Compound interest

Money grows on money — and the magic is in the last few years.

Compound interest is the most-quoted concept in personal finance, and the most misunderstood. The boring middle decades fool people into giving up before the magic happens. Here's what the curve actually looks like, why timing beats amount, and how inflation quietly eats the headline number.

What compounding actually is

If you put $100 in an account earning 5% per year, after one year you have $105. After two years you don't have $110 — you have $110.25. The extra 25 cents is interest on the interest from year one. That's compounding, and over short periods it looks trivial.

The contrast is with simple interest, where you'd earn 5% on the original $100 every year forever, no matter how much had built up. Simple interest grows in a straight line. Compound interest grows in a curve that bends upward over time. Over a year or two, the curve and the line look almost the same. Over forty, they look like different worlds.

The math is unfussy: each year's earnings join the principal, and next year's earnings are calculated on the new, larger balance. It's how almost every savings account, retirement account, and investment portfolio actually grows.

Why the curve looks the way it does

If you graph compound growth over 40 years, the line is almost flat for the first 15. Then it tilts. By year 25 the slope is steeper than the line; by year 35 it's no contest. Most of the wealth in any compound-growth account is made in the last third of the timeline.

This is the part people quietly give up on. After ten years of saving, the balance feels disappointing — barely more than what was contributed. The temptation is to assume the math doesn't work, or that compounding is just a story financial salespeople tell. It's the opposite. The first ten years are the boring part. The math doesn't kick in visually until the curve has had enough time to bend.

Stop too early and you confirm your suspicion that it was a story. Stay the course past the bend and you discover that the story was conservative.

Why starting early beats saving more

The most counter-intuitive thing about compound interest is that when you save matters more than how much. Two example savers make this concrete. Both earn 7% per year on their savings. Both stop at age 65. The only difference is when they start.

Sarah saved a third of what James saved, and finished with $90,000 more. The reason is that Sarah's money had a decade longer to compound. Her contributions made between ages 25 and 35 spent the next thirty years multiplying. James's contributions, however disciplined, never got the same amount of runway.

This is the practical take-away from the curve. Time in the market is the most valuable thing you can give your future self. Even small early contributions, left alone, often beat much larger later ones.

Where this shows up in your life

Working for you

Superannuation. The 12% of your salary that gets paid into super compounds inside the fund — typically over 30 to 40 years for most workers. This is the largest compounding pool most Australians ever have, by a wide margin.
Long-term investments. Index funds, ETFs, and other diversified portfolios held over decades. The historical returns of major share indices are largely a compound-interest story.
High-interest savings accounts. Smaller scale, but the math works the same. Cash compounding at 4–5% over many years still grows meaningfully.
Reinvested dividends. When dividend payments get automatically reinvested instead of taken as cash, they buy more shares, which earn more dividends. The compounding mechanism in action.

Working against you

Mortgage debt. The bank's interest compounds against you. Every dollar of interest you pay this year was on a balance partly inflated by interest from previous years. This is why the offset account trick is powerful.
Credit card debt. Compounding at 18–22% per year. Paying only the minimum keeps you almost stationary against the interest charge. This is the most aggressive compounding most consumers ever face.
Investment fees. A 1% annual fee doesn't sound like much, but over 40 years it compounds against your return. A 1% fee on a 7% return cuts your final balance by roughly 25% over a working lifetime.
Buy-now-pay-later interest. Often advertised as "interest-free" but with late fees that, if missed payments roll, compound aggressively.

Three things that quietly mess with the math

Inflation

The widget above shows the gap. A 7% nominal return at 2.5% inflation is really only ~4.4% in real terms — that is, in terms of what your money can actually buy. Compounding still works, just slower than the headline number suggests. The inflation page goes into this in detail.

Sequence of returns

Compound interest examples assume a steady rate. Real markets don't deliver steady rates — they deliver an average over time, with big years and bad years mixed in. The order of those years matters more than people expect, especially when you're drawing down (retirement) rather than accumulating. A 30% loss early in retirement is much harder to recover from than the same loss in mid-career.

Tax and fees

The headline return is gross. Your actual experience depends on what tax you pay on the gains (capital gains, income tax on dividends, super contribution and earnings tax) and what fees the fund or platform charges. A 7% gross return at 1% fees in a 30% tax environment delivers closer to 4% net. Worth modelling honestly rather than working off the gross number.

None of these break compounding. They just slow it down — and they slow it down across the whole curve, not just the last bit. Realistic compounding is still extraordinary, just less extraordinary than a textbook example might suggest.

The honest summary

Compound interest is real, the math is correct, and the long-run effects are larger than most people emotionally accept. The reason it stays underused isn't that it's complicated — it's that the visible payoff takes longer than feels intuitive. Saving for ten years and watching your balance grow to slightly more than you contributed is psychologically very different from the textbook image of a steep upward curve. Both pictures are right; the textbook just shows you year 40.

The two practical lessons are: start as early as you can, even with small amounts, and don't stop in the boring middle. Almost everything else — picking the perfect fund, optimising the perfect tax structure — matters less than these two behaviours. Time and consistency are doing most of the work.

The widget above can show you what your own situation looks like. The real and nominal numbers together are more honest than just the nominal — start with those, decide what feels meaningful, and check back in twenty years.

The inflation explainer goes deeper on the real-vs-nominal gap shown in the widget. The offset accounts explainer shows the same compounding mechanism running in reverse against mortgage interest. Both are useful counterparts.