A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Mass measures how much matter is in an object, using grams (g) and kilograms (kg) as metric units. A paperclip has a mass of about 1 gram; a textbook has a mass of about 1 kilogram. There are 1,000 grams in a kilogram. Students estimate masses, use a balance scale to measure, and solve word problems involving mass.
Holding known masses (1 g and 1 kg reference objects) builds physical intuition. Estimating before measuring develops number sense. Balance scales make the comparison of masses concrete.
You already know how to compare the heaviness of objects — you can tell a book is heavier than an eraser just by picking them up. Mass is the measure of how much matter is in an object, and measuring it means assigning a number with a unit so that different people can communicate exactly how heavy something is without having to pass it back and forth. The metric system gives us two everyday units for mass: grams and kilograms.
A gram (g) is a very small unit — a single standard paperclip has a mass of about 1 gram. Think of a gram as the unit for light, small things: a coin, a piece of candy, a pencil eraser. A kilogram (kg) is 1,000 times heavier than a gram — a textbook, a bag of sugar, or a liter bottle of water each have a mass of about 1 kilogram. The prefix *kilo-* always means 1,000 in the metric system, so 1 kg = 1,000 g. This relationship is the anchor for all metric mass calculations.
When you encounter a mass problem, the first step is deciding which unit makes sense. A strawberry is measured in grams; a person is measured in kilograms. Estimating before measuring trains your intuition: "Is this object more like a paperclip or more like a textbook?" If the answer is closer to paperclip, use grams; if closer to textbook, use kilograms. A balance scale makes mass concrete — you place the object on one side and known masses on the other until it balances, showing that the two sides have equal mass.
Word problems involving mass usually require adding or subtracting masses in the same unit. If one object has a mass of 350 g and another has 420 g, their combined mass is 770 g. If the total is more than 1,000 g, you can express it in kilograms (1,000 g = 1 kg, so 1,200 g = 1 kg 200 g). Keeping your units consistent — never mixing grams and kilograms in the same calculation without converting — is the key habit that carries forward into all measurement work.