Compound interest is interest earned on both your original money and the interest it has already earned. It is the reason starting to save and invest early is so powerful, and the reason high-interest debt can spiral if you ignore it.
Simple vs compound interest
| Year | Simple interest (5% on $1,000) | Compound interest (5% on $1,000) |
|---|---|---|
| 1 | $1,050 | $1,050 |
| 5 | $1,250 | $1,276 |
| 10 | $1,500 | $1,629 |
| 20 | $2,000 | $2,653 |
| 30 | $2,500 | $4,322 |
The gap grows larger every year because the interest itself earns interest.
The compound interest formula
The basic formula is A = P(1 + r/n)nt, where P is your starting amount, r is the annual rate, n is how often interest compounds each year, and t is the number of years. You do not need to calculate it by hand: our Compound Interest Calculator does it for you, including monthly contributions.
Why starting early matters
Consider two savers investing at a 7% average annual return (not guaranteed):
- Ava invests $200 a month from age 22 to 32, then stops. She contributes $24,000.
- Ben invests $200 a month from 32 to 62. He contributes $72,000.
By 62, Ava’s balance can be similar to or larger than Ben’s, despite contributing a third as much, because her money had an extra decade to compound. Time is the most powerful ingredient.
The rule of 72
To estimate how long money takes to double, divide 72 by the annual return. At 6%, money doubles in about 12 years; at 9%, about 8 years. It is a quick mental shortcut, not an exact figure.
Compounding frequency
Interest can compound daily, monthly, quarterly or yearly. More frequent compounding produces slightly more growth. When comparing savings accounts, look at the APY or AER, which already includes compounding.
When compound interest works against you
Credit cards and some loans compound too. A $2,000 credit card balance at about 24% APR can grow quickly if you pay only the minimum, which is why paying off high-interest debt is such a strong “return”. Plan payoff with our Loan Repayment Calculator.
How to make compound interest work for you
- Start investing as early as possible, even with small amounts.
- Contribute regularly with automatic transfers.
- Reinvest dividends and interest.
- Keep fees low, because fees compound against you too.
- Stay invested through ups and downs.
- Pay off high-interest debt first.
Compound interest in everyday accounts
High-yield savings accounts, money market funds, retirement accounts and index funds all benefit from compounding. In Nigeria, money market funds and treasury bills compound when you reinvest returns. Always compare returns with inflation to understand your real growth.
Compound interest by starting age
This table shows how compound interest rewards starting early. Each person invests $150 a month until 65, at an illustrative 7% average return:
| Start age | Total contributed | Approx. value at 65 |
|---|---|---|
| 20 | $81,000 | about $570,000 |
| 30 | $63,000 | about $270,000 |
| 40 | $45,000 | about $120,000 |
Starting at 20 instead of 30 adds only $18,000 in contributions but roughly doubles the final balance. Even if you can only invest a small amount now, starting is the most powerful step you can take.
For official, up-to-date guidance, see Investor.gov’s compound interest calculator.
Frequently asked questions
What is the difference between APR and APY?
APR is the annual rate without compounding; APY includes compounding, so it shows your true yearly return or cost.
How often should interest compound?
More frequent compounding is slightly better for savers. Compare APYs to see the full effect.
Does compound interest apply to stocks?
Stocks do not pay interest, but reinvested dividends and growth compound in a similar way over time.
Compound interest: key takeaways
- Compound interest earns interest on interest.
- Starting early matters more than the amount.
- Use the rule of 72 to estimate doubling time.
- Compounding also makes high-interest debt grow fast.
Last reviewed: October 2026. This article is general education, not personal financial, tax or legal advice. Rules, rates and limits change, so check official sources or speak to a qualified professional before making decisions. See our Financial Disclaimer.