How much investing a fixed amount every month grows
The question sounds simple, but the answer surprises: the final figure depends far less on how much you put in and far more on how long you leave it.
The numbers
Investing a fixed amount monthly at about 0.8% a month, after 5 years interest is roughly 19% of the total; after 20 years it is about 64%. Most of the balance stops being your deposits and becomes interest on interest.
Why time beats the amount
Doubling the monthly amount for ten years yields less than half the amount for twenty. Compound growth is exponential in time and only linear in the amount, so starting early with less beats starting late with more.
What the simulation leaves out
Inflation (use a real rate for purchasing power), income tax on returns, and consistency — the simulation assumes you never skip a contribution or withdraw early.
Running your own numbers
Use the compound interest calculator, which charts contributions separately from interest, or the goal-specific versions for retirement and emergency funds.