🔥 TOP SEARCHES:
Geometry & Triangles • Lesson 22 ⏱ 5 min read 📅 Updated Nov 2026

Circle Formulas: Radius, Diameter, Circumference, and Area Explained

Learn how pi (π ≈ 3.14159) connects radius, diameter, circumference, and area, with step-by-step solved circle problems.

Core Constant

Pi (( pi approx 3.14159 )) connects the linear distance across a circle to the distance around its rim. No matter how large or small the circle is, ( C / d = pi ) is always constant!

Radius (r) Area A = πr² Circumference C = 2πr
Circle Anatomy: Radius (r), Diameter (2r), Perimeter (2πr), and Area (πr²)

Circle Formulas Reference Card

MetricSymbolFormulaExample (r = 7 cm)
Radiusrr = d / 27 cm
Diameterdd = 2r14 cm
Circumference (Perimeter)CC = 2πr = πd14π ≈ 43.98 cm
AreaAA = πr249π ≈ 153.94 cm2
Try It Yourself

Practice with Our Interactive Circle Calculator

Check your calculations or explore full step-by-step solutions instantly.

Open Circle Calculator →
Solved Step-by-Step Examples:Area of Circle (r = 7)Interactive Circle Calculator
Your Turn!

Check Your Understanding (3 Quick Questions)

Score: 0 / 3

Question 1: If a circle has a radius r = 5 cm, what is its diameter?

Question 2: What is the formula for the Area of a circle?

Question 3: If you double radius of a circle, what happens to Area?