How to teach comparing and measuring length

Pre-K to Grade 1

Before rulers, children compare and measure length directly and with informal units. They line objects up to see which is longer or shorter, then measure by laying identical units (paper clips, blocks) end to end and counting them. The key rules are a fair start line and no gaps or overlaps between the units.

How to teach it

1. Compare two objects directly first: line up one end and see which reaches further, teaching longer, shorter and the same.
2. Introduce informal units (paper clips, cubes) and lay them end to end with no gaps and no overlaps.
3. Insist every unit is identical, because mixing big and small units gives a meaningless count.
4. Always start measuring from the very end of the object, giving a fair start line.
5. Count the units to give the length (the pencil is 6 cubes long), then compare two lengths by their counts.

Common mistakes

• Not lining up the starting ends, so the comparison is unfair.
• Leaving gaps or overlapping the units while measuring.
• Using units of different sizes and still counting them the same.
• Starting the count from one instead of the first whole unit.