Say you're comparing eg. a FTE with $115k salary, and 4 weeks vacation. Naively, a contractor working those hours is 115 / 48 weeks / 48 hours = $60/hr for the same salary.

But the employer is also paying out payroll taxes, insurance, office space/supply expenses, retirement plans (401k) - this typically adds up to 25-30% on top of the base salary.

Meanwhile, the contractor also isn't expecting their position to be a long-term gig, and (at least if you're not at an agency) also is managing a business so needs to account for business overhead (legal/accounting) and time spent lining up more business, which is un-billable. If they worked 4 days weekly at the contract gig and 1 day as overhead, already they need to bump salary expectation by 25% to keep even. And if you want to account for downtime between gigs, that number gets higher much quicker.