💲 Simple Interest Calculator

What is Simple Interest?

Simple interest is the most straightforward method of calculating the cost of borrowing or the return on lending money. It is computed solely on the original principal — the interest does not accumulate and earn further interest as it does with compound interest. The formula is: I = P × R × T, where I is the total interest, P is the principal amount, R is the annual interest rate expressed as a decimal, and T is the time period in years.

For example, if you deposit $5,000 in a savings account that pays 4% simple interest for 3 years, your interest earned is $5,000 × 0.04 × 3 = $600, giving a total of $5,600 at maturity. The calculation is transparent and predictable — you always know exactly what you'll earn or owe.

When is Simple Interest Used?

• Auto loans: Most car loans in the United States use simple interest, calculated daily on the outstanding balance. Making payments early reduces the principal faster and saves on total interest.
• Short-term personal loans: Payday loans and bridge loans typically use simple interest, though the rates can be very high.
• Treasury Bills (T-Bills): U.S. government short-term securities use discount-based simple interest calculations.
• Some savings bonds and CDs: Fixed-term deposit products sometimes advertise simple interest rates for clarity.
• Peer-to-peer lending: Many P2P platforms calculate borrower interest simply to make terms transparent.

Simple Interest vs Compound Interest

The key difference lies in what the interest is calculated on. With simple interest, you earn (or pay) the same dollar amount each period. With compound interest, prior interest is added to the principal before the next period's interest is calculated. Over short periods, the difference is negligible. Over long periods, it becomes enormous:

• $10,000 at 6% simple interest for 20 years: $10,000 + $12,000 interest = $22,000
• $10,000 at 6% compound interest (monthly) for 20 years: $33,102

This is why simple interest is favored for borrowing (lower total cost for short durations) while compound interest is favored for investing (higher total growth over time).

Calculating Daily Simple Interest

Many loans calculate interest daily based on the outstanding balance. The daily rate is your annual rate divided by 365. If your loan has an 8% annual rate and a $10,000 balance, you accrue $10,000 × (0.08/365) = $2.19 in interest per day. Each payment first covers accrued interest, then reduces principal. Paying even a few days early on a simple interest loan saves real money.

Frequently Asked Questions

Is simple interest better for borrowers or savers? It's better for borrowers (you pay less total interest than with compounding) and worse for savers (you earn less than with compounding). For short durations, the difference is small. For multi-decade horizons, compounding is dramatically more beneficial for investors.

How do I convert a simple interest rate to APY? Annual Percentage Yield (APY) accounts for compounding. For true simple interest products, APY equals the stated annual rate. If a product compounds, the APY will be higher than the stated rate — always compare APY when shopping savings accounts.

Can I use simple interest to estimate compound interest? Yes, as a quick approximation. For short periods (1–3 years) and modest rates, simple interest gives a reasonable estimate. For longer durations, use the compound interest formula for accuracy.