A savings account calculator that is useful in Belgium has to do two things the advertised rate does not: apply the deposit cap and count the loyalty bonus (prime de fidélité) only once it is earned. Over twelve months, €10,000 then earns €61.13 in a capped account at 3.15%, and €200 in an uncapped one at 2.00%.
3.15% on one side, 2.00% on the other, and the second wins
The tool below sets two accounts side by side and answers in euros. It is preset to two opposite structures found on the Belgian market: a high-rate account limited to €500 a month, and a lower-rate account that takes the whole capital at once. Change the capital, the monthly deposit, the period or any rate, and the result follows.
Two savings accounts, one figure that matters: euros
Your savings
From 1 to 120 months. The loyalty bonus is only earned after 12 full months.
Account A
Default: KBC Start2Save, 1.65% + 1.50%, €500 per month. Source: KBC Start2Save product page, re-read 14 October 2026.
Account B
Default: Keytrade High Fidelity, 0.50% + 1.50%, no cap. Source: Keytrade Bank product page, accessed 14 October 2026, and Test-Achats survey of 21 September 2026.
Every value can be edited: replace them with the figures on your own account’s key information sheet.
Result at the end of the period
| KBC Start2Save | Keytrade High Fidelity | |
|---|---|---|
| Amount that entered the account | €6,000.00 | €10,000.00 |
| Amount left outside (cap) | €4,000.00 | €0.00 |
| Base interest | €53.63 | €50.00 |
| Loyalty bonuses earned | €7.50 | €150.00 |
| Interest earned, gross | €61.13 | €200.00 |
| Annual yield on all the money available | 0.61% | 2.00% |
| Bonus still accruing, lost if you withdraw that day | €41.25 | €0.00 |
Keytrade High Fidelity earns €138.87 more over 12 months.
Tipping point: below €1,525.00 of starting capital, KBC Start2Save earns more; above it, Keytrade High Fidelity moves ahead (same monthly deposit, same period).
Note: €4,000.00 never entered KBC Start2Save because of the deposit cap. That money is counted here as earning nothing.
Assumptions behind the calculation
- Starting capital deposited in month 0, monthly deposit from month 1 (the convention used by Wikifin’s savings account comparator).
- Each month, all available money is deposited, up to the cap. The surplus waits for the next month and earns nothing meanwhile.
- Base rate: deposit × rate × months held ÷ 12. Banks count in days: the gap is no more than one or two days of base interest per deposit.
- Loyalty bonus: deposit × rate, for each complete 12-month period. It is paid on the first day of the quarter after it is earned.
- Rates assumed constant. In practice the base rate can change at any time; the bonus is locked for 12 months from each deposit.
- Interest not reinvested, no withdrawals, total balance caps not handled, withholding tax not deducted.
An estimate, not a statement
Only your bank’s own statement and the key information sheet for savers are authoritative. This tool compares conditions; it recommends no investment and is not personal advice. To compare every account on the market, the official Wikifin (FSMA) comparator is the reference.
The default values are those of the KBC Start2Save (1.65% base rate, 1.50% bonus, €500 a month, page read on 14 October 2026) and the Keytrade High Fidelity (0.50% and 1.50%, no cap, surveyed by the consumer organisation Test-Achats on 21 September 2026 and confirmed on Keytrade Bank's own page on 14 October 2026). They are examples, not a selection. The ranking of Belgian banks judges institutions on their full annual cost, current account included.
How does the calculator work out the interest?
It adds two lines that do not run on the same clock, then sets a third one apart.
Base interest. For each deposit: amount × base rate × months held ÷ 12. The rule comes from Wikifin, the financial education site of the FSMA, Belgium's Financial Services and Markets Authority: base interest starts no later than the day after the deposit and is paid once a year, around 1 January.
Loyalty bonuses earned. For each deposit: amount × bonus rate, as many times as the deposit has spent twelve full months in the account. According to Wikifin again, the bonus rate is set for twelve months at the time of the deposit and paid quarterly, on 1 January, 1 April, 1 July or 1 October.
The bonus still accruing. This is the share of bonus built up by deposits that have not yet reached their anniversary. It is not part of the total. It sits on its own line because it has a price: a withdrawal makes it disappear.
The cap comes in before all of that. Each month, the calculator deposits the money available up to the permitted limit and carries the rest over to the next month. The starting capital arrives in month 0 and the first monthly deposit one month later, a convention taken from Wikifin's savings account comparator, which describes it in its explanatory note.
Three simplifications are deliberate. A month counts as one twelfth of a year, whereas the bank counts days. Rates stay constant for the whole period. Interest paid out is not reinvested.
What does the calculation by hand give on €10,000?
€61.13 for the capped account, €200 for the other. Here is the working, which you can redo on a pocket calculator.
In account A, the €500 cap lets twelve deposits in over a year, or €6,000. The remaining €4,000 never gets in. The first deposit stays twelve months, the second eleven, and so on down to the last, which stays one month: 12 + 11 + … + 1 = 78 months held in all. Base interest is therefore 500 × 1.65% × 78 ÷ 12, or €53.63. A single deposit reaches its twelve months, the first: 500 × 1.50% = €7.50 of bonus. Total: €61.13, or 0.61% of the €10,000 you had to place.
In account B, everything goes in on day one. 10,000 × 0.50% = €50 of base interest, 10,000 × 1.50% = €150 of bonus, earned in the twelfth month. Total: €200.
Gap: €138.87 in favour of the lower rate.
Account A does hold €41.25 of bonus still accruing at that date, which will mature if the money stays. Over twenty-four months the gap narrows (€304.38 against €400) without reversing.
And for €500 deposited every month for two years?
The ranking turns over. With no starting capital and €500 a month for twenty-four months, the cap no longer gets in anyone's way: both accounts receive the same €11,500, in twenty-three deposits.
The months held add up from 23 down to 1, which makes 276. In account A: 500 × 1.65% × 276 ÷ 12 = €189.75 of base interest. In account B, at a base rate of 0.50%: €57.50. The bonus is identical on both sides, since the bonus rate is the same and twelve deposits have passed their anniversary: 12 × €7.50 = €90.
Result: €279.75 against €147.50. A capped account rewards a regular deposit, not capital you already hold.
From what capital does the uncapped account move ahead?
Very early over one year, much later over three. The calculator looks for this tipping point at every setting; here are its results for capital placed in one go, with no monthly deposit, against the Keytrade High Fidelity at 2.00%.
| Account capped at €500 a month | Tipping point over 12 months | Tipping point over 24 months | Tipping point over 36 months |
|---|---|---|---|
| KBC Start2Save, 1.65% + 1.50% | €1,525 | €6,736 | €10,823 |
| NIBC Opti, 2.00% + 1.50% | €2,375 | €8,187 | €13,500 |
Our own calculation of 14 October 2026, rates assumed constant. How to read it: with €5,000 to place for two years, the Start2Save earns more; with €8,000, the uncapped account moves ahead.
The period changes everything. The longer you give the capped account to fill up, the more capital it can absorb. But during those thirty-six months its base rate can be changed at any time, and the base rate is what carries its advantage.
What do five Belgian accounts earn over 12 and 24 months?
The table below works as a reference even if you never touch the tool. Each line carries its base rate, its bonus, its cap and the date the rate was recorded.
| Savings account (date recorded) | Base + bonus | Cap | €10,000 over 12 months | €10,000 over 24 months | €300 a month over 24 months |
|---|---|---|---|---|---|
| KBC Start2Save (14 October 2026) | 1.65% + 1.50% | €500/month | €61.13 | €304.38 | €167.85 |
| Belfius Flow (18 September 2026) | 1.83% + 1.50% | €600/month | €80.37 | €372.44 | €180.27 |
| NIBC Opti (19 August 2026) | 2.00% + 1.50% | €500/month | €72.50 | €346.67 | €192.00 |
| Keytrade High Fidelity (21 September 2026) | 0.50% + 1.50% | none | €200.00 | €400.00 | €88.50 |
| Belfius Fidelity (18 September 2026) | 0.15% + 1.50% | none | €165.00 | €330.00 | €64.35 |
Gross interest earned, calculated on 14 October 2026 with the method described above. The Belfius and NIBC rates are those of our survey of 3 October 2026 and have not been re-read since.
Two columns, two opposite rankings. The account that comes last on €10,000 of capital comes near the top on a €300 monthly deposit, and the NIBC Opti, first on the monthly deposit, stays far behind on capital. None of these accounts is the best in absolute terms.
Why does the result differ from the bank's statement?
Because it is an estimate, resting on assumptions that reality does not always honour.
The first concerns the calendar. A bank counts in days and applies a value date; the calculator counts in months. The gap is no more than one or two days of base interest per deposit. The second concerns rates: the bank can change the base rate whenever it likes, including on money already deposited, and nothing tells you what it will be in eighteen months. The bonus is locked for twelve months from each deposit, but the next one will be set at the rate of the day.
Then come the items the tool leaves aside: base interest reinvested every 1 January, the total balance caps that some accounts add to the monthly cap, any fees on the current account the bank requires, and the payment delay on the bonus, which lands on the first day of the quarter after it is earned.
I would rather have a simple calculation whose every assumption is visible than a to-the-day calculation that shows none. For the figure that counts, look at the bank's statement. And to go through every account on the market, the Wikifin comparator remains the official reference: it does a different job from this tool, which pits two accounts against each other and shows how much the cap leaves outside.
Which misreadings come up most often?
- Taking the accruing bonus for money earned. It only becomes yours on each deposit's anniversary. Our article on the cost of moving a savings account puts a figure on what leaving early costs.
- Forgetting the amount left outside. The €4,000 in the example is sitting somewhere, often in a current account at 0%. If you place it elsewhere, add that return yourself.
- Entering today's advertised rate for an older deposit. Your bonus is the one in force on the day of the deposit, as our article on the loyalty bonus explains.
- Comparing over a period that is not yours. Over six months no bonus is earned, and only base rates can be compared.
- Reading the result as a guaranteed yield. Only the bonus on deposits already made is guaranteed, for twelve months.
Should you deduct withholding tax?
Rarely. In 2026, interest from a regulated savings account is exempt up to €1,020 per person per year, €2,040 for a married or legally cohabiting couple, and the 15% withholding tax (précompte mobilier) only applies to the excess. At 2.00%, you need €51,000 in the account to get there.
The calculator shows a reminder when the annual pace of interest passes €1,020. It does not make the deduction: the exemption is calculated per taxpayer, across all accounts, and the tool only knows the two you show it.
If the money can stay locked away, the trade-off is a different one, and term deposit or savings account sets out the figures. To put the savings rate next to the cost of the same bank's current account, the Belgian bank comparator shows them side by side.
Sources
- Wikifin (FSMA), What does your savings account earn you? (in French): start of base interest, how the bonus is earned and paid, order of withdrawals, €1,020 exemption in 2026. Page updated 12 May 2026, accessed 7 October 2026.
- Wikifin (FSMA), Savings account comparator and its explanatory note: initial amount with same-day value date, first regular deposit one month later. Accessed 7 October 2026.
- KBC, KBC-Start2Save savings account: 1.65% base rate, 1.50% loyalty bonus, €500 a month at most. Page re-read 14 October 2026.
- Test-Achats, Rising rates: 5 ways to earn more on short-term savings: 21 September 2026, Keytrade High Fidelity at 0.50% + 1.50%. Rate confirmed on 14 October 2026 on Keytrade Bank's High Fidelity page (category A, no deposit cap).
- Belfius, press release of 11 September 2026 (Belfius Flow at 1.83% + 1.50% from 18 September, Belfius Fidelity unchanged), and Test-Achats, NIBC Opti: savings rate rises to 3.50%, 19 August 2026. Recorded on 3 October 2026.
- FPS Finance, Movable income: 15% withholding tax on the taxable share of regulated accounts. The 2026 amounts quoted here are those published by Wikifin.
- Ombudsfin, Submit a complaint: free mediation for banking disputes.
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Frequently asked questions
Marion a travaillé six ans en agence, à Namur puis à Bruxelles, avant de passer côté rédaction en 2020. Elle a ouvert, fermé et transféré assez de comptes à vue pour savoir où se cachent les frais : le forfait « tout compris » qui exclut justement le virement instantané, la carte de crédit offerte la première année, le compte d’épargne dont la prime de fidélité s’évapore au premier retrait. Depuis, elle relit chaque janvier et chaque juillet les documents d’information tarifaire des banques belges, recalcule le coût annuel réel par profil, et vérifie ce que rapporte vraiment un taux affiché quand le plafond est de 500 € par mois. Ce qui l’agace : les comparatifs qui classent une banque première sur la qualité de son application alors que le client, lui, paie ses frais en euros.