Comparing lengths by measuring
Measure to find how much longer one object is than another. Give the length difference in a standard unit of length.
What a learner can do afterwards
- Measure two objects and calculate the difference in length
- Express the difference using the correct unit (e.g. '7 cm longer')
- Solve a comparison problem: 'How much taller is the bookshelf than the desk?'
1 · Read
Measure both objects in the same unit. Compare length with length, width with width, or height with height. Subtract the smaller measurement from the larger one to find the difference. Say the unit and which object is longer, wider or taller. If one is longer by an amount, the other is shorter by that same amount.
Mia's book is 22 cm long and her notebook is 15 cm long. Count up from the shorter length: 15 to 20 adds 5, then 20 to 22 adds 2. The gap is 5 + 2 = 7 cm. Her book is 7 cm longer.
For larger gaps, count up in easy parts. From 30 cm to 54 cm, add 10 to reach 40, then add 14 to reach 54. The difference is 10 + 14 = 24 cm. Check by addition: 30 + 24 = 54. The shorter length plus the difference should equal the longer length.
A comparison can help you find a missing height. A box is 14 cm tall. A plant is 6 cm taller, so its height is 14 + 6 = 20 cm. A shelf is 53 cm tall. A stool is 21 cm shorter. Subtract 20 to reach 33, then 1 to reach 32 cm.
Equal measurements have a difference of zero. Neither object is longer. If measurements use different units, measure both again in one unit before subtracting. A quick look may show which object is taller, but it does not give the exact measured difference.
Measure both objects in the same unit, then subtract to find how much longer one is.
Lesson, questions and flashcards written for this dataset.
Questions and readings written for this dataset. Linked videos are credited to Khan Academy; their content licenses are not established here.
More to read
Compare matching measurements
Decide what you are comparing: length, width or height. Measure that same dimension on both objects. Put the tool's zero mark at one end and read the mark at the other end.
Record both measurements in one unit. Find the gap by subtracting the smaller measurement from the larger one. Name the difference, its unit and which object is longer, wider or taller.
Use the same unit
Two children may count different numbers of hand-widths along the same table because their hands differ in size. Standard units give a shared reference.
For a length difference, measure both objects in the same unit. Equal measurements have a difference of zero. For unequal measurements, count up from the shorter one to the longer one. Add the parts of the gap. Check that the shorter measurement plus the gap equals the longer measurement.
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2 · Watch
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Where it sits
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Where this leads
14 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.