What each Weight unit is
Gram g
Grams are the unit of recipes and postage outside the United States, and the unit nutrition labels use nearly everywhere.
Kilogram kg
The kilogram was the last unit defined by a physical object — a cylinder in a vault near Paris that was slowly losing mass. Since 2019 it has been fixed to the Planck constant instead.
Tonne t
A tonne is 1,000 kilograms, and it is not the same as either the US short ton (907 kg) or the UK long ton (1,016 kg). Where the difference matters, this one is sometimes written "metric ton".
Ounce oz
An ounce is a sixteenth of a pound. It is not the fluid ounce, which measures volume, and the two are only loosely related — a fluid ounce of water weighs about an ounce, which is where the name came from and where the confusion starts.
Pound lb
A pound is exactly 0.45359237 kilograms by the same 1959 agreement that fixed the inch. The abbreviation "lb" comes from libra, the Roman unit it descends from.
Stone st
A stone is fourteen pounds, and it is used for body weight in Britain and Ireland and essentially nowhere else — which is why an American reading a British weight usually needs this conversion.
Notes on the common Weight conversions
kg to lbs
Double it and add a tenth
The shortcut worth carrying is multiplication by 2.2, done in two easy steps rather than one hard one: double the kilograms, then add ten per cent of the doubled figure. 70 kg doubles to 140, ten per cent of 140 is 14, and 154 is the answer — against 154.32 exactly. 60 kg gives 120 + 12 = 132 against 132.28; 90 kg gives 180 + 18 = 198 against 198.42.
It is short by 0.21 % every time, which is a consistent bias rather than a random error and is therefore easy to allow for: whatever the shortcut says, the real number is a whisker higher. Below about 43 kg the gap is under a fifth of a pound and invisible. Above about 108 kg it passes half a pound, which is where a figure being quoted rather than estimated should come from the field above instead.
kg to stone
The decimal is not how the answer is said
Dividing by 6.35029318 gives a number that is arithmetically right and conversationally wrong. 92 kg comes out as 14.487 stone, and there is no context in Britain or Ireland where somebody would say that. Weight in stone is spoken as two numbers — a whole stone and a remainder in pounds — the way height is spoken as feet and inches.
The second step is the one to remember: keep the whole number, multiply the decimal by 14. Here 0.487 × 14 = 6.8, so 92 kg is fourteen stone seven, rounding to the nearest pound the way a person would. The trap is reading 14.5 as "fourteen and a half stone" and hearing seven pounds when it means seven — which happens to be right, and is right by accident.
lbs to kg
Divide by 2.2 and the error you have accepted
The number nearly everybody carries is 2.2, and dividing by it gives an answer about 0.2 % high — 200 lb becomes 90.91 kg against a true 90.72. On a suitcase that is 190 grams and nobody cares. On a figure that will be multiplied by something else it is the first of two errors, and the second one rarely cancels it.
The multiplication is the safer habit in this direction. Pounds times 0.45359237 needs no division and no reciprocal, and if you want a short version, 0.4536 is right to a part in ten thousand. The field above carries the full factor and rounds only when it prints, which is why its answer and a hand-worked one can differ in the last place shown without either being wrong.
grams to kg
The size at which grams stop being readable
Metric units are designed to be swapped so the number stays in a range a person can hold, and the accepted band is roughly 0.1 to 1,000. A figure of 1,500 g is inside no rule that forbids it and outside the habit — it reads as a number with a zero on the end doing nothing, where 1.5 kg reads as a quantity. That is the whole of the decision on most documents.
The exception is any field that will be sorted, summed or compared by machine. A column holding 850 g, 1.2 kg and 40 kg is three units and no arithmetic; a column holding 850, 1200 and 40000 grams is one unit and any arithmetic you like. Store in the small unit, display in the readable one, and do the conversion at the last possible moment.
grams to lbs
Two pounds twelve, not 2.7558 pounds
The answer a gram figure produces is a decimal pound, and decimal pounds are almost never what the receiving form wants. A US shipping label, a customs declaration and a marketplace weight field are all pounds-and-ounces fields, and 1,250 g is 2 lb 12 oz rather than the 2.76 the arithmetic hands you.
The step is division by sixteen, not by ten, and that is the whole of the trap. Multiply the fractional part by 16: 0.7558 × 16 = 12.09, so twelve ounces with a tenth left over. Read the same 2.76 as "two pounds seventy-six" and the parcel is declared four and a half ounces light, which is enough to move it into the wrong price step on a light package.
grams to ounces
Three things called an ounce, and a recipe means one of them
The ounce this page converts is the avoirdupois ounce, a sixteenth of a pound, 28.349523125 g exactly. The fluid ounce is a volume — 29.57 ml in the US, 28.41 ml in the UK — and appears in the same recipes, sometimes in the same line. The troy ounce, 31.1034768 g, prices gold and silver and turns up in a kitchen only by accident, but it is the reason a gram-to-ounce answer copied from a bullion site will not match this one.
The names collided because an imperial fluid ounce of water was defined to weigh about an ounce, which made the two interchangeable for the one substance and for nothing else. A US recipe asking for 8 oz of chocolate means a mass and one asking for 8 oz of milk means a cup — and the only reliable tell is what is being measured, not how it is written.