Convert weight units between units, using exact SI factors.
One international pound is exactly 0.453 592 37 kilograms. This too is a definition rather than a measurement, in force since 1959. The SI base unit here is the kilogram. Eighty kilograms is therefore about 176.4 pounds. An ounce is one sixteenth of a pound, or 28.349 grams.
Mass is the amount of matter and does not change anywhere — on Earth or on the Moon. Weight is the force gravity exerts on that mass, and it varies with gravity. Everyday speech treats them as synonyms, but in SI, per the BIPM brochure, the unit of mass is the kilogram and the unit of force is the newton.
A metric tonne is 1,000 kilograms. It should not be confused with the US short ton (907.18 kg) or the British long ton (1,016.05 kg), both still found in freight paperwork. The tonne is not an SI base unit, but the BIPM brochure accepts it for use alongside SI.
Until 2019 the kilogram was the last SI unit defined by a physical object — a platinum-iridium cylinder in Paris. Its mass drifted slightly over decades, which meant the unit itself was drifting. BIPM now defines it through the Planck constant, so it no longer depends on any artefact.