The Power of Compound Interest (and the Rule of 72)
Albert Einstein probably never actually called compound interest the eighth wonder of the world — but the saying stuck because the idea really is that powerful. Compounding is the engine behind almost all long-term wealth, and it rewards one thing above all: time. Here’s how it works and how to think about it.
What compounding actually is
Simple interest earns a return on your original money only. Compound interest earns a return on your original money and on all the returns you’ve already earned. Those returns start earning returns of their own, and the snowball grows faster and faster.
Year one looks boring — a few percent on your balance. But each year the base is bigger, so the same percentage adds more dollars. Decades in, most of your balance is growth, not contributions. That late-stage acceleration is why starting early matters so much.
The Rule of 72: doubling in your head
Want a quick sense of how fast money grows? Use the Rule of 72: divide 72 by your annual return to estimate the years it takes to double.
| Annual return | Years to double (72 ÷ rate) |
|---|---|
| 3% | ~24 years |
| 6% | ~12 years |
| 8% | ~9 years |
| 9% | ~8 years |
| 12% | ~6 years |
So money growing at 6% doubles roughly every 12 years: $10,000 → $20,000 by year 12, → $40,000 by year 24, → $80,000 by year 36. The same rule works in reverse for inflation — at 3% inflation, prices double (and your cash halves in value) in about 24 years.
Time is the biggest lever — more than amount
Here’s the classic example that surprises everyone. Two people each earn about 7% a year:
- Early Erin invests $5,000 a year from age 25 to 35 — ten years, $50,000 total — then never contributes another dollar and just lets it grow.
- Late Liam waits, then invests $5,000 a year from age 35 to 65 — thirty years, $150,000 total.
At 65, who has more?
| Contributed | Approx. value at 65 (~7%) | |
|---|---|---|
| Early Erin | $50,000 | ~$525,000 |
| Late Liam | $150,000 | ~$470,000 |
Erin invested a third of what Liam did and still came out ahead — purely because her money had more time to compound. You can’t out-contribute a head start. Test it yourself in the compound interest calculator.
Why small differences become huge
Because compounding multiplies over many years, small changes in the inputs have outsized effects on the end result:
- A slightly higher return compounds into a dramatically bigger balance over decades.
- A slightly higher fee does the same in reverse — this is why a “small” 1% or 2% fund fee quietly costs so much (see what a 1% MER really costs).
- A few more years of growth at the end — when the balance is largest — adds more than years at the start.
The flip side: compounding against you
The same math that builds wealth can bury you. Credit card debt compounds against you at ~20% (Rule of 72: it would double in under four years if you let it), which is why minimum payments feel hopeless. Clearing high-interest debt is really just compounding working for you again — see the debt payoff calculator.
The takeaway
- Compounding is returns earning returns — it starts slow and accelerates.
- The Rule of 72 (72 ÷ rate = years to double) is a handy mental shortcut.
- Time beats amount: starting early is the single biggest advantage, because the final years of growth are the most powerful.
- The same force works against you on fees and high-interest debt — keep both low.
Watch compounding build over time in the compound interest calculator, or model a real portfolio in the ETF portfolio calculator.
This is general education, not financial advice. Examples use assumed returns for illustration — real returns vary.