Enter the gross dividends for the year and the marginal IRS rate you would have under aggregation. For companies in Portugal or the EU, only half the dividend is taxed under aggregation; from outside the EU, the full amount is.
Flat 28% (taxa autónoma)
Do nothing: the 28% withholding is the final tax.
Aggregation (englobamento)
Opt to aggregate: only 50% of the dividend is taxed (Art. 40-A CIRS).
Here it pays to opt for aggregation: you save €105.00 of tax versus the flat 28%.
Note: aggregation forces you to combine all income in the same category and can raise the rate on the rest of your income. Decide with your full IRS in mind.
Educational estimate, not tax advice. It compares the tax on dividends by each route; it does not compute your full IRS or the withholding at source on foreign dividends (double-taxation credit).