Wealthy PotCompound Interest: How It Works, With Real Numbers
The SEC's investor education site puts it in one line: "Compound interest is the interest you earn on interest." Each period, interest is added to your balance, and the next period's interest is calculated on that larger balance. Over a few years the effect is small; over decades it dominates. $10,000 earning 5% a year grows to about $16,289 in 10 years, $26,533 in 20, and $43,219 in 30, without another dollar added. This page shows the formula, why time matters more than anything else, what compounding frequency and APY really change, and how the same arithmetic works against you on debt.
The Short Answer
- Compound interest pays interest on your original deposit and on the interest already earned. Simple interest pays only on the original deposit.
- Time is the biggest lever. In our example below, $200 a month from age 25 grows to about twice as much as the same $200 a month from age 35, although only a third more money went in.
- Compounding frequency matters less than people think. At 5%, daily compounding adds about 0.13 percentage points a year over annual compounding.
- The rate matters a lot. The FDIC's national average savings rate was 0.37% in September 2026. At that rate money takes about 195 years to double.
- The Rule of 72: divide 72 by the annual rate to estimate how many years it takes to double.
- Debt compounds too. At the 22.15% average rate on credit card accounts charged interest (Federal Reserve, Q2 2026), an unpaid balance grows by more than a fifth in a year.
How It Works
Investor.gov's own example: "if you have $100 and it earns 5% interest each year, you'll have $105 at the end of the first year. At the end of the second year, you'll have $110.25." The extra 25 cents is interest on the first year's $5 of interest. It says that "in 10 years you'll have more than $162" without adding anything; the exact figure is $162.89.
Compare simple interest, which is paid only on the original amount:
| $10,000 at 5% | Simple interest | Compound interest (annual) | Difference |
|---|---|---|---|
| After 10 years | $15,000 | $16,289 | $1,289 |
| After 20 years | $20,000 | $26,533 | $6,533 |
| After 30 years | $25,000 | $43,219 | $18,219 |
The gap is small at 10 years and larger than the original deposit at 30. That shape, slow at first and then steep, is the whole story of compounding.
The Formula
For a single deposit:
A = P × (1 + r/n)n × t
- A: the amount at the end
- P: the starting principal
- r: the annual interest rate as a decimal (5% = 0.05)
- n: how many times a year interest is compounded (1 for annually, 12 for monthly, 365 for daily)
- t: the number of years
Worked through: $10,000 at 5% compounded monthly for 10 years is 10,000 × (1 + 0.05/12)120 = $16,470.
For regular contributions, the future value of a fixed monthly deposit is PMT × [(1 + i)N − 1] ÷ i, where i is the monthly rate and N the number of months. Our compound interest calculator does both at once, with any starting balance, contribution and rate.
Why Time Matters Most
Hypothetical illustration. Three people each invest $200 a month until age 65, earning a steady 6% a year compounded monthly. The only difference is when they start.
| Starts at | Years investing | Total put in | Value at 65 |
|---|---|---|---|
| 25 | 40 | $96,000 | $398,298 |
| 35 | 30 | $72,000 | $200,903 |
| 45 | 20 | $48,000 | $92,408 |
The 25-year-old contributes a third more than the 35-year-old but ends with almost twice as much. The extra ten years at the start, when the balance is small, matter less for what they add than for how long they leave everything else to compound. It is also why the tax-free growth in a Roth IRA or an employer plan counts for most when you are young.
Compounding Frequency and APY
Banks compound daily, monthly or quarterly, and advertise the result as an annual percentage yield (APY). Federal rules define it: under Regulation DD, the APY "measures the total amount of interest paid on an account based on the interest rate and the frequency of compounding," calculated as if the money stays on deposit for a year. That is why you should compare accounts by APY, not by the stated interest rate.
| 5% interest rate, $10,000 for 10 years | Effective annual yield | Balance after 10 years |
|---|---|---|
| Compounded annually | 5.000% | $16,289 |
| Compounded monthly | 5.116% | $16,470 |
| Compounded daily | 5.127% | $16,487 |
Daily compounding beats annual by about $198 over ten years on $10,000. Real, but a rounding error next to the rate itself: the same money at 6% compounded annually would reach $17,908.
The Rule of 72
Investor.gov's shortcut: "Simply divide the number 72 by your investment's expected rate of return." At 9%, money doubles "about every 8 years." It is an approximation that works well between roughly 4% and 12%:
| Annual rate | Rule of 72 estimate | Exact doubling time |
|---|---|---|
| 0.37% (national average savings) | 195 years | 188 years |
| 4% | 18 years | 17.7 years |
| 6% | 12 years | 11.9 years |
| 9% | 8 years | 8.0 years |
| 22.15% (average card rate charged) | 3.3 years | 3.5 years |
The Rate You Actually Get
Compounding only works as hard as the rate allows. The FDIC publishes national average deposit rates each month; the figures it listed as of 21 September 2026 were:
| Account | National average rate |
|---|---|
| Savings | 0.37% |
| Interest checking | 0.07% |
| Money market deposit account | 0.63% |
| 12-month CD | 1.73% |
| 60-month CD | 1.38% |
Averages hide a wide range. Hypothetical illustration: $10,000 left for 10 years at the 0.37% national savings average grows to about $10,377; at 4% compounded monthly it grows to about $14,908. Where you keep cash matters far more than how often it compounds. Our guide to high-yield savings accounts covers where the higher rates are, and inflation will erode a rate that is lower than it.
When Compounding Works Against You
The same maths runs in reverse on debt. The Federal Reserve's G.19 release put the average rate on credit card accounts that were charged interest at 22.15% in the second quarter of 2026.
Hypothetical illustration: a $5,000 card balance at 22.15%, compounded monthly, with no payments and no new spending, grows to about $6,227 in a year, $1,227 of it interest. At that rate an unpaid balance roughly doubles in three and a half years. Minimum payments slow this down but, because they are mostly interest at first, barely dent the balance.
That is why paying off high-rate debt is usually the best "investment" available: clearing a 22% card balance is a guaranteed, tax-free 22% return. Our guides to debt avalanche vs snowball and debt consolidation cover how to do it, and the debt payoff calculator shows how long it takes.
Sources & Methodology
- SEC Investor.gov, What is compound interest?, for the definition, the $100 example and the Rule of 72.
- Regulation DD (12 CFR Part 1030), Appendix A, for the definition and calculation of APY.
- FDIC, National Rates and Rate Caps, for average deposit rates (as of 21 September 2026).
- Federal Reserve G.19, Consumer Credit, for the Q2 2026 credit card rate on accounts assessed interest.
All growth figures are our own calculations using the formulas above, with constant rates and no taxes, fees or inflation. They are illustrations, not forecasts. Rates cited here are refreshed on this page's rates cadence.
FAQ
What is compound interest in simple terms?
Interest earned on interest. Each period's interest is added to the balance, so the next period's interest is calculated on a bigger number.
How much will $10,000 grow with compound interest?
At 5% compounded annually, about $16,289 after 10 years, $26,533 after 20 and $43,219 after 30, with no further deposits. Returns on real investments vary.
Is daily or monthly compounding better?
Daily, slightly. At a 5% rate the yield is 5.127% with daily compounding against 5.116% monthly. Compare accounts by APY, which already includes the compounding effect.
What is the Rule of 72?
Divide 72 by the annual rate to estimate the years it takes to double. At 6%, about 12 years; at 9%, about 8.
What is the difference between APR and APY?
APY includes the effect of compounding over a year, as defined in Regulation DD for deposit accounts. The stated rate does not, so at the same stated rate, more frequent compounding means a higher APY.
Do stocks earn compound interest?
Not interest as such, but reinvested dividends and growth compound in the same way. Unlike a savings account, the rate is not fixed and can be negative in any given year.
Does compound interest apply to credit cards?
Yes. Unpaid interest is added to the balance and itself charged interest. At the Q2 2026 average of 22.15% on accounts charged interest, a balance left unpaid grows by more than a fifth in a year.
This article is for general information and is not investment advice. Definitions are from the SEC's Investor.gov and Regulation DD; rates are from the FDIC and Federal Reserve, checked on 2026-09-30. All growth examples are hypothetical and assume constant rates; actual returns will differ.
