Simple Interest vs Compound Interest

The One Difference That Matters

Simple interest is calculated only on the original principal, every period, for the life of the loan or deposit. Compound interest is calculated on the principal plus whatever interest has already accumulated, so each period's interest is calculated on a slightly larger base than the last. That single difference — whether interest earns interest — is what separates the two formulas:

Simple Interest: SI = (P × R × T) ÷ 100

Compound Interest: A = P × (1 + R ÷ 100)^T, Interest = A − P

Side by Side: ₹1,00,000 at 8% for 5 Years

Both start identically in Year 1 — the gap only opens up once compound interest starts earning interest on the interest already added in Year 1. By Year 5, the same principal, rate, and time produces ₹6,933 more under compounding. Extend the time horizon and the gap grows faster, not linearly — this is why compounding is described as accelerating over time rather than adding a fixed extra amount each year.

Where Each One Actually Shows Up

• Simple interest is common in short-term loans, certain fixed-deposit products, and some basic personal loans where the lender calculates interest on the original amount only.
• Compound interest is the default for savings accounts, most mutual fund and SIP growth, and the vast majority of long-term loans and credit products — which is also why carrying credit card debt or a long-tenure loan costs more than simple interest math would suggest.
• EMI-based loans (home loans, car loans, most personal loans) use a reducing-balance compound method: interest is recalculated each period on the outstanding principal, which itself shrinks as you repay. This is different from both formulas above — see the EMI calculator for that specific structure.

What This Means When Borrowing or Saving

As a borrower, compound interest working against you means the cost of delaying repayment grows faster the longer you wait — a debt left untouched doesn't grow at a flat rate. As a saver or investor, the same mechanism works in your favor: money left invested longer doesn't just earn more, it earns proportionally more per year than it did the year before, which is why starting early matters more than the specific rate in many long-term savings comparisons.

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