TL;DR — Compound interest is interest you earn on your interest. Each period your balance grows, and the next round of interest is calculated on the bigger number, so growth speeds up on its own. It is the reason small, boring deposits become six-figure balances over decades — and the reason a credit card balance grows even after you stop spending.
Compound interest is what happens when the interest your money earns starts earning interest of its own. Instead of growing in a straight line, your balance grows on a curve — slowly at first, then faster every year. The one-sentence answer: compound interest is interest calculated on your original deposit plus all interest already added, which is why time matters more than the size of any single deposit.
That sounds like textbook trivia. It isn't. It decides whether $200 a month becomes $34,000 or $525,000 — and whether a $5,000 credit card balance quietly becomes $9,600 while you're not looking. The U.S. Federal Reserve put the average credit card rate at 21.6% in 2024, so balances compound at a pace no savings account can match. The problem for most people is that this math happens invisibly, inside accounts they never track. Vault — a free budgeting app that works without a bank login — makes those numbers visible: every dollar gets a job, including the ones meant to grow.
What is compound interest?
Compound interest is interest calculated on both your original money and every dollar of interest it has already earned. A $10,000 deposit at 5% earns $500 in year one; in year two it earns interest on $10,500, and the interest line keeps growing even if you never add another cent. The SEC's free compound interest calculator on Investor.gov lets you run the numbers yourself.
Simple vs. compound interest: same money, two endings
Simple interest pays only on the original amount — a straight line. Compound interest pays on the running balance — a curve that pulls away. Here is the same $10,000 at the same 5% rate, with no extra deposits:
| Years invested | Simple interest at 5% | Compound interest at 5% |
|---|---|---|
| 10 | $15,000 | $16,289 |
| 20 | $20,000 | $26,533 |
| 30 | $25,000 | $43,219 |
Same deposit, same rate — the only difference is whether earned interest gets to earn its own. Over 30 years that one difference pays $18,219 extra. And the gap keeps widening: most of the compound column's growth arrives in the final years, which is exactly why people who start late find the math so unforgiving.
How does compound interest work?
Each period — daily or monthly for real savings accounts, once a year in textbooks — the rate is applied to the entire balance, not just the original deposit. The new interest is added to the balance, so the next period's interest is calculated on a slightly bigger number. Repeated over years, that loop is what bends the growth curve upward.
The compound interest formula that produces the table above is:
A = P × (1 + r/n)^(n×t)
In plain English, each letter means:
- A — what you end up with
- P — the amount you start with
- r — the annual rate, as a decimal (5% = 0.05)
- n — how many times a year interest is added (12 for monthly)
- t — years you leave it alone
You don't need the formula to benefit from it — you need the loop it describes: add the interest, then let the interest earn interest. Frequency matters a little (daily compounding beats annual), but time matters enormously more. A few months of extra compounding is trivia; a few extra years is most of the outcome.
What compound interest does to $200 a month
One deposit is the textbook version. Real savers add money monthly, and that's where compound interest gets dramatic — each new deposit starts its own compounding clock. At 7% a year, in the range of what broadly diversified index funds have historically returned over long stretches, here is $200 a month:
| Years | You put in | Balance becomes | Growth added |
|---|---|---|---|
| 10 | $24,000 | $34,617 | $10,617 |
| 20 | $48,000 | $104,185 | $56,185 |
| 30 | $72,000 | $243,994 | $171,994 |
| 40 | $96,000 | $524,963 | $428,963 |
Read the growth column like a story. In the first decade, compounding feels like a rounding error — $10,617 on top of $24,000 saved. In the final decade alone, growth added about $281,000 without a single new dollar from you. That's not a reward for being rich; you never contributed more than $200 a month. It's a reward for starting and not stopping. If you're wondering what your own number should be, start with how much to save each month, then let time do the rest.
The Rule of 72: how fast your money doubles
The Rule of 72 is compound interest you can do in your head: divide 72 by the annual rate, and that's roughly how many years your money takes to double.
| Annual rate | Years to double |
|---|---|
| 2% | 36 |
| 4% | 18 |
| 6% | 12 |
| 8% | 9 |
| 10% | about 7 |
The table explains why the rate you earn matters so much. A typical savings account paying 0.4% takes roughly 180 years to double your money — useless. A high-yield account at 4% takes 18 years, and a stock-index fund averaging 8% takes about 9. Same money, different doubling clocks — which is why parking cash in a real account is one of the few free wins in personal finance. The comparison between account types is laid out in high-yield savings accounts.
Compound interest in reverse: what it does to debt
The same math runs against you when you owe. Interest on a credit card compounds onto the balance the same way it compounds onto savings — except now the curve is a trap. According to the Federal Reserve's G.19 consumer credit release, commercial banks charged an average rate of 21.6% on credit card plans in 2024, and about 22.3% on accounts actually carrying a balance.
Here's a $5,000 balance at a 22% APR, left alone:
| Time | You owe |
|---|---|
| Today | $5,000 |
| 1 year | $6,218 |
| 2 years | $7,733 |
| 3 years | $9,616 |
Nothing was spent, nothing was added — the balance nearly doubled in three years purely through compounding. This is why the standard advice to pay off credit card debt before investing isn't really about discipline; it's about compound interest pointed the right way. Killing a 22% debt is a guaranteed 22% return, which no portfolio can promise. The full tradeoff is in paying off debt vs. saving first.
How to put compound interest to work: 5 steps
- Start earlier than feels necessary. The table showed where growth lives: in the late years, which only arrive if the early years happened. A decade of delay deletes the decade that pays best.
- Automate the deposit for payday. Compounding needs contributions that never pause. Set the transfer for the day after payday, before spending gets a vote — the habit behind paying yourself first.
- Earn a real rate where it's safe. For money that must stay stable, a high-yield savings account compounds at many times the typical rate. For long horizons, low-cost index funds have historically compounded faster still.
- Leave it alone. Every withdrawal shrinks the base that future interest is calculated on. The curve is boring in the middle years — that's the price of the steep ones.
- Clear high-rate debt first. Owing 22% while earning 4% is compound interest running a losing race in both lanes. Retire the expensive debt; then point the same payments at savings.
This is where Vault does the quiet work. Every savings goal gets its own envelope — a named pot funded on payday before anything else spends the money — so step 2 runs whether or not you feel like it that week. The reports view shows your true monthly surplus against actual spending, which is exactly the number your automatic deposit should be set to, and because Vault syncs across devices, the plan you build at your desk is the same one in your pocket at the store. The guide to budgets and goals walks through the setup in about fifteen minutes.
Frequently asked questions
What is compound interest in simple terms?
It's interest on your interest. You earn a return on your money, that return is added to the balance, and future returns are calculated on the bigger amount. Your money grows on a curve instead of a straight line, which is why the last years of saving contribute far more than the first ones.
How do you calculate compound interest?
Multiply the principal by (1 + the rate divided by compounding periods per year), raised to the power of periods times years. For a quick estimate, the Rule of 72 works: 72 divided by the rate gives the years to double. The SEC's Investor.gov calculator does the exact math for any amount.
What is the Rule of 72?
Divide 72 by an annual rate to estimate how many years money takes to double: at 6%, about 12 years; at 9%, about 8. It's compound interest compressed into mental math, accurate enough for comparing accounts and sanity-checking any projection someone puts in front of you.
Is compound interest good or bad?
Neither — it's just math that amplifies whichever side of the transaction you're on. On savings, investments, and retirement accounts it works for you and rewards patience. On credit cards and most personal loans it works against you and punishes delay. The practical job is keeping yourself on the earning side of it.
How much will $1,000 be worth in 10 years with compound interest?
At 5%, about $1,629; at 8%, about $2,159. Larger amounts scale identically, so $10,000 becomes roughly $16,290 to $21,590. Notice what the numbers imply: the rate doubles the outcome over one decade, while doubling the time roughly triples it — which is why starting a year earlier beats chasing an extra percent.
The bottom line
Compound interest is a loop: interest earns interest, the balance grows, and the next round grows on it. Given decades, $200 a month becomes half a million; given a 22% credit card, $5,000 becomes $9,600 in three years of doing nothing. Same math, two directions. Point it your way by starting early, automating the deposit, and never interrupting it — and get the expensive debt out of the race.
Give compound interest a head start — open your first savings goal in Vault.
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