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Savings & Compound Interest Calculator

Compound interest with Portuguese IRS

Estimate how much your savings can grow with monthly contributions, monthly compounding and Portugal's 28% IRS withholding on interest. See the final balance, gross interest, tax withheld and net gain.

By Elijah·UPDATED AUGUST 2026·4 MIN·OFFICIAL SOURCES
KEY FACTS
Tax on interest
28% final withholding tax
Net TAN with 28%
Net TAN = 0.72 × gross TAN
Compounding
monthly, with contributions at month-end
Savings certificates
check the series and rate at IGCP
€
€
Assumido no fim de cada mês.
%
anos
TRATAMENTO FISCAL
Em depósitos, compare a TANB/TANL e a ficha do produto. Para certificados de aforro, use o simulador oficial do IGCP.
SALDO FINAL ESTIMADO
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ContribuiçõesJuro líquido
Total investido-
Juros brutos-
IRS retido-
Ganho líquido-
Taxa anual efetiva líquida-
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How this works

Compound interest only works on what stays invested. In Portugal, when deposit interest and many capital-income payments have 28% final withholding, it is the net interest that keeps compounding. This tool shows the difference between simulating the full gross rate and simulating the net rate after IRS.

  1. 1
    Enter capital, contribution and term
    The tool adds the starting amount and monthly contributions over the chosen term. Contributions enter at the end of each month, so they start earning interest in the following month.
  2. 2
    Use the gross annual rate
    Enter the TANB or gross annual rate promised by the deposit, interest account or savings product. The simulation divides that rate by 12 to estimate monthly compounding.
  3. 3
    Choose whether to deduct IRS
    With the IRS option on, each monthly interest amount has 28% withheld and only 72% compounds. This follows the net-rate logic described by Banco de Portugal for deposits where no special regime applies.
  4. 4
    Read the balance as an estimate
    Real deposits may pay interest at a different frequency, use a 30/360 day count or have early-withdrawal rules. Savings certificates and treasury certificates have their own series, bonuses and redemption rules.

Frequently asked

Is interest in Portugal always taxed at 28%?
For many deposit interest payments and other capital income paid by Portuguese entities, the IRS Code sets final withholding at 28%. Special regimes, autonomous-region residents, optional aggregation and international cases can change the result.
What are TANB and TANL?
TANB is the gross nominal annual rate, before tax. TANL is the net nominal annual rate, after withholding. With 28% withholding, Banco de Portugal explains the simple relation: net TAN = 0.72 × gross TAN.
Can I use this for certificados de aforro?
Only as an approximation. Certificados de aforro have series, limits, permanence bonuses, indexation and quarterly capitalization rules set by IGCP. For an official savings-certificate simulation, use the IGCP simulator and the technical sheet for the series.
Why does tax reduce compounding so much?
Because tax is deducted before the interest compounds. If a deposit yields 4% gross and has 28% withheld, only 2.88% nominal net remains to earn interest in the next period, before compounding.
Does this replace a bank quote?
No. The tool gives an educational estimate. Before subscribing, read the product information sheet, the technical sheet, the interest-payment frequency, withdrawal conditions and capital guarantee.
OFFICIAL SOURCES
DISCLAIMER
Informational estimate, checked on 18 August 2026. It assumes monthly compounding, contributions at month-end and, when selected, 28% IRS withholding on each monthly interest amount. It does not calculate inflation, risk, fees, stamp duty, regional regimes, aggregation, non-residents, variable-rate products, permanence bonuses, early withdrawal or specific savings/treasury-certificate rules. Not financial or tax advice.