How the rate is worked out
Your profit has to cover your tax before anything reaches you, so the calculator grosses up the income you want to keep by your tax rate. Business costs are added on top of that, since they come out of what you bill rather than out of your pocket. The result is the revenue you need for the year, divided by the hours you can actually bill.
Billable hours are the number people get wrong
A full working week is around 40 hours, but very little of that is billable. Pitching, invoicing, email, admin, learning and chasing payment all take real time that no client pays for. Twenty to twenty-five billable hours a week is realistic for most solo freelancers. Setting that field to 40 is the fastest way to arrive at a rate that quietly underpays you.
Weeks off matter just as much
Employees are paid during holidays and sick leave. Freelancers are not, so the cost of that time has to be spread across the weeks you do work. Dropping from 52 weeks to 46 raises your required rate by more than ten percent on its own.
Is this tax advice?
No. The tax field is a single flat percentage, which is a rough stand-in for progressive brackets, deductions and local rules. Treat the result as a floor to negotiate up from, and check the tax side with an accountant who knows your situation.