Compound Interest Calculator

What you can solve

How much will I have at retirement if I save $500 per month?

Enter your current savings as the principal, 500 as the monthly contribution, your expected annual return (try 7% for a diversified index fund), and the number of years until retirement. For example, $0 initial, $500/month, 7%, 30 years → approximately $567,000 at retirement.

How long does it take to double my money?

The Rule of 72 approximates the doubling time: divide 72 by the annual interest rate. At 7%, money roughly doubles every 10.3 years (72 ÷ 7). Verify with this calculator — enter your amount, the rate, and 10 years and check whether the result is close to 2× your principal.

How much interest will I earn in a high-yield savings account?

Enter your deposit as the principal, the account's APY as the annual rate, your planned holding period in years, and any monthly deposits. Use Daily or Monthly compounding frequency — most HYSAs compound daily. The Interest Earned card shows total interest over the period.

Is it really that much better to start investing earlier?

Yes — dramatically. Enter $10,000 at 7% for 40 years: result is ~$150,000. Change to 30 years: ~$76,000. The extra 10 years nearly doubles the outcome. This is the compounding effect — early years grow slowly but later years accelerate exponentially.

Does it matter if interest compounds monthly vs annually?

Yes, but less than most people expect. On $10,000 at 5% over 20 years: annual compounding yields $26,533 while monthly compounding yields $27,126 — a $593 difference. The rate and time period matter far more than compounding frequency in most real-world scenarios.

What is

What is Compound Interest Calculator?

The Compound Interest Calculator shows exactly how money grows when interest compounds over time — visually, year by year. Enter a starting principal, annual interest rate, and time period, and see the final balance, total contributions, and total interest earned broken out clearly. An optional monthly contribution field lets you model regular savings deposits on top of the initial amount.

Choose from five compounding frequencies — annually, semi-annually, quarterly, monthly, or daily — to understand how frequency affects final returns. The year-by-year bar chart makes the exponential growth curve visible and intuitive, helping you see why starting early matters more than the exact rate.

How to use

How to use Compound Interest Calculator

1. Enter your Initial Principal — the amount you're starting with.

2. Enter the Annual Interest Rate as a percentage (e.g., 7 for 7%).

3. Enter the Time Period in years.

4. Optionally enter a Monthly Contribution if you plan to add money regularly.

5. Select a Compound Frequency — Monthly is most common for savings accounts and investments.

6. The result cards and bar chart update instantly.

Example

Principal: $10,000  |  Rate: 7%  |  Years: 20  |  Monthly: $200  |  Frequency: Monthly

Final Amount:        $117,292
Total Contributions: $58,000
Interest Earned:     $59,292

Year 1:  $12,682  |  Year 5:  $27,481
Year 10: $48,609  |  Year 20: $117,292

Common use cases

1. Retirement planning: Model how a 401(k) or IRA grows with monthly contributions over 20–40 years at historical market returns.

2. Savings goal planning: Find out how long it takes to reach a target amount at a given savings rate.

3. Education fund: Calculate how much to save monthly to reach a college fund goal in 10–18 years.

4. Comparing savings accounts: Compare the effect of different compounding frequencies and interest rates.

5. Understanding inflation impact: Run the calculator with an inflation rate to see how purchasing power erodes over time.

Frequently asked questions

What is the difference between simple and compound interest?v

Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus all previously earned interest — so interest earns interest. Over long periods, the difference is dramatic. A $10,000 investment at 7% for 30 years yields $30,000 simple but $76,000 compound (monthly).

What compounding frequency should I choose?v

Use Monthly for most savings accounts and many investment accounts — it's the most common real-world frequency. Daily compounding is used by some high-yield accounts and gives slightly higher returns. Annually matches how bonds and some CDs compound.

What interest rate should I use for a realistic projection?v

The S&P 500 has averaged approximately 7–10% annually over the long term (before inflation). For conservative savings, use 4–5% (high-yield savings or CDs). For inflation-adjusted projections, subtract 2–3% from the nominal rate.

How are monthly contributions handled?v

The calculator adds the monthly contribution at each compounding period (divided to match the compound frequency). For monthly compounding, contributions are added each month. The formula used is the future value of an annuity combined with the future value of the lump sum.

Why does compounding frequency matter?v

More frequent compounding means interest is calculated and added to the principal more often, giving that new total more time to earn interest. The difference between annual and monthly compounding at 7% over 30 years on $10,000 is about $5,000 — meaningful but smaller than the effect of rate or time.