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
- Compare two objects directly first: line up one end and see which reaches further, teaching longer, shorter and the same.
- Introduce informal units (paper clips, cubes) and lay them end to end with no gaps and no overlaps.
- Insist every unit is identical, because mixing big and small units gives a meaningless count.
- Always start measuring from the very end of the object, giving a fair start line.
- 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.
Frequently asked questions
How do children measure before they use rulers?
They compare lengths directly, lining objects up to see which is longer or shorter, then measure with informal units by laying identical items like paper clips or blocks end to end and counting them. This builds the ideas a ruler later formalises.
What age is comparing and measuring length taught?
Comparing and measuring length is usually taught from Pre-K to Grade 1. Children first compare two objects directly, learning longer, shorter and the same, then measure with informal units before moving on to standard units and rulers in later years.
What are the rules for measuring with informal units?
Two rules matter most: start from the very end of the object for a fair start line, and lay the units end to end with no gaps and no overlaps. The units must also be identical, because mixing big and small units gives a meaningless count.
How do you compare which of two objects is longer?
Line up one end of both objects, the start line, and see which reaches further. Comparing directly like this, with a fair start, teaches the words longer, shorter and the same before any counting of units begins.
Why must the measuring units all be the same size?
Because counting mixes only make sense if each unit is identical. If you measure with a mix of big and small paper clips but count them all the same, the number tells you nothing. Using identical units is what makes the count a fair measure of length.
Why does my child measure length incorrectly?
Common errors are not lining up the starting ends so the comparison is unfair, leaving gaps or overlapping the units, using units of different sizes, and starting the count from one instead of the first whole unit. A fair start line and evenly placed identical units fix these.
What comes after measuring with informal units?
Children move on to standard units and rulers, measuring in centimetres and metres, and later to converting between metric units. The ideas of a fair start line and no gaps carry straight over to reading a ruler correctly.
Practise with free worksheets
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