- Definition
The radius is a line that goes from the very center of a circle to any point on its edge. It's also exactly half the length of the circle's diameter, which is a line that goes all the way across the circle through its center.
If a circle has a diameter of 10 cm, its radius is 5 cm because the radius is half the diameter.
How to teach this in 30 seconds
Try this analogy: A radius is like a single spoke on a bicycle wheel, going from the center hub to the outer rim.
Ask your child: If a sprinkler is watering in a circle, how far is the water spraying?
Common mix-up: Kids often confuse the radius with the diameter, which goes all the way across. The radius is the 'halfway across' measurement, starting from the exact center.
Examples
- The minute hand of a clock is a radius of the circle made by the clock's face.
- On a pizza, the straight line from the very center to the crust of any slice is a radius.
radius is a term used in Ontario’s Kindergarten to Grade 8 curriculum.
Where this shows up
The descriptions on this page are SmartStudy’s own original, plain-language writing — not the Ministry’s wording. They are aligned to the Ontario Curriculum, © King’s Printer for Ontario. For the Ministry’s official wording, see the source link below. SmartStudy is an independent service and is not affiliated with, endorsed by, or sponsored by the Government of Ontario or the Ministry of Education. Ontario Curriculum and Resources ↗
