An annulus is the flat ring-shaped region between two concentric circles — think of a washer or a donut shape.

How is annulus area calculated?

Area = π × (R² − r²), where R is the outer radius and r is the inner radius.

What's the difference between annulus and circular ring perimeter?

The annulus has two perimeters — outer circumference (2πR) and inner circumference (2πr) — both of which this calculator can show separately.

Annulus Calculator - CalculatorLabs.net

Annulus Calculator

An annulus is the flat, ring-shaped region between two circles that share the same center — like a washer, a CD, or the rim around a manhole cover.

Outer Radius (R)

Inner Radius (r)

Instant Result

Ring Area -

Outer Circumference -

Inner Circumference -

What Is a Annulus (Ring) Area Calculator?

An annulus is the flat, ring-shaped region between two circles that share the same center — like a washer, a CD, or the rim around a manhole cover. This calculator finds the area of that ring and the circumference of both circles from the outer and inner radii.

How to Use This Annulus (Ring) Area Calculator

Measure the outer radius R (edge of the whole ring) and inner radius r (edge of the hole).
Enter both values into the fields above.
Click Calculate to get the ring's area and both circumferences.

Formula

Ring Area = π × (R² − r²)
Outer Circumference = 2 × π × R
Inner Circumference = 2 × π × r

Worked Example

A metal washer has an outer radius of 10 mm and an inner radius (hole) of 4 mm. Ring Area = π(10² − 4²) = π(84) ≈ 263.89 mm². Outer Circumference ≈ 62.83 mm, Inner Circumference ≈ 25.13 mm.