Compound Interest Calculator

✓ Fact-checked – Last updated September 2026 – results are estimates, not financial advice.

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How often interest is added to the balance.
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How it works · The formula · FAQs

Compound interest is often called the eighth wonder of the world - your money earns returns, and those returns earn their own returns. Our free compound interest calculator projects how your savings grow over time. Enter your initial deposit, monthly contributions, annual interest rate and timeframe to see your future balance year by year.

How the compound interest calculator works

Five inputs, one click:

  • Step 1: Enter your initial deposit - the lump sum you start with.
  • Step 2: Enter your monthly contribution (use 0 if you are not adding regularly).
  • Step 3: Enter the annual interest rate as a percentage.
  • Step 4: Choose how many years to project and how often interest compounds.
  • Step 5: Hit Calculate to see your future balance, total contributions and interest earned.

The results include a year-by-year table and a growth chart. Your inputs are saved on your device, and the share link encodes them in the URL.

The compound interest formula

For a single lump sum, the future value follows the classic formula:

A = P x (1 + r/n)^(n x t)

Where P is the starting principal, r is the annual rate, n is compounding periods per year, and t is years. Regular monthly contributions are added on top, each compounding for the remaining time. More frequent compounding gives slightly faster growth.

The power of compounding over time

Take $10,000 invested once, plus $500 a month, at 7% compounded monthly:

  • After 10 years: about $106,600 - you contributed $70,000.
  • After 20 years: about $256,000 - you contributed $130,000.
  • After 30 years: about $566,000 - you contributed $190,000.

Doubling the timeframe from 10 to 20 years more than doubles the balance, because later growth builds on earlier gains.

Why starting early matters

Time is the most powerful input. Compare investing $500 a month at 7% from age 25 versus age 35, both to age 65:

  • Starting at 25 (40 years): about $1,197,000 total.
  • Starting at 35 (30 years): about $566,000 total.

Starting ten years earlier more than doubles the outcome. The earlier contributions compound the longest.

Want to see what decades of compounding look like for your own retirement? Try our retirement savings calculator.

Compound interest FAQs

What is compound interest?

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. It makes your balance grow faster over time compared to simple interest.

How often should interest compound?

More frequent compounding gives slightly higher returns. Monthly compounding is standard for savings accounts; daily is marginally better. The difference is small compared to the rate and time.

What is a good interest rate for savings?

High-yield savings accounts offer 4-5% as of 2026. Stock market index funds have averaged about 7% annually after inflation over the long term, with higher volatility.

How much do I need to save monthly to reach $1 million?

At 7% annual returns, saving about $380 a month for 40 years grows to roughly $1 million. Starting earlier or earning a higher rate reduces the monthly amount needed.

Disclaimer: Figures are projections for guidance before tax and fees, not financial advice. Actual returns vary.

See the flip side of compounding with our personal loan calculator — it shows what debt costs you.

Saving up for a big trip or purchase abroad? Convert your target with our currency converter to see what it really costs in the local currency.

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