Simple Interest vs Compound Interest: Which Calculator Should You Use?

Simple vs Compound Interest: What's the Difference?

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus previously earned interest — which is why compound interest grows money much faster over time.

Choosing between the two matters everywhere in personal finance: loans, FDs, PPF, EPF, and even late-fee calculations.

The Core Formulas

Simple Interest = (Principal × Rate × Time) / 100 Compound Amount = Principal × (1 + Rate/100)^Time

For example, ₹1,00,000 at 8% for 10 years:

	Simple	Compound (yearly)
Interest	₹80,000	₹1,15,892
Total	₹1,80,000	₹2,15,892

The difference grows with time — after 20 years the gap is around ₹1,15,000 on the same ₹1,00,000.

When Lenders Use Simple Interest

• Personal loans between friends — almost always simple interest.
• Old-style loan agreements — some chit funds and private lenders still use simple interest.
• Fixed-deposit calculations listed on bank websites — they show simple interest figures in examples.

For these, the simple interest calculator gives you the exact amount.

When Compounding Applies

• FDs and PPF — banks compound interest quarterly, PPF yearly.
• EPF and mutual funds — the entire fund value earns returns year after year.
• Credit card late fees and loan arrears — interest on interest is charged on overdue EMIs.

Use the compound interest calculator for these scenarios to see the real picture.

Which Calculator Should You Use?

Ask one question: does the interest earn its own interest?

• No → simple interest calculator
• Yes → compound interest calculator

Many Indian situations mix both — for example a loan that compounds overdue interest monthly. Model it with the compound calculator to avoid surprises, and check your actual EPF growth with the EPF interest calculator.

Frequently Asked Questions

• Are FDs simple or compound? Banks advertise the simple-interest equivalent, but they actually compound — the compound calculator is more accurate for planning.
• Which one should I use for a borrowed sum? Read your agreement. Default to compound if your lender charges "interest on overdue interest".
• Is the calculator free? Both tools are 100% free: simple interest and compound interest.

Understand which interest is working against you (loans) or for you (savings) — calculate both properly.