Perimeter
Perimeter is the total distance all the way around the outside of a flat shape. For a rectangle 5 cm long and 3 cm wide, it is 5 + 3 + 5 + 3 = 16 cm.
Maths · Ages 7–8 · 50 minutes
Measuring length in centimetres and metres
Children measure lengths accurately with a ruler starting from zero, record them in centimetres and metres, and add lengths to find a total.
Success criteria
- I can line up the zero with the end of the object
- I can read a length in cm
- I can add lengths together
Key vocabulary
- length
- measure
- ruler
- centimetre
- cm
- metre
- m
- estimate
- total
- perimeter
You will need
- 30 cm rulers
- metre sticks
- tape measures
- a set of classroom objects to measure (pencils, books, scissors)
- string
- mini whiteboards
Curriculum links
- Year 3· England
- Grade 2· United States
- Primary 3· Nigeria
- Class 3· India
Practice sheets
Generated fresh each time you press a button, and marked instantly on screen.
Number line: hop on
Number line: hop on
Perimeter
1.Starter
8 minutesHold up a pencil. "How long do you think this is?" Children write an estimate in cm on whiteboards. Take a few, then ask: "How could we find out for sure?"
Show a ruler that has a gap before the zero. "Where does the measuring start: at the very end of the ruler, or somewhere else?" Leave the question for the teach.
2.Teach
12 minutesPut a ruler under a visualiser or draw a large one on the board.
"A ruler is a number line. Each gap between numbers is one centimetre. The measuring starts at zero, not at the end of the plastic."
Line up a pencil with the zero. "The end of the pencil lines up with 0. The tip reaches 14. So it's 14 centimetres. We write 14 cm."
Now show a common mistake: line the pencil up at 1. "My friend got 15. Why?" (They started at 1, so they counted one centimetre too many.)
Step by step
- Line up the zeroThe end of the object sits exactly on 0.
- Keep it straightThe object lies along the ruler, not at an angle.
- Read the number at the other endAnd write the unit: cm or m.
Introduce metres with a metre stick. "100 centimetres make 1 metre. We use metres for bigger things, like the length of the classroom."
Then add lengths. "A table is 80 cm long. Another table is 70 cm. Pushed together, how long?" Count on in tens on a 0–200 number line: 80, 90, 100... 150. "150 cm. That's 1 metre and 50 centimetres."
