Compound Interest Calculator
Starting amount, monthly deposits, and time — see what growth on growth really does.
How this is calculated
FV = P(1+r)ⁿ + D × ((1+r)ⁿ − 1) ÷ r where r = annual rate ÷ 12, n = years × 12
Worked example: R10,000 starting, R500 monthly, 8% for 10 years → the lump sum grows to about R22,196 and the deposits stack to about R91,473 — roughly R113,670 total from R70,000 contributed. The gap is compounding: every deposit earns, then its earnings earn.
The honest caveats
Projections assume a constant rate; real returns wobble. Inflation quietly shrinks what the final number buys, and tax or fees can take a slice of the growth. None of that changes the core lesson the slider makes obvious: years beat rate, and both beat waiting.
Frequently asked questions
How does compound interest work?
Interest is added to your balance, and the next period earns interest on that bigger balance — growth on growth. With monthly compounding, a year has twelve small growth steps instead of one, which is why the final value beats simple interest at the same quoted rate.
What does this calculator assume?
Monthly compounding (the norm for savings accounts and unit trusts), and deposits added at the end of each month. The formula is FV = P(1+r)ⁿ + D·((1+r)ⁿ − 1)/r with r the monthly rate and n the number of months — shown under the calculator with a worked example.
Why do small monthly deposits matter so much?
Because each deposit gets its own compounding runway. In the worked example below, R500 a month at 6% becomes about R81,900 in ten years — R60,000 of deposits and roughly R21,900 of pure growth. Start five years later and the growth part shrinks by more than half.
Is the rate guaranteed?
No — this is a projection at the rate you enter, not a promise. Savings rates float and investment returns vary year to year. Use a conservative rate for planning, and remember the result ignores tax and fees, which reduce real-world growth.