What the numbers separate
The useful output is not the final figure — it is the split between what you contributed and what the interest added. Early on the first dominates completely; late on the second does. Seeing where the balance tips is more informative than the total.
Time beats rate
Compounding is exponential in time and linear in the rate, so an extra decade usually outweighs an extra percentage point. Someone contributing for 30 years at 6% ends up ahead of someone contributing the same for 20 years at 9%.
This is the one genuinely useful conclusion from a compound interest calculator, and it argues for starting rather than for optimising.
What the model does not know
A constant rate. Real returns arrive unevenly, and the order matters when you are also withdrawing. Fees compound too, in the wrong direction — a 1% annual fee removes a substantial share of a lifetime's growth. Tax is not modelled at all.
Treat the output as a shape, not a forecast.
Questions
Why does the contribution matter more than the rate early on?+
Because compounding needs a balance to work on. In the first years almost all growth comes from what you put in; the interest only overtakes contributions once the balance is large. That crossover is usually a decade or more away, which is why starting matters more than optimising the rate.
How often is it compounded here?+
Monthly, with contributions added at the end of each month. Compounding frequency has a smaller effect than people expect: at 8%, monthly compounding beats annual by about 0.3 percentage points a year.
Is this in real or nominal terms?+
Nominal. An 8% return with 6% inflation is 2% in purchasing power, and over decades that difference dominates everything else. Run the result through the [inflation calculator](/t/inflation-calculator) before drawing conclusions.
Is this financial advice?+
No. It is arithmetic on the numbers you type. It assumes a constant return, which no real investment provides, and it ignores fees, tax and the sequence of returns.