An Angestellter's payslip already has the hard parts done for them. Their employer runs Lohnsteuer withholding every month, splits the four social insurance contributions roughly 50/50, and nothing lands in their account until tax, Rentenversicherung, Arbeitslosenversicherung, Krankenversicherung, and Pflegeversicherung have all already been taken out. A Freiberufler gets the gross figure in full, then has to handle every one of those pieces themselves.
That starts with the Finanzamt. Freelancers pay income tax via vierteljährliche Vorauszahlungen, quarterly advance payments based on last year's profit, then true it up (or get a refund) with the annual Steuererklärung. Real business costs, Betriebsausgaben such as a laptop, software, a coworking desk, professional insurance, and accountant fees, reduce taxable profit first. This calculator uses a flat percentage of revenue as a simplified stand-in for that, since your actual ratio depends entirely on your own setup.
Health insurance is where the numbers really diverge. An Angestellter's employer pays half of their GKV contribution; a Freiberufler who stays in the statutory system as freiwillig versichert pays the FULL combined rate alone, 17.5% of income, floored at a minimum monthly assessment base and capped at the same contribution ceiling employees use. Private insurance (PKV) is an alternative, but it prices on your age and health at entry rather than your income, so it can't be reduced to a single formula the way GKV can.
The part that's easy to miss entirely is pension. An Angestellter's 9.3% Rentenversicherung contribution isn't a tax, it's compulsory retirement saving their employer matches. A Freiberufler in most professions isn't required to pay into it at all, and neither into the 1.3% Arbeitslosenversicherung. Skip both and your monthly net figure looks bigger by exactly that much, but you've also skipped the retirement saving, not just the deduction. For a genuinely fair comparison, price in whatever you'd actually set aside voluntarily, whether a statutory scheme, a Versorgungswerk, or a private pension, before trusting either net number on its own.