Quiz prep
Key ideas to know
For the Compound Interest Quiz
- Compound interest earns interest on your interest. Simple interest pays only on your original deposit; with compounding, each period's interest joins the balance, so the next period earns on a bigger amount.
- Each year's interest is figured on the new balance. $1,000 at 10% becomes $1,100 after year 1, so year 2 earns $110, not $100.
- The Rule of 72 estimates how long money takes to double. Divide 72 by the annual rate: at 8%, money doubles in about 72 ÷ 8 = 9 years.
- Starting early beats starting big. In the example below, starting at 25 instead of 35 ends with more than twice the balance for only $36,000 more in contributions.
- Compounding works against you on debt. Unpaid credit card interest is added to the balance, so you pay interest on interest, which is why high-interest debt usually comes first.
For the Grow Your Net Worth learning path
This tool is step 2 of the path: Let time do the heavy lifting.
Continue the learning pathWhat Is Compound Interest?
Here's the simplest way to think about it: you earn interest on your interest. Every year, your interest gets added to your balance — and the following year you earn interest on that larger number. The longer this goes on, the faster your money grows.
A simple example — $1,000 at 10% per year
| Year | Balance | Interest earned this year |
|---|---|---|
| Year 1 | $1,100 | $100 |
| Year 2 | $1,210 | $110 |
| Year 3 | $1,331 | $121 |
| Year 5 | $1,611 | $146 |
| Year 10 | $2,594 | $236 |
| Year 20 | $6,727 | $611 |
| Year 30 | $17,449 | $1,586 |
Notice how the interest earned each year keeps growing — from $100 in year 1 to $1,586 in year 30. That's compounding at work. You put in $1,000 and did nothing else. After 30 years you have $17,449.
Compare that to simple interest, where the bank only ever pays you 10% of your original $1,000 — so you'd earn $100 every single year, no matter what. After 30 years with simple interest you'd have $4,000. With compound interest you'd have $17,449. Same rate, same starting amount, same 30 years — the only difference is whether your interest earns interest.
This is why starting early matters so much, and why leaving money invested for longer is almost always better than pulling it out. The growth isn't linear — it accelerates. The last 10 years of a 30-year investment produce far more than the first 10 years.
The Compound Interest Formula
FV = P × (1 + r/n)n×t
- FV — Future value (your ending balance)
- P — Principal (your initial investment)
- r — Annual interest rate as a decimal (e.g., 10% = 0.10)
- n — Compounding frequency per year (12 = monthly, 365 = daily)
- t — Time in years
Don't worry about memorising the formula — that's what this calculator is for. But it's useful to know that n (how often interest compounds) and t (time) are the exponents, which is why small increases in either have a large effect on the result.
Compound interest works in your favour in savings accounts, CDs, money market accounts, index funds, and retirement accounts like a 401k or IRA. It works against you on debt — credit card balances and student loans grow the exact same way if you don't pay them down.
How to Use This Calculator
- 1 Enter your initial investment — the amount you're starting with today. This could be existing savings, a bonus, or a lump-sum transfer.
- 2 Set your annual rate of return — use 7% for an inflation-adjusted stock portfolio historical average, 10% for nominal S&P 500 returns, or 4–5% for a high-yield savings account.
- 3 Choose your time horizon — how many years you plan to stay invested. Extending by even five years can significantly change the outcome.
- 4 Add recurring contributions — regular deposits of any amount and frequency. Monthly contributions of even $100–$200 dramatically accelerate growth.
- 5 Expand Advanced Options to choose how often interest compounds (annually, quarterly, monthly, or daily). Most savings accounts compound daily; index funds effectively compound continuously through reinvested dividends.
- 6 Explore the Graph and Grid tabs — hover over the chart to inspect any year's breakdown, or switch to Grid for a sortable year-by-year table.
Real-World Examples
Starting at 25 vs. 35
Suppose two people each invest $5,000 as a lump sum and contribute $300 per month at a 7% annual rate compounded monthly until age 65.
| Scenario | Years Invested | Total Contributed | Final Balance | Interest Earned |
|---|---|---|---|---|
| Start at age 25 | 40 | $149,000 | $798,200 | $649,200 |
| Start at age 35 | 30 | $113,000 | $366,700 | $253,700 |
Starting just 10 years earlier results in a final balance more than twice as large — despite investing only $36,000 more. That extra decade of compounding makes the difference.
Monthly vs. Annual Contributions
Contributing $1,200 annually versus $100 monthly (same total per year) at 7% over 30 years yields noticeably different results because monthly contributions enter the market earlier and compound sooner. The monthly contributor ends up with roughly 5–8% more, depending on timing. Small habit changes create meaningful long-term differences.
Factors That Affect Compound Interest Growth
Principal
A larger starting balance means a larger base earning interest each period. $10,000 at 7% for 20 years becomes ~$38,696. $1,000 at the same rate becomes ~$3,870 — exactly 10— less, proportional to the starting amount.
Interest Rate
Even a 1–2 percentage point difference is enormous over decades. $10,000 at 6% for 30 years = $57,435. At 8% = $100,627. Always account for fund fees and expense ratios — a 1% annual fee effectively reduces your net return by 1% every year.
Time
Time is your biggest lever, especially early in life. Starting earlier with smaller amounts consistently outperforms starting later with larger amounts over typical 30–40 year investment horizons.
Compounding Frequency
Annual vs. monthly compounding on $10,000 at 7% for 30 years: $76,123 vs. $81,165. Daily compounding pushes it slightly further. More frequent compounding helps — but consistent contributions have a far greater impact.
Ways to Earn Compound Interest
- ✓ High-Yield Savings Accounts (HYSA) — Often compounding daily, current rates range from 4–5% APY. FDIC-insured and fully liquid.
- ✓ Certificates of Deposit (CDs) — Fixed rates for a set term. Slightly higher rates than HYSAs in exchange for locking up funds.
- ✓ Money Market Accounts — Similar to HYSAs with tiered rates based on balance; often offered by brokerages and credit unions.
- ✓ Index Funds and ETFs — Reinvesting dividends in low-cost index funds effectively compounds returns at historically strong rates over decades.
- ✓ Retirement Accounts (401k, IRA) — Tax-advantaged accounts where compounding is amplified because taxes don't erode gains year over year.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original principal and the interest already earned. That compounding effect makes growth accelerate over time.
What is the compound interest formula?
The core formula is FV = P × (1 + r/n)n×t, where P is principal, r is the annual rate, n is the number of compounding periods per year, and t is time in years. Regular contributions add a future-value annuity component.
What is the difference between compound interest and simple interest?
Simple interest is calculated only on the original principal. Compound interest is added to the balance each period, so the next period earns interest on a larger amount.
How often should interest be compounded?
More frequent compounding increases returns slightly: daily beats monthly, and monthly beats annual. Your rate of return and investment time horizon generally matter much more.
What rate of return should I use?
Use a rate that fits the investment: roughly 7% for a diversified stock portfolio after inflation, 10% for the S&P 500 before inflation, and 4–5% for a high-yield savings account. Conservative long-term projections often use 6–7%.
How does starting early affect compound interest?
Starting early gives each contribution more time to grow. A decade of additional compounding can make a dramatic difference even when monthly contributions stay the same.
Does compound interest apply to debt?
Yes. Credit card balances, student loans, and some personal loans can compound against you. Paying off high-interest debt usually beats investing until that debt is cleared.
What is APY and how is it different from APR?
APR is the stated annual rate before compounding. APY reflects the actual annual return after compounding, making it the better comparison for savings accounts.
How does inflation affect compound interest calculations?
Inflation reduces purchasing power. A 7% return with 3% inflation is roughly a 4% real return, so subtract expected inflation when projecting future buying power.
Should I pay off debt or invest?
A useful rule is to pay off debt when its interest rate exceeds your expected investment return. High-rate credit card debt almost always comes first.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate how long money takes to double. At 6%, it is about 12 years; at 9%, about 8 years.
How do taxes affect compound interest growth?
Taxes can reduce the amount that remains invested and therefore slow compounding. Tax-advantaged accounts such as IRAs and 401(k)s help investments compound with less annual tax drag.
Related Articles & Resources
Law 15 — Understand the Power of Compound Interest
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How to Make Your Money Work for You with Compound Interest
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The High Cost of Delaying Retirement Contributions
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For additional reference, see Investor.gov's compound interest resources (U.S. Securities and Exchange Commission).