What is a Circle Calculator?

Circle Calculator is a free and easy tool to calculate a circle's area, circumference (or perimeter), radius, and diameter - in less than a few seconds! It is great for geometry homework, design and fabrication work, crafting projects, or anything else that requires quick, accurate circle measurements. Simply enter one known value, and the calculator determines the rest - there are not any formulas or logarithmic tables to look up.

How Does the Circle Calculator Work?

1. Enter one known value: Radius (r), Diameter (D), Circumference (C), or Area (A).
2. Click Calculate. The calculator calculates the other properties, instantly.
3. Check the results. You will see area, circumference, diameter, and radius. Units are maintained (i.e., cm in -> cm out).

Example: Enter radius = 7 cm → Area ≈ 153.94 cm², Circumference ≈ 43.98 cm, Diameter = 14 cm.

Circle Calculator Formulas Used

→ Core identities:

• Area: A = πr²
• Circumference: C = 2πr
• Diameter: D = 2r

→ Useful rearrangements:

• r = D/2
• r = C/(2π)
• A = π(D/2)²
• D = C/π

Use π ≈ 3.14159 (rounded as needed). With any one value known (A, C, r, or D), the others are derived directly.

Key Benefits of Using the Circle Calculator Online

• Speedy & precise: Will save time and avoid manual errors.
• Multi-functional: Calculate area, circumference, diameter, & radius from one input.
• Supported: Useful for geometry applications (practicum), machining, woodworking, sewing, signage.
• Cross-device compatibility: Use it on your mobile, tablet, or desktop—no app installations required.
• Educational: Reinforces the relationships in formulae when generating immediate results.

Example Use-Case Scenario

• In a geometry classroom: If r = 7 cm, the calculator provides Area ≈ 153.94 cm², Circumference ≈ 43.98 cm, Diameter = 14 cm (which you can copy into your assignment).
• In fabrication: If the tabletop has a diameter of 1.2 m, input D = 1.2 m → and get (correct for trim length around the edge of the circle) C ≈ 3.77 m (length of edge trim), and A ≈ 1.13 m² (surface material).
• In crafting: If you want to make a wreath ring with circumference 80 cm, input C = 80 cm → making a wreath ring would yield D ≈ 25.46 cm, r ≈ 12.73 cm.