Compound interest is the most powerful force in personal finance — and it's almost always wildly misjudged by people looking at it for the first time. The phrase sounds abstract, but the underlying idea is simple: when your money earns a return, that return also earns a returnnext period. Stretched over decades, this small-looking effect moves wealth in a way that linear thinking can't predict.
Simple vs compound, one paragraph
With simple interest, every year's interest is calculated on the original principal only. Put $10,000 into a simple 5% account and you earn $500 every year — $500 after year 1, another $500 after year 2, and so on. After 30 years you have $25,000.
With compound interest, each year's interest is added to the balance, so next year earns interest on the larger number. Same $10,000 at 5% compounded annually becomes $43,219 after 30 years. Same rate, same time, same initial deposit — and you ended up with 73% more money. That gap is the compounding effect.
The formula, in plain English
The future value of a lump sum compounded at rate r for n periods is FV = P × (1 + r)n. If you also add a regular contribution each period, a second term joins the party: FV = P × (1 + r)n + PMT × ((1 + r)n − 1) / r. Our Compound Interest Calculator evaluates that formula for you, shows a year-by-year table, and lets you toggle compounding frequency.
A worked example that usually surprises people
Take someone who saves $500 per monthfrom age 25 to 55 — 30 years of contributions. Let's assume a 7% annual return (roughly the long-run real return of a global equity portfolio).
- Total contributed: $500 × 12 × 30 = $180,000
- Final balance: ~$611,700
- Interest earned: ~$431,700
They put in $180k and ended up with over $600k. Roughly 70% of the final numberwas growth, not savings. That's why personal-finance writers sound like broken records on this topic.
Time matters far more than rate
A common mistake is obsessing over squeezing an extra percent of return. The obsession should really be on time. Compare:
- Ana invests $500 / month from age 25 to 35 (10 years, $60k contributed), then stops. At 7% she ends at ~$612k by age 65.
- Ben starts at 35 and contributes $500 / month for 30 years straight to age 65 ($180k contributed). He ends at ~$612k too.
Ana contributed a third of what Ben did and ended up with roughly the same amount. The ten years she had on him mattered more than tripling the contributions. Starting early is the single biggest lever a saver has.
The rule of 72
Here's a shortcut worth memorising: 72 divided by your rate equals the number of years for your money to double. At 8%, money doubles every 9 years. At 6%, every 12. At 4%, every 18. Useful for back-of-the-envelope math without pulling out a calculator.
Compounding frequency matters less than you'd think
Annual vs monthly vs daily compounding is a smaller knob than the internet suggests. At 7% over 30 years, monthly beats annual by about 3% — meaningful but not life-changing. Daily beats monthly by another fraction of a percent. Where frequency does matter: very high rates (credit cards) and short horizons (payday-style products).
The inflation gotcha
A projection in nominal dollars can feel bigger than it really is. If prices rise at 3% per year for 30 years, $600k in 2056 only buys what ~$247k buys today. Run your projections in real(inflation- adjusted) returns — typically 4–6% instead of 6–8% nominal — and the number stays anchored to today's purchasing power. Our Inflation Calculator translates between the two.
Where this shows up in other tools
Three tools on this site are built on top of compound-interest math and it's worth knowing which does what:
- The Retirement Calculator projects your nest egg at a target age and translates it into sustainable annual income using a 4% withdrawal rule.
- The FIRE Calculatorflips the question: instead of "what will I have by 65?", it asks "how soon can my portfolio sustain my expenses without a paycheque?"
- The Savings Goal Calculator solves for the monthly contribution required to hit a specific number by a specific date.
The one-line takeaway
Start early, let it run, and check the math once a year rather than once a week. The compounding happens whether you watch it or not — the main way to blow up long-term wealth is to interrupt the process.