One pound is exactly 453.59237 grams. Every digit of that number is definitional: the international avoirdupois pound was defined in 1959 as exactly 0.45359237 kilograms, and the gram figure is the same number moved three decimal places.
About the units
The gram is a thousandth of a kilogram, and it is the unit European kitchens, laboratories and nutrition labels are built around. The pound is the anchor of the avoirdupois system, divided into sixteen ounces and grouped into stones and hundredweights above. Before 1959 the American and British pounds differed very slightly — the American pound was defined through the metre by the Mendenhall Order, the British through a physical prototype — and the difference was around one part in ten million. The 1959 agreement chose 0.45359237 kg because it was close to both and could be stated exactly.
The exact factor
The factor is exactly 453.59237, which makes pounds to grams an exact conversion with no rounding. Going the other way, 1 g = 0.00220462262 lb. The useful approximation is 450 g to the pound, which is low by 0.8 per cent, or the sharper 454 g, which is high by less than a tenth of a per cent. Two derived values worth knowing: half a pound is 226.8 g, and a quarter pound is 113.4 g — the last of which is where the quarter-pound hamburger gets its metric equivalent.
Where you meet this conversion
Recipe conversion is the everyday case and the one where accuracy matters most, because baking depends on ratios rather than absolute amounts. American recipes give flour, sugar and butter in pounds and cups while European ones give grams, and butter in particular is sold in different standard blocks: an American stick is a quarter pound, 113.4 g, while a European block is typically 250 g. Meat, produce and coffee cross the same boundary in trade, and specification sheets for exported goods carry both figures.