Go to the website to calculate Profit Margin:

finance.cch.com/sohoApplets/ProfitMargin.asp

Formula to calculate Selling price using Gross Margin

Selling Price = Cost / (1-GM%)
40% Margin
For example, if your product costs $100 and the required gross margin is 40%, then your Selling Price = $100/(1-0.4) = $100/0.6 = $166.6
 
Example 2
35% Margin
 
You want to sell a bag that costs $3.25 with a 1 color screen print: You want to make 35% Gross Margin.
 
Selling Price = $3.25/ (1-.35) = $3.25/.65= $5.00
 
 
Example 3: 
30% Margin
 
You want to sell a sreenprinted shirt that costs $2.50 with a 2 color print: You want to make 30%
 
Selling Price = $2.50/ (1-.30) =  $2.50/.70 = $3.57 each
 
Example 4:
25% Margin
 
You want to sell a coffee mug that costs $3.00 with a 1 color print. You want to make 25% margin
 
Selling Price = $3.00/ (1-.25) = $3.00/.75= $4.00 each