Investing guide · Reviewed 27 August 2026

Compound interest is a relationship between time, money and an assumption.

A future-value result is not a promise. It is a transparent answer to a specific question: what would happen if the entered rate, contribution and timing stayed unchanged?

What compounds

With simple interest, the original principal earns interest. With compound interest, previously calculated interest remains in the balance and can itself earn a later return. Recurring deposits add another layer: each deposit has a different amount of time to grow.

That is why two people contributing the same total amount can finish with different balances when one starts earlier or contributes more consistently.

The three assumptions to inspect

• Rate: an annual rate is an input, not a forecast. Markets and bank products do not deliver a constant return.
• Timing: this site’s recurring-contribution examples use end-of-month contributions unless a page says otherwise.
• Fees and tax: they reduce a real outcome but are excluded from the basic illustration unless explicitly added.

Use the result as a sensitivity tool

Change one input at a time. Compare a lower and higher assumed rate, then compare the effect of delaying contributions. This reveals which assumptions drive the result instead of turning one attractive number into a target.

Test the compound interest calculator ↗ Apply the idea to retirement modelling ↗

Source note: The explanation uses standard future-value and ordinary-annuity mathematics. It is educational information, not investment advice, a guarantee or a product recommendation.