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CircleLab
Enter a radius — get area, circumference, and diameter with the exact formulas used.
A circle's area scales with the square of the radius — double the radius and you quadruple the area, not just double it. That's because area is fundamentally two-dimensional; π·r² comes from summing up infinitely thin rings from the center out to the edge.
Circle area shows up constantly: sizing a pizza to compare value per dollar, figuring out how much material covers a round table, estimating paint or fertilizer for a circular garden bed, or sanity-checking a pipe's cross-sectional flow capacity.
Area
78.54
A = π × r²
What you entered
Diameter
2 × 5= 10Circumference
2π × 5= 31.4159Area
π × 5²= 78.5398Result
Area: 78.54
A circle with radius 5 has an area of 78.54 square units and a circumference of 31.416 units.