Compound Interest Calculator

See how your savings grow with compound interest, set a starting balance, monthly contributions, rate, and compounding frequency to project the future value. Runs entirely in your browser.

Future value

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Starting balance-
Total contributions-
Interest earned-

Show the math

A lump sum compounds while level contributions grow as an annuity:

FV = P(1 + i)n + PMT · ((1 + i)n − 1) / i

Contributions are added monthly, and the chosen compounding frequency is applied through an effective monthly rate so its APY is preserved. Interest earned is the final balance minus everything you put in, your starting balance plus all contributions.

What this does

A compound interest calculator projects how savings grow when interest earns interest. Set a starting balance, monthly contributions, a rate, and a compounding frequency, and it shows the future value split into what you put in versus what it earned.

How to use it

  1. Enter your starting balance and monthly contribution.
  2. Enter the interest rate and choose a compounding frequency.
  3. Set the number of years to project.
  4. Read the future value and the contributions-versus-growth split.

How it works

Each period, interest is added to the balance, and that interest then earns interest too. More frequent compounding raises the effective yield (APY) on the same nominal rate, and regular contributions compound alongside the starting balance.

Understanding your result

Contributions usually drive most long-term growth, while time and rate determine how much the interest compounds on top. The split shows how much of the final balance is earnings rather than deposits.

Example

$10,000 at 7% compounded monthly with $200 added each month grows to roughly $300,000 in 30 years, about $82,000 contributed and the rest investment growth.

Sources & methodology

Last updated .

Frequently asked questions

How does compound interest work?

Each period you earn interest on your balance, and that interest then earns interest too. Over time this snowball effect grows your money far faster than simple interest, especially with regular contributions.

Does compounding frequency matter?

Yes, the more often interest is added (daily vs. yearly), the higher the effective yield (APY) on the same nominal rate. This tool lets you pick the frequency and applies it on a monthly contribution timeline.

How much do contributions add?

Regular deposits are the biggest driver of long-term growth. The result splits your final balance into the money you put in versus the interest it earned.

Do you store my numbers?

No. Everything is calculated locally in your browser, nothing you enter is uploaded or saved.