What is simple interest?

Simple interest is calculated only on the original principal, not on accumulated interest. The formula is I = P * r * t, where P is principal, r is the annual rate, and t is time in years.

What is the difference between simple and compound interest?

Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus accumulated interest, resulting in faster growth over time.

When is simple interest used?

Simple interest is commonly used for short-term loans, auto loans, and some personal loans. It is also used to calculate interest on savings bonds and US Treasury bills. Most mortgages and long-term investments use compound interest.

Is simple interest better for borrowers or lenders?

Simple interest is better for borrowers because you only pay interest on the original loan balance, not on accumulated interest. For lenders or investors, compound interest is more advantageous since returns grow faster.

Free Simple Interest Calculator

Calculate simple interest on loans or investments. Find the interest amount and total value using principal, rate, and time.

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Principal Amount

USD

Annual Interest Rate

Time Period

years

Total Interest

$1,500.00

Total Value$11,500.00

Total Interest vs Time Period

View formula & how it works →

How Simple Interest Works

Simple interest charges you a flat percentage on the original amount only. Borrow $1,000 at 5% simple interest for 3 years and you owe $150 in interest, period. The interest doesn't compound. This makes simple interest easy to calculate and predict, which is why some personal loans, auto loans, and short-term lending still use it.

The Formula

Interest = Principal x Rate x Time

Principal = Amount borrowed or invested

Rate = Annual interest rate (as a decimal, so 5% = 0.05)

Time = Time in years

Total Amount = Principal + Interest

Simple vs. Compound Interest

The difference matters. $10,000 at 6% simple interest for 10 years earns $6,000 in interest. The same amount at 6% compound interest (annually) earns $7,908. Over short periods the gap is small. Over decades it gets large. Credit cards, savings accounts, and most investments use compound interest. Simple interest shows up in some car loans, personal loans, and bonds.

When to Use This

Calculating interest on short-term loans, estimating bond yields, or understanding the interest portion of a flat-rate personal loan. Also useful as a quick estimate when compound interest would give a similar result (short time periods at low rates).

Common Mistakes

Applying simple interest math to a credit card balance. Credit cards use compound interest, usually daily. Simple interest would significantly understate what you owe.

Forgetting to convert the rate to a decimal. 5% in the formula is 0.05, not 5.

Confusing time units. If the rate is annual but the time is in months, divide months by 12 first.

Example Calculation

You deposit $10,000 at 5% simple interest for 3 years.

01Interest = $10,000 * 0.05 * 3 = $1,500
02Total value = $10,000 + $1,500 = $11,500

Example Calculations

How much interest on $5,000 at 8% for 3 years?

$1,200

1.Principal (P): $5,000
2.Rate (r): 8% = 0.08
3.Time (t): 3 years
4.Interest = P × r × t = 5,000 × 0.08 × 3 = $1,200

How much interest on $10,000 at 5% for 2 years?

$1,000

1.Principal: $10,000
2.Rate: 5% = 0.05
3.Time: 2 years
4.Interest = 10,000 × 0.05 × 2 = $1,000

How much interest on $20,000 at 4% for 5 years?

$4,000

1.Principal: $20,000
2.Rate: 4% = 0.04
3.Time: 5 years
4.Interest = 20,000 × 0.04 × 5 = $4,000
5.Total amount: $20,000 + $4,000 = $24,000

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