Circle – Circumference
Circumference is the distance around the outside of a circle, found with C = 2πr.
This is a free explanation of Circle – Circumference — a preview, not the full interactive lesson. The lesson, practice, quizzes, and AI Tutor are inside the app.
A circle is a perfectly round flat figure where every point on the edge is the same distance from the center. Use circumference when a problem involves going around the edge — fencing, trim, or the distance a tire rolls. The formula is C = 2πr, or equivalently C = πd, where d is the diameter. Use π ≈ 3.14 unless told otherwise.
The radius (r) runs from the center to the edge; the diameter (d) crosses the full circle through the center, so d = 2r. Because 2r equals the diameter, C = 2πr and C = πd give the same result. If a problem gives you the diameter, you can use πd directly.
Keep circumference separate from area: circumference is measured in regular linear units (feet, cm), while area is measured in square units. If your answer’s units look wrong, you may have used the wrong formula.
Worked example
A circle has radius 7. Find its circumference. (π ≈ 3.14)
- Write the formula: C = 2πr.
- Substitute the values: C = 2 × 3.14 × 7.
- Multiply: 2 × 3.14 = 6.28, and 6.28 × 7 = 43.96.
43.96 units
On the test: Circumference is measured in linear units; use C = πd when the problem gives the diameter.
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