Calculate simple interest earned or owed over a period of time, with a full breakdown of the formula.
Simple interest = principal × rate × time = 5,000 × 4% × 3 years = 600.
Interest is the cost of borrowing money or the return earned on money saved or invested. There are two fundamental methods of calculating it: simple interest, which grows at a constant linear rate, and compound interest, which grows exponentially because interest is earned on previously accumulated interest as well as the principal.
I is interest earned, P is principal, r is the annual rate (decimal), and t is time in years. $5,000 at 5% simple interest for 3 years earns exactly $750, regardless of how the calculation is split into sub-periods.
The same $5,000 at 5%, compounded annually for 3 years, grows to about $5,788 — $38 more than simple interest, because year 2 and 3 also earn interest on the prior interest.
| Years | Simple interest total | Compound interest total |
|---|---|---|
| 5 | $6,250 | $6,381 |
| 10 | $7,500 | $8,144 |
| 20 | $10,000 | $13,266 |
Simple vs. compound interest — $5,000 at 5% over 20 years
Example 1 — Short-term simple interest loan: A $2,000 loan at 8% simple interest for 9 months (0.75 years): I = 2,000 × 0.08 × 0.75 = $120 in interest.
Example 2 — Savings account, compound: $3,000 in a savings account at 4.5% compounded monthly for 4 years grows to about $3,590 — roughly $50 more than the simple-interest equivalent of $3,540.
Example 3 — Comparing loan quotes: A $10,000, 2-year loan quoted at "6% simple" costs $1,200 in interest total; a loan quoted at "6% compounded annually" costs about $1,236 — a small but real difference worth checking before choosing.
| Product | Common interest type |
|---|---|
| Savings accounts, CDs | Compound (often daily/monthly) |
| Mortgages, auto loans | Compound (amortized) |
| Short-term/payday loans | Often simple interest |
| Treasury bills, some bonds | Simple interest to maturity |
Which type of interest do savings accounts typically use? Most savings and investment accounts use compound interest, often compounded daily or monthly.
Which type do short-term loans typically use? Many short-term consumer loans use simple interest calculated on the outstanding balance.
Does more frequent compounding always matter a lot? The practical difference between monthly and daily compounding is usually small compared to the effect of the interest rate itself.
Why do simple and compound interest give the same result in year 1? Because there's no prior accumulated interest yet to compound on — the two methods only start to diverge from year 2 onward.
How do I calculate interest for a period shorter than a year? Convert the time period into a fraction of a year (e.g., 6 months = 0.5 years, 90 days = 90/365 years) and use that fraction directly in the formula.
Is APR the same as the simple interest rate? Not always — APR can include certain fees in addition to the base interest rate, and depending on the product, it may reflect a compounding assumption rather than pure simple interest.
What's a quick way to estimate total simple interest without a calculator? Multiply the principal by the rate to find one year's interest, then multiply that figure by the number of years — this works because simple interest grows linearly, not exponentially.
At $8,000 principal, 4% annual simple interest over 6 years earns $8,000 × 0.04 × 6 = $1,920 in interest, for a total of $9,920 — notice the interest earned is the same every year ($320/year), unlike compound interest which grows each year.
Principal: The original sum of money on which interest is calculated.
Simple interest: Interest calculated only on the principal, not on previously accumulated interest.