One kilogram is exactly 1,000 grams. There is nothing to round and no history to untangle — the prefix kilo means a thousand, and this is the metric system doing exactly what it was designed to do.
About the units
The kilogram is the SI base unit of mass, which makes this pair unusual: it is the only base unit whose name already carries a prefix, an accident of history from the 1790s when the gram was defined first and then found to be inconveniently small for practical standards. The gram was originally the mass of a cubic centimetre of water at the ice point, and the kilogram was simply a thousand of them — cast as a physical artefact because a metal cylinder is easier to compare against than a volume of water. Since May 2019 the kilogram has been defined through the Planck constant instead, and the gram is now formally derived from it rather than the other way round.
The exact factor
The factor is exactly 1,000: shift the decimal point three places. 2.5 kg is 2,500 g; 0.75 kg is 750 g; 340 g is 0.34 kg. Because the relationship is exact, precision is never lost in either direction, which matters in laboratory and pharmaceutical work where a chain of conversions must not accumulate error. Worth holding alongside it: a tonne is a thousand kilograms and therefore a million grams, and a milligram is a thousandth of a gram.
Where you meet this conversion
Cooking is the everyday case, where recipes mix both freely — 1.2 kg of potatoes alongside 300 g of butter — and the mental conversion happens without anyone noticing. Nutrition labels give macronutrients in grams and package contents in kilograms. Shipping and postage cross the boundary at around a kilogram, which is where price bands often sit. In science and medicine, doses are calculated per kilogram of body weight but administered in milligrams, making this pair one step in a chain that runs across six orders of magnitude.