Diameter to Radius Calculator r = d ÷ 2. Diameter Radius 5 Step-by-step with your numbers: 1. Values used: 2. Diameter = 10 3. Radius = 5 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! Radius = half diameter. FAQ Reverse? d = 2r. Related calculators Circle Calculator Find a circle's area, circumference and diameter from its radius. Radius of a Cylinder Calculator r from V and h.