How do I find the concentration of a primer?

The starting material will determine the method you need to use to find the concentration of a primer. 

To find the concentration of a primer from lyophilized powder 

Most companies will send you primers as lyophilized powders, with details of how many nmoles of primer are provided. 

First, re-suspend the primer in 100 µl of water or buffer, then use this calculation to find the primer concentration: 

(XX nmol/100 µl) x (1000 pmol/nmol)= pmol/µl = µM

Assuming you have 22 nmol of a DNA primer containing 16 bases, your calculation will be as follows: 

(22 nmol/100 µl) x (1000 pmol/nmol) = 220 pmol/µl = 220 µM

To find the concentration of a primer from liquid

You’ll have to use a different method to find the primer concentration if you didn’t recover the full amount of lyophilized primer that was sent by the company or you froze the tubes of primer without writing the concentrations on them. In these cases, you’ll have to use an OD260 reading to calculate the concentration of a primer. 

Assume the primer was diluted from above 1:200 and the OD260 reading was 0.132. Calculate the primer concentration using these formulas:  

(OD260) x (0.02*) x (dilution factor) = µg/µl       *Conversion factor for single stranded DNA

(µg/µl/330Daltons/nt**) x (106/# of nt) = µM    **Average molecular weight per nucleotide

Example:  (0.132)(0.02)(200)=0.528 µg/µl

(0.528/330) x (106/16)= 100 µM