Interest rate:
interest × 100 ÷ (principal × years)

How to calculate an interest rate from the interest paid

To find a simple interest rate, multiply the interest by 100 and divide by the principal times the years. If you borrowed ₹20,000 and paid ₹1,800 in interest over one year, the rate is 1,800 × 100 ÷ (20,000 × 1) = 9 percent a year. This works when interest is charged on the original amount only.

The simple interest calculator on this site goes the other way: you enter the principal, an annual rate and the time in years, and it returns the interest and the maturity amount. Use it as a check: once you have worked out a rate, enter it with the same principal and time, and the interest it shows should match what you paid.

The interest rate formula, rearranged

The order of the steps matters less than keeping the units consistent. Interest and principal must be in the same currency, and time must be in years. If you have a monthly figure, multiply the monthly rate by 12 to see the yearly equivalent, for example 1 percent a month is 12 percent a year before any compounding.

Part years need a decimal. For ₹25,000 that earned ₹1,500 in 9 months, convert 9 months to 0.75 years. The rate is 1,500 × 100 ÷ (25,000 × 0.75) = 8 percent a year.

Interest chart: what each rate costs on ₹50,000

Typical uses include checking an informal loan between family or friends, comparing a fixed deposit offer with another, or verifying the interest a lender says you owe. In each case write down the principal, the interest and the exact time, and compute the rate yourself before you sign or pay. A rate that looks far from what you were quoted is a reason to ask questions.

To work out the rate from a result you already have, find your interest and years in the table. If you paid ₹8,000 over 2 years on ₹50,000, it sits in the 8 percent row.

When the simple interest rate is not the full story

Simple interest ignores compounding. Many savings accounts and deposits add interest to the balance, so the interest in year two is higher than in year one, and the true annual yield is above the stated rate. Credit cards and many loans also add fees and charges.

A flat-rate loan repaid in monthly instalments is another case. The flat rate is charged on the original principal, but you hold less of the money as you repay, so the effective rate you pay is higher than the quoted figure. Compare loans by their stated annual percentage rate and ask the lender for the total cost. This article is general information, not financial advice.

Common mistakes when finding the interest rate

• Using months where the formula expects years. 9 months is 0.75 years, not 9.
• Using the total repaid instead of the interest. Subtract the principal first.
• Forgetting to multiply by 100, which gives the rate as a decimal such as 0.09 rather than 9 percent.
• Applying the simple formula to compounding or instalment loans.
• Mixing up the rate per month with the rate per year.