How Compound Interest Builds Wealth (And Why Starting Early Wins)
Two people save for retirement. Alex invests $250 a month from age 25 to 35, then stops completely — ten years, $30,000 total. Sam invests nothing until 35, then puts in $250 every month until 65 — thirty years, $90,000 total.
At 65, assuming 7% annual growth, Alex has about $351,000. Sam has about $305,000. Alex contributed a third as much money and finished ahead by $46,000, having stopped saving three decades earlier.
That result isn't a trick. It's the entire argument for starting early, expressed in one comparison.
The short version
- Compound growth pays returns on your returns — so the earliest money does the heaviest lifting.
- Time matters more than amount. A decade of head start is worth more than tripling your contributions later.
- Rule of 72: divide 72 by your return rate to estimate how many years until money doubles.
- Compounding frequency barely matters. Rate and time are what move the needle.
What compounding actually is
Simple interest pays you only on your original deposit. Compound interest pays you on your deposit plus every bit of growth already earned.
$10,000 at 7% earns $700 in year one. In year two you earn 7% on $10,700 — $749. The extra $49 is interest on interest. It looks trivial. But by year 20 that $10,000 has become $40,387. Simple interest would have produced only $14,000 of growth over the same period; compounding produced $30,387. That extra $16,387 is entirely growth earning its own growth.
The formula, if you want it: A = P(1 + r/n)nt, where P is the starting amount, r the annual rate, n the compounding periods per year, and t the years. The exponent is the whole story — it's why the curve bends upward rather than climbing in a straight line.
The Rule of 72
A genuinely useful piece of mental arithmetic: divide 72 by your annual return to get the approximate years to double.
| Annual return | Rule of 72 estimate | Actual |
|---|---|---|
| 3% | 24 years | 23.4 years |
| 5% | 14.4 years | 14.2 years |
| 7% | 10.3 years | 10.2 years |
| 10% | 7.2 years | 7.3 years |
Close enough for planning in your head. It also works in reverse as a warning: at 7% inflation, prices double in about ten years.
Model your own numbers. Set a starting balance, monthly contribution, rate and horizon, and see the year-by-year growth table.
Open the compound interest calculatorWhy the first decade matters disproportionately
Return to Alex and Sam. The reason Alex wins is that his $30,000 spent the maximum possible time compounding. Money invested at 25 has forty years to double roughly four times over at 7%. Money invested at 55 has ten years — one doubling.
Put differently: a dollar invested at 25 is worth roughly four times a dollar invested at 45, before you've saved a single extra cent. You cannot buy that time back later, at any contribution level. This is the one financial advantage available exclusively to people who currently feel too broke to invest.
The long view
$250 a month for 40 years at 7% comes to $656,000, from $120,000 of actual contributions. Roughly 82% of the final balance is growth, not deposits. The saver's job is mostly to start, keep going, and not interrupt it.
Does compounding frequency matter?
Much less than people assume. $10,000 at 7% for 20 years:
- Compounded annually: $38,697
- Compounded monthly: $40,387
A $1,690 difference over two decades — real, but trivial next to what a single percentage point of return or five extra years would do. Don't choose accounts on compounding frequency; choose on rate, fees and tax treatment.
What return should you actually assume?
This is where planning goes wrong most often. Reasonable assumptions:
| Where the money is | Reasonable planning assumption |
|---|---|
| Chequing account | 0% |
| High-interest savings / GIC / CD | Whatever is currently offered |
| Bond funds | 3–5% |
| Diversified stock index funds (long run) | 6–8% before inflation |
Two caveats worth taking seriously. First, that 6–8% is a long-run average, not an annual guarantee — real markets deliver +20% one year and −15% the next, and the average only emerges over decades. Second, it's a nominal figure. Subtract 2–3% for inflation to think in today's purchasing power; planning at 4–5% real is more honest.
The three things that quietly destroy compounding
1. Fees
Fees compound against you exactly as returns compound for you. On $250/month over 40 years at a 7% gross return, a 0.2% index fund finishes at about $620,000 while a 2% managed fund finishes at about $382,000 — a difference of $239,000 on identical contributions. A 2% fee doesn't take 2% of your money — over a lifetime it takes a large fraction of your growth. This is the single most controllable variable in investing.
2. Interrupting it
Withdrawing during a downturn locks in the loss and removes that money from all future compounding. This is the real function of an emergency fund: it isn't just a safety net, it's what protects your long-term investments from being raided at the worst possible moment.
3. Taxes on growth
Money compounding inside a tax-sheltered account (TFSA/RRSP in Canada, ISA in the UK, 401(k)/IRA in the US) keeps everything. Money in a taxable account is skimmed each year, and the skimmed amount never compounds again. Filling tax-sheltered space first is usually worth more than any fund selection.
What to actually do
- Start now, at any amount. $50 a month beginning today beats $500 a month starting in five years, for a long time.
- Automate it on payday, so it's never a monthly decision.
- Use tax-sheltered accounts first, and capture any employer match before anything else.
- Keep fees low — this is free money, and it's entirely within your control.
- Raise contributions with every raise, before lifestyle absorbs it.
- Then leave it alone. Compounding's requirement is time, and its enemy is fiddling.