Compound Interest Calculator

What this calculator does

This calculator projects how a sum of money grows when interest compounds over time. You enter a starting principal, an annual interest rate, the number of years and how often interest is compounded. You can also add a regular contribution that is paid in every compounding period. The tool shows the final balance, the total interest earned and the total amount you contributed, plus a year-by-year table so you can watch the balance build.

Why you might need it

Compound growth is the engine behind savings accounts, fixed deposits and long-term investing, and it is famously hard to estimate in your head. Seeing the numbers makes the effect concrete: a modest rate over many years, or a small regular contribution sustained over a long period, can produce a surprisingly large balance. The calculator is useful for setting savings goals, comparing accounts with different compounding frequencies, and understanding how much of a future balance comes from contributions versus interest.

Concrete questions it can answer include: how large a one-time deposit needs to be today to reach a target figure in ten years, how much a monthly contribution would need to rise to hit a goal sooner, or how a difference of a single percentage point in rate changes the outcome over a long horizon. It is also a useful sanity check on financial-product marketing — when an account quotes a headline rate, you can model the same rate yourself and see what the balance would actually look like at the end of the term rather than trusting a rounded illustration.

How to use it

1. Enter the starting principal — the amount you begin with.
2. Enter the annual interest rate as a percentage.
3. Enter the number of years the money will grow.
4. Choose the compounding frequency: annually, semi-annually, quarterly, monthly or daily.
5. Optionally enter a contribution per period that is added every compounding period.
6. Pick your currency and read the final balance and the year-by-year table.

How it’s calculated

For a lump sum with no contributions, the tool uses the standard compound interest formula:

A = P × (1 + r ÷ n)^(n × t)

where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year and t is the number of years.

When a contribution is included, the calculation steps through every period: each period the balance earns interest at the periodic rate r ÷ n, then the contribution is added, and the new balance carries into the next period. This matches an ordinary annuity contributed once per compounding period. The total interest is the final balance minus everything paid in — the principal plus all the contributions.

As a worked example, take a principal of 10,000 at a 6% annual rate compounded monthly for 5 years. The periodic rate is 0.06 ÷ 12 = 0.005 and there are 60 periods, so the balance grows to 10,000 × 1.005^60, about 13,489 — roughly 3,489 of interest. Adding a contribution of 200 each month would push the final balance well above 27,000, and most of that extra is the contributions themselves plus the interest they in turn earn, which is why the contribution total and interest total are shown separately.

Common pitfalls

A frequent error is comparing accounts on rate alone while ignoring compounding frequency, or the reverse — overestimating how much frequency matters when the rate is what really drives the result. Another is forgetting that the figures are nominal: inflation erodes purchasing power and tax may apply to interest, so the real outcome is lower. Finally, the contribution here is tied to the compounding period, so switching frequency also changes how often you are assumed to contribute.

Tips

To isolate the power of compounding, set the contribution to zero and lengthen the term — the curve steepens noticeably in later years because interest is earning interest. Then add a contribution and compare: regular saving often contributes more to the final balance than the starting principal does. To see the slower alternative, run the same principal and rate through a simple interest calculator. Treat every figure here as an informational projection for planning, not a guaranteed return or financial advice.