Since you said radius, I assume you mean Circle or Sphere.
Circle:
Area = Pi * radius^2
therefore, Radius = Square Root of (Area/Pi).
Sphere:
Area = 4/3 Pi * Radius^3
therefore, Radius = CubeRoot of (3/4 * Area/Pi)
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-- Take the formula for the area of the circle in terms of the radius . . . A = (pi) R2-- Solve that formula for 'R'. You'll then have a formula for the radius in terms of the area,which is exactly what you're looking for.
It is not clear whether you want the formula of some aspect (area or circumference) of a circle when given its radius or a formula for its radius when given some characteristic measure of the circle.
The formula for area of a circle is TTr2.This means that we multiply 3.14(the rounded off nunber for pi) by the radius and radius(3.14xradiusxradius).If the diameter is given,divide it by 2 to get the radius.
The formula for an area of a circle is the greek symbol "pi" times the radius squared. A = π * r2 r = radius π = 3.14 Now, your radius is given to you in millimeters. Generally answers are given in meters. 7 mm (millimeters) is equal to .007 m (meters).
The formula to calculate the area of a circle is A = πr^2, where A is the area and r is the radius. Given a radius of 4 inches, we can substitute r = 4 into the formula to find the area. Therefore, the area of a circle with a 4-inch radius is A = π(4)^2 = 16π square inches, or approximately 50.27 square inches.