Simple Interest Calculator
Calculate simple interest on a principal amount in any currency. Simple interest is calculated only on the original principal, not on accumulated interest. See also Compound Interest Calculator and Loan Calculator.
How to Calculate Simple Interest
Simple interest is calculated only on the original principal amount. Unlike compound interest, it does not earn interest on previously accumulated interest. Simple interest is commonly used for short-term loans, car loans, and some types of bonds. The calculation is straightforward: multiply the principal by the rate and the time period.
Simple Interest Formula
Simple Interest = P × R × T
Total Amount = P + (P × R × T)
Where:
P = Principal amount
R = Annual interest rate (as decimal)
T = Time in years
Example
Principal: $10,000, Rate: 5%, Time: 3 years
Interest = $10,000 × 0.05 × 3 = $1,500
Total = $10,000 + $1,500 = $11,500
Simple vs Compound Interest
The key difference is that simple interest is always calculated on the original principal, while compound interest is calculated on the principal plus accumulated interest. Over short periods, the difference is small. Over long periods, compound interest grows significantly faster. For example, $10,000 at 5% for 10 years: simple interest yields $15,000, while compound interest (monthly) yields $16,470 — a $1,470 difference.
Simple Interest Reference Table
| Principal | Rate | 1 Year | 3 Years | 5 Years |
|---|---|---|---|---|
| $1,000 | 3% | $30 | $90 | $150 |
| $5,000 | 5% | $250 | $750 | $1,250 |
| $10,000 | 5% | $500 | $1,500 | $2,500 |
| $10,000 | 8% | $800 | $2,400 | $4,000 |
| $50,000 | 4% | $2,000 | $6,000 | $10,000 |
| $100,000 | 6% | $6,000 | $18,000 | $30,000 |
Frequently Asked Questions
When is simple interest used?
Simple interest is commonly used for short-term loans, auto loans, some personal loans, and certain types of bonds. It is also used in some savings certificates and fixed deposits for shorter terms.
Is simple interest better than compound interest?
It depends on your perspective. As a borrower, simple interest is better because you pay less. As an investor, compound interest is better because you earn more. Simple interest is always less than compound interest for the same rate and period.
How do I calculate simple interest for months?
Convert months to years by dividing by 12. For example, 6 months = 0.5 years. Then use the formula: Interest = P × R × (months/12). For $10,000 at 5% for 6 months: $10,000 × 0.05 × 0.5 = $250.
Solved Examples
Example 1: Short-term personal loan interest
Solution:
Principal = $5,000, Rate = 8% per year, Time = 2 years
Simple Interest = P × R × T = $5,000 × 0.08 × 2
Simple Interest = $800
Total Amount = $5,000 + $800 = $5,800
Answer: You pay $800 in interest, for a total repayment of $5,800.
Example 2: Fixed deposit for 6 months
Solution:
Principal = $25,000, Rate = 4.5% per year, Time = 6 months = 0.5 years
Simple Interest = $25,000 × 0.045 × 0.5
Simple Interest = $562.50
Maturity Amount = $25,000 + $562.50 = $25,562.50
Answer: The deposit earns $562.50 in 6 months, maturing at $25,562.50.
Example 3: Finding the required interest rate
Solution:
You want to earn $3,000 in interest on $20,000 over 5 years
Rate = Interest / (Principal × Time) = $3,000 / ($20,000 × 5)
Rate = $3,000 / $100,000 = 0.03 = 3%
Answer: You need an annual rate of 3% to earn $3,000 on $20,000 over 5 years.
Practice Questions
Try these on your own:
- Calculate the simple interest on $15,000 at 6% for 4 years. (Answer: $3,600)
- A loan of $8,000 charges 9% simple interest. How much interest is owed after 18 months? (Answer: $1,080)
- What principal earns $2,000 in simple interest at 5% over 8 years? (Answer: $5,000)
- You borrow $12,000 and repay $14,160 after 3 years. What was the annual simple interest rate? (Answer: 6%)
- How long does it take $30,000 to earn $4,500 in simple interest at 3%? (Answer: 5 years)
- Compare simple interest earned on $50,000 at 5% for 10 years vs compound interest monthly for the same terms. (Answer: Simple = $25,000; Compound ≈ $32,348; Difference = $7,348)
Common Mistakes to Avoid
The most common mistake with simple interest is forgetting to convert the time period to years. If you have a 6-month loan, you must use T = 0.5, not T = 6. Similarly, a 90-day loan uses T = 90/365 (or 90/360 for banker's convention). Another frequent error is confusing simple interest with compound interest when comparing loan offers — a 5% simple interest loan for 3 years costs less than a 5% compound interest loan for the same period. People also often forget that the quoted rate is annual even if the loan term is shorter or longer. Finally, when working backwards to find the rate or time, ensure you rearrange the formula correctly: R = I/(P×T) for rate, and T = I/(P×R) for time.
Key Takeaways
- Simple interest is calculated only on the original principal — it grows linearly, not exponentially.
- The formula I = P × R × T is straightforward: multiply principal by rate (as decimal) by time in years.
- Simple interest is commonly used for short-term loans, auto loans, and some fixed deposits.
- Always convert time to years: months ÷ 12, days ÷ 365 (or 360 for some banking calculations).
- Over long periods, simple interest yields significantly less than compound interest at the same rate.
- As a borrower, simple interest means lower total cost; as a saver, compound interest means higher returns.