Radius of a Circle Calculator r from circumference or area. Circumference Radius 5.001 Step-by-step with your numbers: 1. Values used: 2. Circumference = 31.42 3. Radius = 5.001 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! r = C ÷ (2π). FAQ From area? r = √(A/π). Related calculators Circle Calculator Find a circle's area, circumference and diameter from its radius. Diameter to Radius Calculator r = d ÷ 2. Circumference Calculator Find a circle's circumference and diameter from the radius.