Simple Interest Calculator

What Is Simple Interest?

Simple interest is interest calculated only on the original principal amount — it does not compound or accumulate interest on previously earned interest. The formula is: I = P × r × t, where I = Interest, P = Principal, r = Annual interest rate (as a decimal), and t = Time in years. The total future value is: A = P + I = P(1 + rt).

Simple interest produces linear growth — the interest earned each year is always the same fixed dollar amount, regardless of how much interest has already accumulated. This contrasts with compound interest, which produces exponential growth by calculating interest on an ever-growing balance.

Where Is Simple Interest Used?

Simple interest is most commonly used in short-term financial products and loans where compounding would add little practical difference:

• Auto loans: Many car loans use simple interest, which means paying off your loan early actually reduces the total interest paid (since interest is calculated daily on the remaining balance)
• Personal loans: Short-term personal loans often use simple interest for transparency
• US Treasury Bills: Short-term government securities use simple interest calculations
• Bonds: Bond coupon payments are based on simple interest of the face value
• Educational purposes: Simple interest is the foundation for understanding all forms of interest

Simple Interest vs Compound Interest

The difference between simple and compound interest becomes significant over long time periods or at high interest rates. Consider $10,000 at 5% for 10 years:

• Simple interest: $10,000 × 5% × 10 = $5,000 interest → Total: $15,000
• Compound interest (monthly): Total interest ≈ $6,470 → Total: $16,470

Over 10 years, compound interest earns $1,470 more — a 29% advantage. Over 30 years, the gap becomes dramatic: simple interest yields $25,000 while compound interest (at the same 5% rate) yields approximately $43,219. This is why compound interest is used for almost all savings and investment accounts, while simple interest is typically reserved for shorter-term lending.

Practical Tips

• Making extra payments on a simple interest loan reduces your principal faster, which reduces daily interest accrual — always beneficial
• For savings and investments, always seek compound interest accounts to maximize growth
• When borrowing, simple interest loans may be preferable for short terms — but for long-term debt, the difference from compounding is usually small compared to the interest rate itself