Mass and weight

Mass or weight?

Mass is the amount of matter in an object. Weight is the pull of gravity on that matter. In a science lesson the two are different, but in the kitchen and the shop people use the word weight for both. When a recipe says 500 g of flour, it means the mass, and the scale in the kitchen measures exactly that.

A balance scale compares two pans. When the beam stays level, the two pans hold the same mass. Adding an object to one side makes that side sink and the other side rise. The picture shows a level scale: the kilogram on the left balances the gram weights on the right, because the two sides hold the same mass.

Grams and kilograms

The metric units for mass are the gram (g) and the kilogram (kg). One kilogram is 1,000 grams. A paperclip has a mass of about 1 g, a slice of bread about 30 g, an apple about 200 g, and a bag of flour about 1 kg. Grams suit small, light things, and kilograms suit bags, boxes and people. A school bag might have a mass of 5 kg, and a bicycle about 12 kg.

Ounces and pounds

The customary units for weight are the ounce (oz) and the pound (lb). One pound is 16 ounces. A slice of bread weighs about 1 oz, a loaf about 1 lb, and a bag of apples about 3 lb. A parcel at the post office might weigh 40 oz, which is 2 lb 8 oz, because 40 = 32 + 8 and 32 oz is 2 lb.

The two systems do not convert with a neat fact. One kilogram is about 2.2 pounds, and one pound is about 454 grams. For everyday work you only need the facts inside each system: 1,000 grams in a kilogram, and 16 ounces in a pound. Change between systems only when a question asks for it, and use the word about.

Comparing masses

To compare two masses, write both in the same unit. Which is heavier, 700 g or 2 kg? Change the kilograms to grams: 2 kg is 2,000 g. Now the comparison is easy, because 2,000 is more than 700, so 2 kg is heavier. The same method works for a mass like 2,800 g against 3 kg: 3 kg is 3,000 g, so 2,800 g is lighter.

A number written in two units is a sum. A puppy with a mass of 3 kg 500 g has a mass of 3,000 g + 500 g = 3,500 g. Going the other way, 2,500 g is 2 kg 500 g, because the first 2,000 g make 2 kg and 500 g is left over.