Compound interest calculator icon with rising growth bars and a compounding arrow.

Compound Interest Calculator

See how your money grows over time with the power of compound interest. Adjust your starting balance, rate, contributions, and timeline in seconds.

Starting Balance:
$10,000.00
Contributions:
$10,000.00
Interest Earned:
$0.00
Total Balance:
$0.00
Starting Balance Contributions Total

What 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

YearBalanceInterest 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. 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. 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. 3
    Choose your time horizon — how many years you plan to stay invested. Extending by even five years can significantly change the outcome.
  4. 4
    Add recurring contributions — regular deposits of any amount and frequency. Monthly contributions of even $100–$200 dramatically accelerate growth.
  5. 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. 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.

ScenarioYears InvestedTotal ContributedFinal BalanceInterest Earned
Start at age 2540$149,000$798,200$649,200
Start at age 3530$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

Click any question to expand the answer.

What is compound interest?
Compound interest is interest calculated on both your original principal and the interest already earned. Unlike simple interest — which only applies to the principal — compound interest snowballs each period. A $1,000 deposit at 7% simple interest earns $70 every year forever. At 7% compound interest it earns $70 in year 1, $74.90 in year 2, $80.14 in year 3, and keeps accelerating. Over 30 years the compounded balance is nearly double the simple-interest balance.
What is the compound interest formula?
The core formula is FV = P — (1 + r/n)n×t, where:

P = principal (starting balance)
r = annual interest rate as a decimal (7% ? 0.07)
n = compounding periods per year (12 = monthly, 365 = daily)
t = time in years

When you add recurring contributions the calculator combines this with the future-value annuity formula, using the effective-rate-per-contribution-period so the math stays accurate regardless of whether you contribute daily, weekly, monthly, or yearly.
What is the difference between compound interest and simple interest?
With simple interest, you earn the same dollar amount every period because interest is always calculated on the original principal only. With compound interest, each period's interest is added to the running balance, so the next period's interest is larger. Over short horizons the difference is small. Over decades it becomes enormous — which is exactly why long investment horizons are so powerful.
How often should interest be compounded?
More frequently is better when you're earning interest. Daily compounding beats monthly, which beats quarterly, which beats annual. That said, the practical difference between monthly and daily is small for most balances. What matters far more is your rate of return and how long you stay invested. Use the Advanced Options toggle in this calculator to compare compounding frequencies side by side.
What rate of return should I use?
It depends on where your money is:

7% (real, inflation-adjusted) — historical average for a diversified US stock portfolio
10% (nominal) — historical average for the S&P 500 before inflation
4–5% — current high-yield savings account APY
3–4% — typical bond or CD rate

For long-term retirement projections, many planners use 6–7% to be conservative. Remember to subtract fund expense ratios from your expected return — a 1% annual fee permanently reduces your net rate by 1 percentage point.
How does starting early affect compound interest?
More than almost any other factor. Consider two investors who both earn 7% compounded monthly and contribute $300/month. Investor A starts at 25 and stops at 65 (40 years). Investor B starts at 35 and stops at 65 (30 years). Investor A ends up with roughly twice the balance despite contributing only ~$36,000 more. Those first 10 years provide the longest runway for exponential growth and can never be recovered.
Does compound interest apply to debt?
Yes — and it works against you. Credit card balances, student loans, and some personal loans use compound interest. If you carry a balance, interest is added to your principal each cycle, and the next charge is calculated against the larger amount. A $5,000 credit card balance at 20% APR compounded daily grows to over $36,000 in 20 years with no payments. Eliminating high-interest debt almost always beats investing until the debt is gone.
What is APY and how is it different from APR?
APR (Annual Percentage Rate) is the stated interest rate before compounding is taken into account. APY (Annual Percentage Yield) reflects the actual annual return after compounding. For example, a 5% APR compounded monthly has an APY of about 5.12%. When comparing savings accounts or investments, always compare APYs — they tell you the true effective return on a like-for-like basis.
How does inflation affect compound interest calculations?
Inflation erodes purchasing power over time. A 7% nominal return during a period of 3% inflation gives you only about 4% real return (roughly: nominal rate - inflation rate). For long-term projections, use the real rate if you want to know what your money will actually buy in today's dollars. Use the nominal rate if you want to know the raw account balance. This calculator uses whatever rate you enter — subtract your expected inflation rate from the nominal return to get a real-terms projection.
Should I pay off debt or invest — which is better for compound interest?
The general rule: if your debt's interest rate is higher than your expected investment return, pay off the debt first. Credit cards at 20% APR compound against you faster than almost any investment can compound for you. Once high-interest debt is cleared, redirect those payments to investments to let compound interest work in your favour. For low-rate debt (e.g., a 3% mortgage), the calculus is closer — many investors do both simultaneously.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes for your money to double. At 6%, money doubles in ~12 years. At 9%, ~8 years. At 12%, ~6 years. It's an approximation (it works best for rates between 6–10%), but it's a fast way to sanity-check a compound interest projection without a calculator.
How do taxes affect compound interest growth?
Taxes reduce the effective compounding rate because gains taxed annually shrink the base. A simple example: at a 25% annual capital gains tax rate on a 8% return, your effective after-tax rate is only 6%. In a tax-advantaged account (Roth IRA, traditional 401k, 529) gains compound without annual tax drag, which is a significant long-term advantage. For Roth accounts, qualified withdrawals are tax-free, meaning the entire compounded balance is yours.

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For additional reference, see Investor.gov's compound interest resources (U.S. Securities and Exchange Commission).