Simple vs compound interest
The difference sounds like a technicality and is the single most important idea in personal finance. One grows in a straight line. The other curves upward without limit, and over a working lifetime the gap between them exceeds everything you contributed.
The distinction
Simple interest is calculated only on the original amount. Deposit $10,000 at 7% simple interest and you earn $700 every year, forever. After 30 years you have earned $21,000, for a total of $31,000.
Compound interest is calculated on the original amount plus all interest already earned. That same $10,000 at 7% compounded annually reaches about $76,000 after 30 years.
Same deposit, same rate, same period. The difference is $45,000 — more than four times the original investment. The only difference is whether interest itself earns interest.
Why the curve steepens
In year one, both approaches earn $700. In year two, compounding earns 7% of $10,700 — an extra $49. Trivial.
By year twenty, the compounding balance is around $38,700, so that year earns roughly $2,700 against the simple version's fixed $700. The annual gap is now nearly four times the original annual interest, and it keeps widening.
This is why compounding feels like nothing is happening for years and then appears to accelerate suddenly. The mechanism was working the whole time; it simply had a small base to work on.
The rule of 72
A quick mental tool: divide 72 by the annual return to get the approximate number of years for money to double.
- At 3%, money doubles in about 24 years.
- At 6%, about 12 years.
- At 9%, about 8 years.
- At 12%, about 6 years.
The tool is more useful than it looks, because it reframes returns as doubling speed. Doubling the rate does not double the outcome — it doubles the number of doublings, which is a far larger effect. Over 36 years, 6% gives three doublings (8x) while 12% gives six (64x).
Compounding frequency matters less than you think
Annual, quarterly, monthly or daily compounding all produce slightly different results, and the difference is small.
On a 7% nominal rate, annual compounding gives exactly 7%. Monthly gives about 7.23% effective. Daily gives about 7.25%. The entire range from annual to daily is roughly a quarter of a percentage point.
The size and consistency of contributions matter enormously more. Do not choose a product for its compounding frequency; choose it for its rate, its costs and its risk.
It works against you too
Every property of compounding applies identically to debt. Credit card interest compounds, typically monthly, at rates around 20% or more. At 20%, the rule of 72 says the balance doubles in under four years if left untouched.
Fees compound against you as well. A 1% annual management fee does not cost 1% — it costs 1% of a balance that would otherwise have grown, every year, compounding. Over 30 years that is roughly a quarter of the final outcome.
Which produces the practical hierarchy: clear compounding debt first, minimise compounding fees second, and give compounding returns as many years as you can.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original amount, so it grows in a straight line. Compound interest is calculated on the original amount plus interest already earned, so it curves upward. Over 30 years at 7%, $10,000 grows to $31,000 with simple interest and about $76,000 with compounding.
What is the rule of 72?
Divide 72 by the annual return to estimate how many years it takes money to double. At 6% that is about 12 years; at 9%, about 8. It is useful because it reframes a rate as a doubling speed, which makes the effect of higher returns far more visible.
Does compounding frequency make a big difference?
Less than most people expect. On a 7% nominal rate, the difference between annual and daily compounding is roughly a quarter of a percentage point of effective return. Contribution size, costs and time horizon all matter far more.