i already have the diameter of the circle and i know the length i need but somehow its tough to get the exact length, is there some way to do this in rhino?

Start Arc.

Pick a center point

Pick a start point

Click on the Length command line option

Type your desired length

yes but it doesn’t then stick to the circle its apart of, its like a have a circle but i need a section of that circle but a particular length

Use the Center osnap to pick the center of the circle.

Use another osnap (like Near or Quad), to pick the arc start point on the circle.

Then click the Length option, check the clockwise/anticlockwise direction, and enter your desired length.

easy peasy

yes sir! that’s what i needed, thanks John!!