How To Convert Weight Units
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Mass versus weight
Weight conversion hides a surprising amount of nuance behind what looks like simple multiplication. Mass and weight are technically different—weight is the force gravity exerts on mass, mass is the amount of matter—but in daily use the two words are used interchangeably when gravity is approximately constant (i.e., on Earth). Beyond that, pounds come in two flavors (avoirdupois for everyday, troy for precious metals), ounces in three (avoirdupois, troy, fluid), and grains show up in ammunition and pharmaceutical contexts. Kitchen scales use grams; bathroom scales use pounds or kilograms depending on the country; shipping uses kilograms or pounds; bullets use grains; gold uses troy ounces. This guide covers metric and imperial mass units, the troy versus avoirdupois distinction, grains, carats, and where each is the professional standard.
The metric ladder
Avoirdupois is the everyday system used for grocery scales, body weight, shipping, and cooking in the US.
The imperial ladder (avoirdupois)
The exact conversion was fixed in 1959 by international agreement: 1 pound = 0.45359237 kg. Every other avoirdupois conversion follows from that.
Metric ↔ imperial
US recipes often use volume (cups, tablespoons) for solids, which is imprecise because flour packs differently. European recipes use weight (grams), which is reproducible. Common conversions:
Troy vs. avoirdupois
Serious bakers use digital scales and weight-based recipes for consistency. Volume measurements of solids can vary by 20% depending on packing.
Grains
The US uses pounds. The UK uses stones and pounds (“12 stone 5” = 173 lb). Most of the rest of the world uses kilograms. BMI calculations demand consistent units—BMI = kg / m², and the imperial version is different (lb / in² × 703). Mixing units in a BMI calculation is a common source of laughable results.
Carats and karats
SI uses the kilogram. Chemistry uses grams and molar masses. Physics uses kilograms in formulas and tolerates grams for small samples. Pharmacology uses milligrams and micrograms for drug doses (mcg, mg, g), where a factor-of-1000 error is a dispensing disaster. Always carry units through calculations; never divide a gram by a kilogram without noting the factor-of-1000.