Compound Interest Calculator explained with worked examples

Compound interest means you earn interest on your interest. In the first year, interest is calculated on your principal alone. From the second year on, it is calculated on the principal plus everything earned so far. That small difference is why the growth curve bends upward instead of climbing in a straight line.

Worked example one: ₹10,000 at 8% a year for 10 years. After year one you have ₹10,800. After year five, ₹14,693. After year ten, ₹21,589 — of which ₹11,589 is interest, more than the ₹10,000 you started with. The last five years add far more than the first five, because each year’s interest is calculated on a bigger balance.

Worked example two: ₹5,000 at 12% a year for 15 years. The higher rate and longer time combine to ₹5,000 × 1.12^15 = ₹27,368, of which ₹22,368 is interest. Compare that with the same ₹5,000 at 12% for only 5 years: ₹8,812. Ten extra years more than triple the result, even though the rate never changed.

The lesson both examples point to is the same: time does more work than most people expect. Starting five years earlier often beats finding a slightly better rate, because every extra year compounds everything that came before it. That is why the calculator rewards playing with the years field first, before hunting for a higher rate.

Frequently asked questions

How is compound interest different from simple interest?

Simple interest is always calculated on the original principal, so it grows in a straight line. Compound interest is calculated on the growing balance, so it curves upward over time.

Does a higher rate always beat a longer time?

Not always. A modest rate over a long time can beat a high rate over a short time, because compounding needs years to do its work. Try both in the calculator before deciding.

Why does the growth look slow at first?

Because the balance is small early on, so each year’s interest is small in absolute terms. The curve steepens as the balance grows — the slow start is normal, not a sign that something is wrong.