Converting and Calculating Units of Mass

The four metric units of mass are the milligram (mg), gram (g), kilogram (kg) and tonne (t), from smallest to largest. Each is 1,000 times the one below it: 1 g = 1,000 mg, 1 kg = 1,000 g and 1 t = 1,000 kg. Medicine doses are in milligrams, pasta packs in grams, people in kilograms and lorry loads in tonnes.

Multiply by 1,000 to move to a smaller unit and divide by 1,000 to move to a larger one, because each unit is 1,000 times the next. To change 560 kg into tonnes, a larger unit, divide: 560 ÷ 1,000 = 0.56 t. To change kilograms into grams, a smaller unit, multiply by 1,000 instead.

Multiply by 1,000, because 1 kg = 1,000 g and grams are the smaller unit. So 21 kg = 21 × 1,000 = 21,000 g, and 5.6 kg = 5,600 g. Whenever you move from kilograms down to grams you multiply; going back up from grams to kilograms you divide by 1,000.

Divide by 1,000, because there are 1,000 g in 1 kg and kilograms are the larger unit. So 3,250 g = 3,250 ÷ 1,000 = 3.25 kg, and 198 g = 0.198 kg. If the answer is less than 1 kg, write it as a decimal rather than a fraction.

Change both masses to the same unit first, then add or subtract. To add 3.25 kg and 30 g, change 30 g to 0.03 kg and add: 3.28 kg. To subtract 30 g from 3.25 kg, you could instead change 3.25 kg to 3,250 g: 3,250 − 30 = 3,220 g, which is 3.22 kg. Use whichever unit is easier; never mix them.