Tonnes & Choosing a Unit

You can weigh a truck in kilograms and get 8000. It is correct and it is clumsy, in exactly the way measuring a road trip in metres was clumsy. So for very heavy things there is a bigger unit: the tonne, which is 1000 kilograms. The truck becomes 8 tonnes. Notice the pattern you already know continuing: 1000 grams make a kilogram, and 1000 kilograms make a tonne. Every step up the ladder is a thousand.

Here is the full ladder. Grams are for small things: a paperclip, a grape, a teaspoon of sugar. Kilograms are for everyday things: a bag of flour, your school bag, a cat. Tonnes are for the very heavy: a car, a truck, a whale. Each step up is a thousand times bigger, so a tonne is a million grams — which is exactly why nobody weighs a truck in grams.

Converting works exactly as it did for kilometres. Going down the ladder to a smaller unit, multiply by 1000: 4 tonnes is 4000 kilograms, and 2 kilograms is 2000 grams. Going up to a bigger unit, divide by 1000: 5000 kilograms is 5 tonnes, and 3500 grams is 3.5 kilograms. Use the same sanity check every time — a smaller unit must give a bigger number, because it takes more small things to make the same weight.

Sometimes you need to jump two rungs, from grams all the way to tonnes. Do it one rung at a time rather than trying to leap. Two and a half million grams divided by 1000 gives 2500 kilograms, and dividing again by 1000 gives 2.5 tonnes. Students who try to do it in one go often divide by 2000 by mistake, which is wrong — it is two separate thousands, so two separate steps.

A humpback whale weighs 30,000,000 grams, or 30000 kilograms, or 30 tonnes. All three are exactly the same weight and all three are correct — but only one of them is any use to a person. The rule is the one you met with kilometres: pick the unit that keeps the number small and sensible, usually somewhere between 1 and 1000. Choosing well is part of answering the question properly, not an optional extra at the end.