What Is Compound Interest?
3 min readUpdated June 7, 2026
Compound interest is interest you earn not just on the money you start with, but also on the interest that money has already earned. Each period, the interest is added to your balance, and the next period's interest is calculated on that larger amount. Over time this creates a snowball effect: the balance grows, the interest grows with it, and the growth accelerates.
This is the single most important idea in personal finance, and it cuts both ways. It is what makes long-term saving and investing so powerful, and it is also what makes credit card debt so dangerous. This guide explains what compound interest is, the formula behind it, and why time and compounding frequency matter more than most people expect.
Compound interest vs. simple interest
With simple interest, you only ever earn interest on your original principal. Put $1,000 in an account paying 5% simple interest and you earn $50 every year — year one, year two, year ten, always $50. After 10 years you have $1,500.
Compound interest is different: the interest gets added back to the balance, so each year you earn interest on a slightly bigger number. That same $1,000 at 5% compounded annually earns $50 the first year, but $52.50 the second year (5% of $1,050), and so on. After 10 years you have about $1,629 — and the gap over simple interest keeps widening the longer you leave it.
The formula, in plain words
The standard formula is A = P(1 + r/n)^(nt). It looks intimidating, but each letter is simple. P is the principal — the amount you start with. r is the annual interest rate as a decimal (so 7% is 0.07). n is how many times interest is compounded per year (12 for monthly, 4 for quarterly, 1 for yearly). t is the number of years. A is the amount you end up with.
Reading it out loud: take your rate, divide it into n smaller chunks across the year, add each chunk to the running balance, and repeat that for n times every year over t years. The exponent (n × t) is just the total number of times interest gets applied. The more times it compounds, the more often interest starts earning its own interest.
Why time and frequency matter
Time is the biggest lever, because compounding is exponential, not linear. Take $10,000 at a 7% annual rate compounded monthly. After 10 years it grows to about $20,097 — it roughly doubles. Leave it for another 10 years and it does not just add the same amount again; it grows to about $40,387. The second decade earns far more than the first, purely because it starts from a much larger base.
Compounding frequency matters too, though less dramatically. The same $10,000 at 7% for 10 years grows to about $19,672 compounded annually, $20,097 compounded monthly, and $20,137 compounded daily. More frequent compounding helps, but the effect is small compared to simply giving the money more years to grow.
Everyday examples
On the saving side, compound interest works for you. Savings accounts, certificates of deposit, and long-term investments all rely on it — which is why starting early matters so much, since the earliest dollars get the most years to compound. A modest amount invested in your twenties can outgrow a much larger amount invested in your forties.
On the borrowing side, the same math works against you. Credit cards and loans compound interest on what you owe, so an unpaid balance can grow alarmingly fast. A card charging 24% APR compounded daily is using compound interest to grow your debt the same way a savings account grows your savings — which is exactly why paying balances off quickly saves so much money.
Frequently asked questions
What is the difference between compound and simple interest?+
Simple interest is calculated only on your original principal, so the amount earned each period stays the same. Compound interest is calculated on the principal plus all previously earned interest, so it grows faster over time.
What does each letter in A = P(1 + r/n)^(nt) mean?+
P is the starting principal, r is the annual rate as a decimal, n is how many times per year interest compounds, t is the number of years, and A is the final amount.
Does compounding more often make a big difference?+
It helps, but modestly. $10,000 at 7% for 10 years grows to about $19,672 compounded annually versus $20,097 compounded monthly. Adding more years matters far more than compounding more frequently.
Why is compound interest important for debt?+
Credit cards and loans compound interest on what you owe, so unpaid balances grow quickly. A 24% APR balance compounds against you the same way savings compound for you, which is why paying debt off fast matters.
Why does starting early matter so much?+
Because compounding is exponential, the earliest money you invest has the most time to earn interest on its own interest. Starting a decade earlier can outweigh investing a much larger sum later.