Simple Interest Calculator
Calculate simple interest, total amount, and monthly or daily interest in seconds. Enter your principal, rate, and time period to see the full breakdown.
How Simple Interest Is Calculated
Simple interest is the most straightforward way to calculate the cost of borrowing money or the return on a basic investment. Unlike compound interest, simple interest is always calculated on the original principal — it never grows on itself. The formula is:
Simple Interest (SI) = Principal × Rate × Time / 100
Where Principal (P) is the starting amount, Rate (R) is the annual interest rate as a percentage, and Time (T) is the number of years. The total amount at the end of the period is simply P + SI. For example, a $5,000 loan at 6% per year for 3 years earns $900 in interest, bringing the total to $5,900.
This calculator also breaks down your interest into monthly and daily amounts — useful for understanding the true cost of short-term borrowing or the pace of savings growth.
Simple Interest vs Compound Interest
The key difference between simple and compound interest is what the rate is applied to each period. With simple interest, the rate always applies to the original principal. With compound interest, the rate applies to the principal plus all previously earned interest.
Over short periods (1–2 years), the difference is small. Over long periods, the gap becomes dramatic. A $10,000 investment at 8% over 30 years earns $24,000 in simple interest ($34,000 total), but grows to over $100,000 with annual compounding. For savings and investments, compound interest almost always works in your favour. For loans, simple interest is generally cheaper for the borrower.
Common real-world uses of simple interest include car loans, personal loans, most short-term credit products, US Treasury bills, and some savings certificates. If a lender advertises a flat-rate loan, it is almost certainly using simple interest.
Practical Uses of This Calculator
This simple interest calculator is useful across a wide range of everyday financial decisions:
Loan comparison: Quickly see how much interest you will pay across different loan amounts, rates, and terms before signing. Compare a 12-month offer at 9% against a 24-month offer at 7% in seconds.
Savings estimation: If your bank offers a simple-interest savings account or fixed deposit, enter the balance and rate to see exactly how much you will earn over any period.
Investment due diligence: Short-term bonds, promissory notes, and peer-to-peer loans often use simple interest. Use this tool to verify the returns quoted by a product match the stated rate and term.
Teaching and learning: Simple interest is the foundation of financial literacy. Students can use this calculator to check homework and build intuition about how rate and time affect interest earned.
All calculations run entirely in your browser using the 2026 standard formula. No data is stored or transmitted — your financial figures remain completely private. Last updated: April 2026.
Tips for Borrowers and Savers
When borrowing, the total amount you repay is what matters — not just the rate. A lower rate over a longer term can cost more than a higher rate over a shorter term. Always calculate total interest, not just the monthly payment. When saving, even a small increase in rate has a meaningful impact over several years. A 1% rate increase on a $20,000 deposit held for 5 years adds $1,000 in simple interest — the equivalent of one extra month of returns for free.