MathGrade 6ages 11–12
⭕ Radius and diameter
A child who has seen both words used for "circle measurements" without a clear picture may swap radius and diameter, unsure which one crosses the whole circle and which one stops at the middle.
Try this lesson freeNo card needed. The first lesson is free.
Download on the App StoreWhat your child learns
- The radius runs from the centre to the edge.
- The diameter crosses the circle through the centre, which makes it exactly two radiuses.
- The circumference is the whole edge, a little over three diameters.
- Every point on the edge is the same distance from the centre.
The mistake most kids make here
Swapping radius and diameter.
Try one question
The radius is 5. What is the diameter?
Radius 5
- 10
- 5
- two and a half
Answer
10 — That's it. Five one way and five the other.
How Maya teaches it
Your child tells Maya what they did in class. She teaches this topic at her desk, asks questions with tappable answers, and when the answer is wrong she explains why that answer was tempting instead of saying “try again”. She remembers what did not land and comes back to it.
Your child has it when they can: Say the diameter of a circle whose radius is 5.
How to help at home
The words are easy to confuse without a picture, but the relationship between them is simple and fixed: the diameter is always exactly twice the radius, on every circle, every time.
Take a round plate, lid, or coin. Have your child measure from the very center out to the edge — that is the radius — then measure straight across the whole circle through the center — that is the diameter.
Compare the two numbers they measured. The across-the-whole-circle measurement should come out to almost exactly double the center-to-edge one, showing the two-radiuses rule directly.
A sign they have it: given a radius, they can say the diameter without measuring again, and can explain which one crosses the center of the circle.
Questions parents ask
- Why does my child mix up radius and diameter?
- Both are measurements of a circle and sound similar without a picture to anchor them, so it is easy to lose track of which one runs from the center and which one crosses the whole shape.
- What is a hands-on way to practice radius and diameter?
- Measure a real round object from its center to its edge, then straight across through the center, and compare the two numbers — the second should be double the first.
- Is the diameter always exactly double the radius?
- Yes, on every circle, because the diameter is defined as a line through the center connecting two points on the edge, which is the same as two radiuses placed end to end.
- How does Maya teach radius and diameter?
- Maya has the child measure both distances on a real round object, so the two-radiuses relationship is confirmed by their own numbers rather than stated as a fact to remember.
No card needed. The first lesson is free.
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