How Simple and Compound Interest Actually Differ

By the Toolbench team · Updated September 2026

Both formulas start from the same idea — you are paid (or you pay) a percentage of an amount over time. Where they split is what that percentage applies to after the first period.

Simple interest: always on the original amount

Simple interest is calculated only on the principal, every single period. If you invest Rs 10,000 at 6% simple interest, you earn Rs 600 every year, without exception, regardless of how many years pass. The formula is Interest = Principal × Rate × Time, and it produces a straight line when charted over time.

Compound interest: interest earns interest

Compound interest recalculates the base amount at the end of every compounding period, folding in whatever interest was just earned. That means year two earns interest on the original principal plus year one's interest, and so on. Charted over time, this produces a curve, not a line, and the gap between simple and compound results widens the longer the money sits.

Why compounding frequency matters

Two accounts at the same annual rate can produce different results depending on whether interest compounds annually, quarterly, or monthly. More frequent compounding means interest starts earning interest sooner, so a 6% rate compounded monthly will out-earn the same 6% compounded annually, even though the stated rate is identical.

A rough way to picture it

Over short periods (a year or two) and small amounts, simple and compound interest look similar. Over five, ten, or twenty years, compound interest pulls noticeably ahead, which is the entire reason long-term investing benefits so much from starting early rather than starting with more money later.

Try the calculators

Compare the two directly using the Simple Interest Calculator and the Compound Interest Calculator with the same principal, rate, and time to see the gap for yourself.