Compound Interest Calculator

How this compound interest calculator works

Enter a starting amount, an optional monthly contribution, an annual interest rate, and a time period to see how your money grows with compounding, where you earn interest not just on your original deposit, but on the interest that’s already accumulated.

The compound interest formula

A = P × (1 + r/n)^(n×t)

A is the future value, P is your principal (starting amount), r is the annual interest rate (as a decimal), n is how many times per year interest compounds, and t is the number of years. When you add regular monthly contributions, each contribution grows separately using the future value of an annuity formula, then adds to the lump-sum growth above.

Step-by-step: how to calculate compound growth

Convert your annual interest rate to a decimal (7% becomes 0.07).
Divide the rate by your compounding frequency, and multiply the years by that same frequency.
Raise (1 + rate/frequency) to the power of (frequency × years).
Multiply that result by your starting principal to get the lump-sum future value.
Add the future value of any regular contributions on top.

Worked example

$10,000 invested at 7% annual interest, compounded monthly, with an additional $200 contributed every month for 20 years:

Lump-sum growth: $10,000 growing at 7% monthly compounding for 20 years ≈ $40,387
Contribution growth: $200/month for 240 months at 7% ≈ $104,338
Total future value: ≈ $144,725
Total contributed (principal + deposits): $10,000 + ($200 × 240) = $58,000
Total interest earned: ≈ $144,725 − $58,000 = ≈ $86,725

Nearly 60% of the final balance came from interest, not deposits, that’s the real power of starting early and staying consistent.

Common mistakes

Underestimating the effect of time. A 10-year head start often matters more than a higher contribution amount.
Ignoring compounding frequency. Daily compounding yields slightly more than annual compounding at the same stated rate.
Forgetting that returns aren’t guaranteed. This calculator assumes a constant rate; real investments fluctuate year to year.

Tips for maximizing compound growth

Starting early matters more than almost any other factor, time is the biggest multiplier in this formula.
Automate contributions so compounding works consistently without relying on willpower each month.
Avoid withdrawing early; interrupting the compounding timeline resets much of the long-term benefit.

What’s the difference between simple and compound interest?

Simple interest is earned only on your original principal. Compound interest is earned on your principal plus all previously accumulated interest, which is why it grows faster over time.

Does compounding frequency really make a big difference?

At typical rates, the difference between monthly and daily compounding is small, usually well under 1% of the final total. The interest rate and time invested matter far more than compounding frequency.

Is 7% a realistic return to expect?

Roughly 7% is often cited as a long-term average annual return for a diversified stock market portfolio after inflation, though actual returns vary significantly year to year and aren’t guaranteed.

How much difference does starting 10 years earlier make?

Often an enormous difference, since compounding accelerates over time. Money invested a decade earlier frequently ends up worth far more than a much larger sum invested later, even with identical contribution amounts.