Simple interest is calculated only on the original principal amount — unlike compound interest, which accumulates on both principal and previously earned interest. Simple interest is used in personal loans from friends/family, some government schemes, short-term loans, and as a preliminary tool to quickly estimate interest costs. While banks almost universally use compound/reducing-balance interest, understanding simple interest is foundational to understanding all interest concepts.
Simple Interest Formula
SI = P × R × T / 100, where P is the Principal, R is the annual rate (%), and T is the time in years. Total amount = P + SI. Example: ₹1 lakh at 8% for 5 years: SI = 1,00,000 × 8 × 5 / 100 = ₹40,000. Total = ₹1,40,000. Compare this with compound interest at the same rate — the compound maturity would be ₹1,46,933 — the difference is the reinvested interest.
Simple Interest vs Compound Interest
Simple interest grows linearly — the same fixed amount is added each period. Compound interest grows exponentially — the interest earned is added back to the principal, and future interest is earned on this larger base. Over 1 year, SI and CI (compounded annually) are identical. Over longer periods, the gap widens significantly. At 10% for 20 years, simple interest doubles the principal; compound interest grows it 6.7×.
Where Simple Interest Is Still Used
Certain microfinance institutions and cooperative societies use simple interest for short-term credit. Partial prepayment of loans during a moratorium period often uses simple interest. Some chit funds and informal lending arrangements use SI. The EPF passbook interest calculation for each year's contribution uses a simple interest approximation before the annual credit.
Frequently Asked Questions
Is simple interest better or worse for a borrower?
For short terms (< 1 year), simple and compound interest are nearly identical. For longer terms, simple interest is better for the borrower (you pay less total interest). However, for investors/lenders, compound interest is better (you earn more). When taking a loan, always check whether the bank uses flat rate (simple interest on original principal) or reducing balance — flat rate is significantly more expensive.
What is the flat rate vs reducing balance interest?
Flat rate: interest is calculated on the entire original principal throughout the tenure — making the effective rate nearly double the stated flat rate. Reducing balance: interest is charged only on the outstanding principal, which decreases with each EMI. Always ask for the reducing balance rate when comparing loan offers — flat rate offers are deceptive.