http://www.beckmancoulter.com/resourcecent...s/rotorcalc.asp

Nice, do they have it for eppendorf centrifuges?

The relationship between revolutions per minute (RPM) and relative centrifugal force (xg) is:
g = (1.118 × 10^-5) R S²
where g is the relative centrifugal force, R is the radius of the rotor in centimeters, and S is the speed of the centrifuge in
revolutions per minute.

You can use this for any centrifuge, just measure the radius of the rotor from the center to outer edge.

You can use this one for all centrifuges too, if you're lazy. Even with a Nomogram for it.

...at least our new eppendorf 5430 R centrifuge in the lab has a button "rpm/ rcf" for automatic conversion of the rpm-value in g and vice versa - pretty easy to handle,-)

Go to the Beckman website and check or look at their catalogue. They have the rotors' radius there and also instruction about how to calculate g... If you are using other types of rotor then check the appropriate producer's catalogue. Calculating g may also depends on where your centrifuged particles are, e.g. the g for the pellet at the bottom of the tube is different from the g of the particles at the middle of the tube (if you are doing sucrose cushion or gradient for example)