First a quick geometry lesson:

this is the circumference of a circle

this is the radius

the diameter is two times the radius

you can calculate the circumference from the diameter by multiplying it with pi

lets first create a new comp

then we’ll add a 400 by 400 solid

double click on the circular mask tool to create a circle mask inside the solid

we’ll precompose this just to be able to add some detail inside the circle

this will make it easier for us to see how it is rotating

next we’ll need the diameter of our circle in order to calculate the amount of pixels it should move during a 360 degree rotation

we already know it is 400 but you can also measure it by dragging out two guides and subtracting the smaller value from the bigger one

then we multiply our diameter with pi

finally we can animate our circle one full rotation and for the position we’ll add the calculated pixel amount

let’s also adjust our motion path so that the animation begins off screen

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