The conversions are very accurate (10 decimal places) and are calculated for all common units of weight measurement.
The avoirdupois pound ( 0.45359237 kilogram ) is used in :
United States of America (USA) , South Africa, United Kingdom (UK), Republic of Ireland (ROI) and many former British colonies.
A rough guide when you are measuring liquids: 1 ml of water equals 1 gram, 1 litre of water weighs approx 1 kilo - other liquids may differ.
Wikipedia links covering units of mass show/hide