Compound Interest

How it works

Compound interest is interest calculated on your original principal plus all the interest earned so far. Simple interest pays only on the principal; compound interest pays on a steadily growing base, so it accelerates over time. The frequency of compounding matters — monthly compounding grows faster than annual, because interest is added (and starts earning) more often. The longer the money stays untouched, the more dramatic the effect.

The two faces of compounding

Compound interest is a powerful ally when you invest and a dangerous enemy when you borrow. On the saving side, it's why a PPF account, an FD, or an equity SIP grows so much over decades. On the borrowing side, it's why unpaid credit card balances spiral — card interest compounds monthly at very high effective annual rates, turning a small unpaid bill into a large debt.

In India

Many Indian products compound: PPF and EPF compound annually, FDs and RDs typically compound quarterly, and savings accounts compound on the daily balance. A useful shortcut is the Rule of 72 — divide 72 by the rate to estimate years to double. At 8%, money doubles in roughly nine years; at 12%, in about six.

Common mistakes

Understanding compound interest is the foundation of all personal finance — it explains why starting to invest early beats investing more later, and why clearing high-interest debt fast is one of the best "returns" you can earn.

People underestimate the long-run power of compounding because the early years look unimpressive — most growth comes late, so they give up or start too late. On the debt side, they pay only the minimum on credit cards, letting compounding work brutally against them. The lessons: start saving early, let it compound undisturbed, and never carry high-interest debt where compounding turns against you.