SI base unit of mass. One kilogram equals about 2.20462 pounds.

Until 2019 the kilogram was defined by a physical object — the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept in a vault in Sèvres, France since 1889. Every other mass measurement on Earth was, ultimately, traceable to that artifact.

The problem: the IPK and its official copies were drifting in mass over time, on the order of 50 micrograms per century. For most practical use this was invisible; for the most precise scientific work it was a hard ceiling on accuracy.

In May 2019 the kilogram was redefined using a fixed value of the Planck constant (6.62607015 × 10⁻³⁴ joule-seconds, exact by definition). The new definition is based on a fundamental physical constant rather than an artifact. Any laboratory equipped with a Kibble balance can now realise the kilogram independently — no central reference object needed.

Practical conversions: 1 kg = 1000 g; 1 lb = 0.45359237 kg exactly; 1 stone (UK) = 6.35029 kg; 1 metric ton = 1000 kg; 1 US ton = 907.185 kg; 1 UK long ton = 1016.05 kg.

Mass vs weight — the everyday confusion that physics teachers care about: mass is the amount of matter in an object (kilograms), weight is the force that mass exerts in a gravitational field (newtons). A 70-kg person has 70 kg of mass on Earth, on Mars, and in deep space — but weighs roughly 686 N on Earth, 261 N on Mars, and effectively 0 N in free fall. Bathroom scales calibrated for Earth’s gravity report mass-in-kilograms, but they are actually measuring force and inferring mass via g = 9.80665 m/s². On a precise scale at the top of a mountain (lower gravity, by about 0.3%) the reading would drop slightly — which is occasionally an issue in scientific weighing and is handled with regional gravity corrections.

Why the redefinition mattered in practice: for industry and consumers nothing changed — the new kilogram was constructed to equal the old kilogram to within measurement noise. For metrology laboratories, the change ended a quiet crisis. National copies of the IPK had been drifting differently from the original — the discrepancy grew by tens of micrograms over a century, which on a 1-kg artifact is a relative drift of parts in 10⁸. After 2019, any lab with a Kibble balance can realise a kilogram from the Planck constant directly, without ever touching a copy of a Paris cylinder. See our kg to lb converter for everyday mass conversions. Reference: BIPM — SI base units.

Worked example

Convert a 165-lb adult to kilograms using the exact factor: 165 × 0.45359237 = 74.84 kg. Now compute that person’s weight (force) on the surface of Earth: F = m × g = 74.84 × 9.80665 = 734.0 N. On Mars (g ≈ 3.711 m/s²) the same person would weigh 74.84 × 3.711 = 277.7 N, equivalent to standing on Earth carrying about 38% of your body mass. Going the other direction: a 2-kg dumbbell labelled in grams reads 2000 g; the same dumbbell costed by the gram in materials at $0.08/g of stainless steel = $160 of stock. The exactness of the pound conversion factor is the key reason the US-customary system can be exchanged with metric without rounding drift in trade documents.

When and why it matters

Dosage calculations in medicine are quoted “per kilogram body weight” specifically because the kilogram is invariant — paracetamol at 15 mg/kg gives a 70-kg adult 1050 mg per dose, and confusing pounds for kilograms is a documented cause of paediatric overdose errors (the WHO recommends weights be recorded in kg only on hospital charts to remove the ambiguity). Aviation freight and shipping container loading work in kg for the same reason. The kilogram’s 2019 redefinition matters most to national metrology labs (NIST, NPL, PTB), pharmaceutical reference-standard manufacturing, and the semiconductor industry where wafer-mass measurements feed into atomic-density calculations. For consumers nothing changed, but the change closes a long-standing scientific embarrassment that the world’s mass standard was an object that gained or lost mass when it was cleaned. Reference: NIST — Kilogram: Mass and Planck’s Constant.