The rule to estimate the circumference:
Explain to the child that to measure the circumference the formula is: Circumference = 3.14 x diameter. To estimate the circumference they can multiply (3 x diameter). In fourth grade they are only required to estimate the circumference.
For example: The diameter of the circle is 7 cm. Therefore, circumference = 3 x 7. Circumference = 21 cm
If the radius is given, you can find the circumference by multiplying radius by 6.
For example: The radius of the circle is 2 cm. Therefore, circumference = 6 x 2. Circumference = 12 cm
The rule to calculate radius and diameter:
Diameter = (2 x radius)
Radius = (diameter ÷ 2)