Rotational Speed?

So what I'm looking to do is have a ball roll on a plane at the right rotational speed.

The diameter of the ball is 2 (radius=1).
The circumference of the ball is Pi*diamter=6.28.

Now say the ball is rolling in a straight line at a speed of 5. And you also want to keep the ball rolling in sync with the frame rate (which would be 0.018).

To recap:
Diameter = 2.0
Circumference = 6.28
Speed = 5.0
delta_time = 0.018

So it's gonna be a value * delta_time

For some reason I can't get the ball rotating at the right speed to match it's linear speed. I can hack it but I'd rather get the formula right.

Assuming 1 unit of speed = 1 units of diameter then I would have thought that the ball would need to roll about 79.62% of the way round, which would be about 286.632 degrees.

So I guess it would be either:

286.632 * delta
or speed*57.3264*delta

(57.3264 / diameter) * delta

The above equation works spot on. Thanks Nexic.