The withholding is 20% of €1,000.00, which is 20% of the fee. Your client pays you the net and remits the withholding to the Agenzia delle Entrate, Italy's tax agency, by the 16th of the following month.
| Fee | €1,000.00 |
| VAT base | €1,000.00 |
| VAT (22%) | €220.00 |
| Invoice total | €1,220.00 |
| Withholding base | €1,000.00 |
| Withholding (20% on 100% of the base) | −€200.00 |
| Net payable | €1,020.00 |
This is a withholding on account (a titolo d'acconto): not an extra tax but an advance on the IRPEF you will declare. You recover it in your annual tax return, where it is set against the tax due.
The withholding is not charged on the invoice total: the 4% INPS rivalsa and undocumented allowances are inside the base, while a professional fund's contribution and expenses documented and advanced for the client are not.
The two 4% contributions look alike on an invoice and behave in opposite ways. The INPS rivalsa for the gestione separata counts as fee income for tax, so it enters both the VAT base and the withholding base: on a 1,000 euro fee with the rivalsa the withholding is 208 euro, not 200. A private professional fund's contributo integrativo (Cassa Forense, Inarcassa, ENPAM) is instead collected on the fund's behalf: it enters VAT but stays outside the withholding. Likewise only expenses analytically documented and advanced in the client's name and on their behalf (article 15 of DPR 633/1972) stay outside both bases; a flat allowance with no documents behind it is fee income.
Educational estimate, not tax advice. The rate and the base depend on the type of service and on the invoicing party's tax regime; special cases such as withholding on building works in a condominio, travel agencies and agricultural businesses follow their own rules. Check with the Agenzia delle Entrate or your accountant.