Need equation to work out frequency!

I am currently working on a project that involves using 555 ICs; I need to change the size of C1 R1 and R2 to get the right frequency, so that the clock pulses are not to slow or to fast. Does anyone know the mathematical equation that I need to do this???

Any help would be great,

Thanks

**broken link removed**. The formulas are on page 8 in the "applications" section.

When I started playing with 555 chips this site was quite informative. Has the formulas and a little more explanation than the datasheets.

Thank You...