The best way is to measure a bracelet that you wear and know that you like the fit.

Or:

Measure your wrist between the wrist bone and hand.  Use a flexible measuring tape or strip of paper.

This is your wrist size.  Use the following to determine how your bracelet will fit.

For a lobster/figure 8 clasp, I usually increase the wrist measurement by 1".  

The guidelines are below:

snug fit:  add 1/2"

comfort fit:  add 3/4"-1"

loose fit: add 1 1/4"

For a toggle clasp, I usually increase the wrist measurement by 1 1/2".

The guidelines are below:

snug fit:  add 1 1/4"

For a comfort toggle clasp fit:  add  1 1/2"-1 3/4"

For a loose toggle clasp fit:  add 2"