Lead conversion rate measures the percentage of your leads that end up converting to opportunities.  To calculate lead conversion rate, you take the number of leads converted to opportunities in a period, and divide that by the number of leads created in that period.
For example, if you had 100 leads that were created in March, and you had 18 leads converted to opportunities in March, then your lead conversion rate would be 18 / 100 = 18%
Formula (Leads converted to Sales/Total Leads)x100