What gets deducted from the gross salary
Two things are mainly deducted from the gross salary: Social Security contributions (a fixed percentage, approximately 6.35% for most permanent contracts in Spain) and income tax withholding (IRPF), which is an advance payment of income tax and varies according to your personal situation.
Why you enter the tax rate yourself
The income tax withholding percentage depends on factors such as your total annual income, number of dependents, disability, contract type, region, and other personal circumstances — the Spanish tax agency calculates it individually. Trying to automate this here would give unreliable results; that's why we ask for the percentage that already appears on your payslip, so the calculation is done correctly on that real figure.
If you don't have a payslip yet and want a rough estimate beforehand, many accounting firms and the Spanish Tax Agency itself offer withholding simulators — this tool is mainly useful for "translating" a tax percentage you already know into concrete net euros.