Numbers · geometry · annulus (ring)Area of a ring — formula and calculator
Work out the area of a ring with the formula S = π(R² − r²) — your numbers substituted and a diagram drawn. The both circumferences is calculated too. Units can be mixed, and the answer comes in the units you need, hectares and acres included.
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Calculator
enter the sizes — the formula and diagram follow02
Formula
and what each letter meansArea
S = π(R² − r²)
Both circumferences
P = 2π(R + r)
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Worked example
with real numbersGivenR = 5 cm, r = 3 cm
Solution
S = π(R² − r²) = 3.1416 × (5² − 3²) = 50.2655 cm²
P = 2π(R + r) = 2 × 3.1416 × (5 + 3) = 50.2655 cm
Answer: area — 50.27 cm², both circumferences — 50.27 cm.
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Common mistakes
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Nearby shapes
CircleS = πr²Circular sectorS = πr² × α / 360°CylinderV = πr²hCalculatorAll 19 shapes, the composite shape and hectares →
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