A walkthrough, end to end.
- 1
Enter the principal, annual interest rate (as a percentage), and time in years.
- 2
The calculator returns the simple interest earned/owed and the total amount (principal + interest).
- 3
Compare against compound interest to see how much faster compound grows over the same period.
Simple interest formula
Simple interest is calculated only on the principal, never on previously-earned interest. Used for short-term loans, some bonds, and many introductory finance problems. For periods over 1–2 years, compound interest is what nearly every real product uses.
What you can do with this.
Short-term personal loans
Many short-term loans (under 1 year) use simple interest. The math is straightforward and total cost is easy to verify against the lender's quote.
Treasury bills and short bonds
T-Bills with maturity under 1 year are typically quoted as simple interest. The discount rate × time on a $10,000 T-Bill at 4.5% for 6 months earns $225.
Auto loan interest math
Most US auto loans use simple monthly interest — interest calculated daily on the outstanding balance. Total interest is similar to but slightly different from a simple-interest formula calc.
Late payment fees
Late payment penalties are commonly framed as simple interest on the overdue amount per day. Use the calculator to estimate the cost of paying late.
Simple vs compound comparison
On $10,000 at 6% over 10 years: simple interest earns $6,000 (total $16K); monthly-compound earns $8,194 (total $18.2K). The gap widens dramatically at longer durations.
Calculating a loan in days
If your loan term is given in days (common for short-term commercial loans), divide days by 365 (or 360 for some bond markets) to get years and feed into the calculator.
Simple interest 2026 — what's current
Most consumer products (savings, mortgages, credit cards) use compound interest. Simple interest persists for short-term lending, factoring, and some commercial paper.
Frequently asked.
Use simple for short-term lending and quick estimates. Use compound for anything multi-year — that's what every real bank, brokerage, and credit card uses.
Yes — convert days to years by dividing by 365 (or 360 for some bond conventions) and use that as time. Banker's year (360 days) gives slightly higher interest than calendar year (365).
Compounding earns 'interest on interest'. Over decades, that's the difference between modestly-growing savings and dramatically-growing savings — Einstein reportedly called it 'the eighth wonder of the world'.
No. Calculations run entirely in your browser.