Idea 4of 22Builds onRough or exact: the job picks the unit
Benchmarks for length
How can you guess a length without a ruler?
Today you'll discover
- four sizes worth remembering
- one for each length unit
- how to estimate with them
Think firstFour things you see every day
About how tall is a doorway, from the floor to the top?
Decide first, then press Next. Picture each thing beside you.
Four boxes: your little fingernail, a hand span, a doorway, a 10-minute walk. Each is marked as a length, then with its size: about 1 cm, about 20 cm, about 2 m, about 1 km. At the bottom, a flagpole beside 4 doorways: 4 × about 2 m, about 8 m. The last step asks about a shelf 6 hand spans long.
See the whole ideathe big idea and the key word, in one place
The big idea
A benchmark is a size you know, like a 2 m doorway, used to estimate other sizes.
One for each unit
One benchmark for each length unit.
- fingernail1 cm
- hand span20 cm
- doorway2 m
- 10-min walk1 km
Easy to get wrongA benchmark gives an estimate, not an exact length. Say “about 8 m”, not “8 m”.
Key word
- benchmark
- an everyday thing whose size you know, used to estimate other sizes
Ready to use it?
- Estimate a length with a benchmark.
- Choose the benchmark that fits the size.