Measure the circumference (C) and use (r=dfrac{C}{2pi})
Measure the diameter (d) and use (r=dfrac{d}{2})
Measure the area (A) and use (r=sqrt{dfrac{A}{pi}})
Use the chord length (L) and distance from the center to the chord (h) (sagitta) and compute (r=dfrac{L^{2}}{8h}+dfrac{h}{2})
Use the chord length (L) and center-to-chord distance (x) (if given) and compute (r=dfrac{(L/2)^2+x^2}{2x})
Use two points on the circle: find the center ((h,k)) first, then compute (r=sqrt{(x-h)^2+(y-k)^2})
Use the general circle equation ((x-h)^2+(y-k)^2=r^2) and take (r=sqrt{r^2}) (the constant term)
If given the circle equation in expanded form (x^2+y^2+Dx+Ey+F=0), compute (r=sqrt{left(dfrac{D}{2}right)^2+left(dfrac{E}{2}right)^2-F})