Rounding Number Line
Number · Year 4–Year 6
Put a number on the line, find halfway, and decide which way it rounds.
About the rounding
The picture a teacher draws by hand every time rounding comes up: the two multiples either side of the number, the halfway mark between them, and the number itself where it belongs. Rounding then stops being a rule about digits and becomes one question, which is which side of halfway the number fell.
Drag the number along the line, or step it with the buttons. The two ends relabel themselves as it moves, so a class sees which two multiples they are working between before they decide anything.
Hide the answer and the board still asks the question. That is the way round to use it: the class decides, then the tool says, and the arrow shows where the number goes.
The same number can be rounded to the nearest 10, then the nearest 100, then the nearest 1,000 without retyping it. That comparison is the Year 5 objective and it is hard to show any other way: 4,732 is 4,730 and 4,700 and 5,000, all at once.
Halfway can be hidden too, once a class no longer needs it drawn for them. Exactly halfway rounds up, which is the convention every English primary scheme teaches, so the tool says so in words rather than leaving it to be guessed from a picture.
How to use it on your whiteboard
- 1Nearest sets what it rounds to: the nearest whole number, 10, 100, 1,000 or 10,000.
- 2Drag the number along the line, or tap the line where you want it.
- 3The buttons step it too. Steps of sets how far one press moves it, so a three digit number is a few taps.
- 4New Number gives one worth asking about: never on a multiple, never exactly halfway.
- 5Answer hides the answer and the arrow, so the class decides first.
- 6Halfway hides the halfway mark, for when they can judge it themselves.
- 7Tap Fullscreen to fill the board.
Questions
- What happens at exactly halfway?
- It rounds up, and the board says so in words: "45 is exactly halfway, so it rounds up to 50". That is the convention English primary schemes teach, and it is the one case a picture cannot settle on its own, so the tool never leaves it to be inferred.
- Why does the line only show one interval?
- Because the halfway mark is the whole question, and it has to be big. A line with ten intervals on it would also show which two multiples a number lies between, which is the other thing children get wrong, but it makes each interval a tenth of the width and halfway too small to argue about. The tool names the two multiples in its caption instead.
- Can I round decimals?
- To the nearest whole number, yes: choose Whole under Nearest and the line runs between two whole numbers with the tenths marked. Rounding to the nearest tenth is not in yet.
- Can I type a number in?
- Not yet. Steps of and the buttons get to any number in a few taps, and dragging along the line is quicker still for the numbers a lesson actually uses. A keyboard at a board is something to avoid where a finger will do.