For more than a century, the kilogram was defined by a single platinum-iridium cylinder kept in a vault near Paris. In 2019 that changed: the kilogram is now defined through a fixed value of the Planck constant, a fundamental physical constant, so it no longer depends on any one object. The pound has been defined since 1959 as exactly 0.45359237 kilograms, and the ounce as one-sixteenth of a pound, about 28.35 grams.
Where you meet it
Most UK packaging is metric, but older recipes, some market stalls and many people's sense of weight still use pounds and ounces. Airline baggage allowances are usually in kilograms, while American websites and products use pounds. And precious metals use their own system: gold and silver are priced per troy ounce, which is about 31.10 grams, heavier than the everyday ounce.
Precision and pitfalls
The troy ounce is the trap most people don't know about. It's about 10% heavier than an ordinary ounce, so converting a gold price per troy ounce using 28.35 grams undervalues the gold. Coins and bars sold as "1 oz" always mean troy ounces. A second pitfall is rounding the pound to 0.45 kg, which introduces an error of about 0.8%, enough to matter for anything sold by weight in large quantities.
Worked conversions
A traditional cake recipe calling for 8 ounces of flour needs about 227 grams. Many cooks round this to 225 grams, which makes little difference. A 23 kg airline bag allowance is about 50.7 pounds, which is why US airlines commonly quote 50 pounds. A one-ounce gold coin contains about 31.1 grams of gold; at an illustrative price of £2,000 per troy ounce, that's about £64 per gram. Using the everyday ounce instead would give about £71 per gram, an error of over 9%.
Postage and luggage weights
Postage is priced by weight bands, so a small conversion error can change the price. Royal Mail sets maximum weights for each letter and parcel size, and items just over a limit move into the next band. If you weigh items at home in ounces, convert to grams before choosing a service. A large letter weighing 9 ounces is about 255 grams, which is over a 250-gram limit, even though "9 ounces" sounds comfortably small. Airline allowances work the same way: being a kilogram over can cost far more than the item is worth.
Measuring accurately
Kitchen and bathroom scales are rarely accurate to better than a few percent at the top and bottom of their range. For postage or luggage, weigh the item twice, and if possible use scales whose readings you've checked against something of known weight, such as a sealed 1 kg bag of sugar. When a result is close to a limit, assume it's over.
Checking your answer
A kilogram is a little over two pounds, about 2.2. A pound is a little under half a kilogram, about 454 grams. An ounce is about 28 grams, roughly the weight of a slice of bread. A litre of water weighs about one kilogram, which gives a handy reference you can check with a jug and kitchen scales.
One more worked example
A bread recipe from an older British cookbook asks for 1 pound 8 ounces of strong flour, 1 ounce of fresh yeast and half an ounce of salt. In metric, that's about 680 grams of flour, 28 grams of fresh yeast and 14 grams of salt. Because fresh yeast is often replaced with dried, which is roughly half as strong by weight, a modern version might use about 14 grams of dried yeast instead.