Weight Conversion Guide

Strictly, weight is the force gravity exerts on a mass — and so it varies slightly with location, since gravity is about 0.5% weaker at the equator than at the poles, and about 17% weaker on the surface of the Moon. In everyday speech and in this category, 'weight' almost always means mass: a kilogram is a kilogram on Earth, on the Moon, and in orbit. We treat the two interchangeably because nearly every weight you'll convert is measured on a calibrated scale that reads in mass-equivalent units. When physics demands the distinction (rocket payloads, planetary geology), the right unit is the newton and the right category is force.

Mass is conserved; a kilogram of feathers and a kilogram of steel weigh the same on any scale. The familiar riddle is a unit-confusion test, not a physics test — the trap is that 'pound of feathers' might mean a troy pound (12 troy ounces) and 'pound of steel' an avoirdupois pound (16 avoirdupois ounces), and those two pounds genuinely are different masses.

Pre-modern weight was even more fragmented than length. Avoirdupois pounds (16 ounces), troy pounds (12 ounces, used for precious metals), apothecary pounds (12 ounces, used by pharmacists), and tower pounds (12 ounces, used by the English mint) all coexisted in Britain alone. The same word — 'pound' — referred to four different masses, depending on what you were weighing and who was weighing it. The French Revolution introduced the gramme and the kilogramme as part of the metric system in the 1790s, anchored at first to the mass of one cubic decimetre of water at its temperature of maximum density.

From 1889 to 2019 the kilogram had a unique status: it was the only SI base unit still defined by a physical artifact, the International Prototype of the Kilogram (the 'Le Grand K' platinum-iridium cylinder kept near Paris). The artifact's mass drifted measurably against its sister copies over the 20th century, and in 2019 the kilogram was redefined in terms of the Planck constant: the value of h is now fixed at 6.62607015 × 10⁻³⁴ J·s, and the kilogram falls out of that definition. The change is invisible at any human-scale measurement but eliminates the artifact-drift problem permanently.

The pound was reconciled with the kilogram in the same 1959 International Yard and Pound Agreement that fixed the inch: 1 avoirdupois pound = 0.45359237 kilograms exactly. That single number defines every imperial weight unit in current use.

Our weight category supports eleven units. The metric family — milligram, gram, kilogram, metric ton — scales decimally and covers almost all scientific, pharmaceutical, and international trade work. The avoirdupois family — ounce, pound, stone, short ton (US, 2,000 lb), long ton (UK, 2,240 lb) — handles everyday US and UK weights. Specialised units include the carat (exactly 200 mg, used in gemstones) and the atomic mass unit u (1.66053906660 × 10⁻²⁷ kg, used in molecular chemistry).

The stone (14 lb, used almost exclusively in the UK and Ireland for body weight) and the short-versus-long ton split are the units most often missed by US-only or metric-only references. Our converter handles all three ton variants — metric, short, long — distinctly so that a freight quotation in 'tons' can be checked in seconds.

Outside this category, you may also encounter the troy ounce (31.1035 g, used for precious metals), the apothecary scruple, the grain (used in ammunition and small-scale pharmacy at 64.79891 mg), and the dram. None of these is in widespread modern use outside its niche, so we keep the everyday set focused. If a niche unit blocks you, the contact page is the fastest way to flag it.

Every weight conversion routes through the kilogram, the SI base unit. Each unit has its exact kilogram equivalent published; Calcflux applies those factors directly. The factors are exact rather than approximate: 1 pound = 0.45359237 kg, 1 ounce = 28.349523125 g (one-sixteenth of a pound), 1 stone = 6.35029318 kg (fourteen pounds), 1 short ton = 907.18474 kg, 1 long ton = 1016.0469088 kg. The methodology page cites NIST SP 811; the underlying exact pound is fixed by the 1959 International Yard and Pound Agreement.

If you're verifying conversions by hand, the most useful exact factors to know are 1 kg ≈ 2.20462 lb (the reciprocal of 0.45359237), 1 oz ≈ 28.35 g, and 1 stone = 14 lb exactly.

Body weight drives most weight searches. Kilograms to pounds and pounds to kilograms are the two highest-volume pair pages in the category — fitness apps, immigration forms, gym programs, and medical charts all need them. Cooking adds two more: ounces to grams and grams to ounces, where Americans read European baking recipes and vice versa. For UK body weight specifically, the stone-to-kilogram pair matters because the NHS still records weights in stones and pounds — '12 stone 4' is a single number a UK GP can read at a glance but a US converter can't.

The full weight comparison table lists every supported unit with its kilogram factor and a worked sample. Use the table when you want to compare more than two units at once — for instance when you're packing a shipping container and need to read net, gross, and tare weights in kilograms, short tons, and metric tonnes simultaneously.

Ton vs tonne vs ton: there are three distinct units called some variant of 'ton'. The metric ton (or tonne) is exactly 1,000 kg. The US short ton is 2,000 avoirdupois pounds (≈ 907.18 kg). The UK long ton is 2,240 pounds (≈ 1,016.05 kg). Shipping documents that say 'ton' without qualifying it are the source of expensive disputes. Always pin down which ton.

Troy vs avoirdupois ounce: when precious metals are quoted in 'ounces', they're troy ounces (31.1034768 g), not the avoirdupois ounce (28.3495 g) used in groceries. A 1-oz gold coin is ~10% heavier than a 1-oz chocolate bar. Our converter exposes the avoirdupois family by default; troy units belong in a separate precious-metals tool.

Mass vs weight: scales report mass (kg or lb), but the underlying physics is force. The difference is invisible for terrestrial measurements and matters only for spacecraft and high-precision metrology. The newton is the SI unit for force; our force category handles those conversions.

Significant figures on body weight: a digital bathroom scale that displays 75.3 kg actually has roughly ±0.2 kg of repeatability. Quoting 75.3 kg as 166.0 lb implies precision the input doesn't have — 166 lb is the honest number.

Side-by-side: the full weight comparison table lists every supported unit with its factor and a worked sample. Definitions: glossary entries for kilogram, gram, pound, ounce, and stone. Method and sources: the methodology page cites NIST SP 811. For baking specifically, weight vs. volume in the kitchen explains why grams beat cups for accuracy.