The tale of two savers
Anna starts putting away $200 a month at age 25. Ben starts the identical $200 a month at 35. Both earn 7% a year and both stop at 65. Run it through the compound interest calculator:
- Anna, 40 years of saving: she put in $96,000 and ends with about $525,000.
- Ben, 30 years of saving: he put in $72,000 and ends with about $245,000.
Anna contributed $24,000 more than Ben. She finished $280,000 ahead. The extra decade didn't add ten years of savings; it added ten years of growth on top of growth, and that's where most of the gap comes from.
Here's the version of this that actually keeps people up at night: if Ben tries to catch Anna by saving more, he needs roughly $430 a month, more than double her rate, for all 30 of his years. Time is the one input you can't buy back.
What compounding actually is
Simple idea, badly named. Year one, your $1,000 earns $70. Year two, you earn 7% on $1,070, which is $74.90. The interest starts earning its own interest. For the first few years this looks like pocket change and people quit, which is the standard mistake. The curve is flat at the start and steep at the end by design. In Anna's example above, her account earns more in its final five years than she contributed in her first twenty.
The rule of 72
Quick way to feel any interest rate in your bones: divide 72 by the rate, and that's roughly how many years money takes to double. At 7%, doubling takes about 10 years. At 3%, about 24 years. At 10%, about 7. It also works in reverse on things working against you: 6% inflation halves your cash's buying power in about 12 years, and an 18% credit card doubles a balance you ignore in just 4. Same math, different direction.
The honest fine print
Real investments don't pay a smooth 7%. Markets drop 20% some years and jump 25% in others; 7% is a long-run stock-market-ish average that plenty of decades beat and some fall short of. The calculator's steady curve is a planning tool, not a promise. Taxes and fees take their cut too, and inflation means Anna's $525,000 won't feel like $525,000 does today. None of that changes the core conclusion, though. Whatever the actual return turns out to be, the person who started earlier gets more of it.
If you're "late"
The second-best time is now, as the tree-planting proverb goes. A 45-year-old saving $400 a month at 7% still has around $200,000 at 65. That's not the fantasy number, but it's a very different retirement than $0. And the doubling math means the last thing you want to do is wait for a "better time to start." There isn't one. There's just earlier and later, and earlier wins by exactly the amounts shown above.
Plug your own age and numbers into the compound interest calculator. Nothing you enter leaves your browser. And if debt is eating the money you'd save, read what a loan really costs first; paying off an 18% card is a guaranteed 18% return, which no market offers.