Compound Interest Calculator with Monthly Contributions

How to use the compound interest calculator

1. Enter your starting amount (use 0 if you're starting from nothing).
2. Enter how much you'll add every month.
3. Set the annual interest rate and the number of years.
4. Choose how often interest compounds — yearly, quarterly, monthly or daily.

You'll see the future balance, how much of it you put in yourself and how much is interest. The chart shows contributions and interest growing year by year, and the table has the exact numbers.

Save, print or share your growth projection

You don't have to start over next time. Under the calculator you can:

• Save the calculation in your browser with a name, then reopen it from "Saved calculations" whenever you like.
• Copy a link that reopens this exact calculation with all your numbers — on your phone, another computer or for someone you're planning with.
• Print a clean one-page report, or download it as a PDF. It shows your future balance, total contributions, interest earned and a year-by-year table, plus a QR code at the top: scan it with your phone camera and the calculation opens again with every number filled in.
• Download a CSV to open in Excel, Google Sheets or Numbers.

Nothing is uploaded: saved calculations stay on your device, and the link and QR code carry your inputs inside the address itself.

The compound interest formula

For a single deposit:

A = P × (1 + r/n)^(n × t)

• A — the amount at the end
• P — the starting amount (principal)
• r — the yearly interest rate as a decimal (7% = 0.07)
• n — how many times a year interest compounds
• t — the number of years

With monthly contributions the calculator converts the yearly rate to an equivalent monthly rate, (1 + r/n)^(n/12) − 1, and then for every month adds interest to the balance followed by your contribution. That handles any compounding frequency correctly.

Why time matters more than anything

Look at the chart: in the early years the grey contribution bars do most of the work. Later, the green interest bars take over. That curve is why starting five years earlier can matter more than saving a bigger amount later.

A simple rule of thumb is the rule of 72: divide 72 by the interest rate to estimate how many years it takes money to double. At 7%, money doubles roughly every 10 years; at 4%, every 18 years.

Small amounts add up

Compounding works on small habits too. A $15 monthly subscription you don't use, invested at 7% instead, becomes about $2,600 in ten years — the subscription cost calculator works this out for everything you pay for. Completing a 52-week savings challenge every year and investing the $1,378 adds up to a meaningful sum over a decade.

Savings goals vs long-term growth

If you are saving for something in the next one to three years, keep the money in a savings account and use the savings goal calculator to plan monthly deposits. Investing for higher returns makes more sense for money you won't need for five years or more, because markets can fall in the short term.

Things this calculator doesn't include

• Taxes on interest or investment gains
• Fees charged by funds or advisers (even 1% a year noticeably lowers the result)
• Inflation — use a lower "real" rate to see the result in today's money
• Variable returns — real investments go up and down; the calculator uses a steady average

Watch your savings grow for real

Nest, the VaultlyApps savings tracker, shows your goals, emergency fund and challenges with the maths built in, and Vault, the personal finance dashboard, tracks savings next to your budget and net worth. Both have a free live demo you can open in your browser.