Estimating lengths
Estimate lengths using units of inches, feet, centimetres, and metres
What a learner can do afterwards
- Estimate the length of a classroom object before measuring it
- Use a measured reference, such as a finger-width checked with a ruler, to make reasonable estimates
- Check estimates by measuring and evaluate how close they were
1 · Read
An estimate is a guess based on a length you know. Look at the object before measuring it. Compare it with a known length, then choose an estimate in inches, feet, centimetres or metres. Measure afterward to check how close your estimate was.
To check a reference, line up a ruler's zero mark with one end and read the other end. Keep the unit. Fingers and steps vary in size, so measure yours first. A finger-width measured as 1 centimetre can stand for 1 cm. A measured one-inch strip, a one-foot ruler or a metre stick also works. Try to picture the same reference size each time.
Ben pictures five one-foot rulers along a shelf and estimates 5 feet. Ada pictures three one-inch strips along a small card and estimates 3 inches. A door looks about as tall as two metre sticks, so Ben estimates 2 metres. Keep the unit of the reference you use.
For repeated references, add their lengths. Four 2 cm spans give 2 + 2 + 2 + 2 = 8 cm. To find how many spans match an estimate, count those additions: four spans reach 8 cm. If you discover that your reference has a different size, estimate again using its measured size.
An estimate need not equal the measurement. To find how far off it was, use the same unit and subtract the smaller number from the larger one. You can count up to find the gap. From 11 cm to 16 cm is 5 cm. This works whether the estimate is too short or too long.
The smaller gap means the closer estimate. Equal gaps mean equally close estimates. Compare gaps in the same unit. Known lengths also set limits: if an object is longer than one reference and shorter than another, estimate between them. A board narrower than a measured wall cannot have a width greater than that wall.
Both guesses have a 5 cm gap from 14 cm, so they are equally close.
A check can also help you find the measurement. If an estimate is 17 cm and the measurement is 2 cm longer, add: 17 + 2 = 19 cm. If the measurement is shorter than the estimate, subtract the gap instead.
Look at the object, compare it to something you know, guess a number, then measure to check.
Lesson, questions and flashcards written for this dataset.
Questions, readings and flashcards written for this dataset. Linked videos are by Khan Academy.
More to read
Choose a measured reference
A reference is a length you already know. Check it with a ruler or tape before using it. Your finger-width may differ from a friend's, and your steps may differ too.
Compare a small object with a short reference. For a larger object, a metre stick or another longer measured reference may be easier to use. Keep the reference's unit when you write your estimate.
Record the estimate first
Choose an object you can measure. Before measuring, compare it with a known length. Write down your estimate and its unit.
Then measure from one end to the other. Keep both results so you can check the gap. The estimate does not have to match exactly. The check tells you how the guess compares with this object's measured length.
Find the gap
Put the tool's zero mark at one end of the object. Read the mark at the other end. Write the estimate and measurement in the same unit.
Find their gap by subtracting the smaller number from the larger one. You can also count up from the smaller number. The same method works when an estimate is too short or too long. Compare two estimates by checking which has the smaller gap.
Use known limits
A known length can help you check an estimate. Suppose a board is narrower than a measured wall. The board's width must be less than the wall's width.
Two references can give two limits. If a strip is longer than one reference but shorter than another, estimate between their lengths. Compare matching dimensions, such as width with width, and keep the measurements in the same unit.
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