There's no formula its just a matter of adding up all the bs that goes with them.
Your going to have unemployment insurance but how much depends on where you live and have you laid anyone off before. We get tagged for the max rate here so that's 6.8%
Then the feds charge .6% for their unemployment insurance.
Workman's comp again these rates vary are you new did you have a claim what is it you do. We pay 4% of the employees payroll.
Then there is your half of social security and medicare 7.3%
Then there is a payroll service if you do not do this yourself for us that's 2k a year.
Then comes time and half for anything over 40 hours and the percentages above are on total hourly wages so when they are on OT the other costs go up accordingly.
Then you got indirect cost like motor vehicle insurance tends to go up when you add more drivers and if they have a bad record it can get ugly.
Hope that helps