# What is my simple interest?

Computes simple interest, I = P × r × t, and the total amount, or solves for the principal, rate, or time from the others.

- Page: https://www.acalculator.org/finance/simple-interest-calculator
- JSON spec: https://www.acalculator.org/finance/simple-interest-calculator.json
- Version: ff70b4873acd

## Default answer

Example with the default inputs (Principal $10,000.00, Interest rate (per year) 5%, Time 10, Time in years): Simple interest on $10,000.00 at 5% a year for 10 years is $5,000.00, for a total of $15,000.00.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| principal | Principal | The amount borrowed or invested. |
| rate | Interest rate (per year) | The simple interest rate per year. |
| years | Time | How long the money is borrowed or invested, in the chosen unit. |
| interest | Interest | The interest earned or paid: principal × rate × time. |
| timeUnit | Time in | The unit of the time: years, months, or days (a 365-day year). |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| principal | Principal | The amount borrowed or invested. |
| rate | Interest rate (per year) | The simple interest rate per year. |
| time | Time | How long the money is borrowed or invested, in the unit chosen under “Time in”. |
| interest | Interest | The interest earned or paid: principal × rate × time. |
| total | Total amount | The principal plus the interest. |

## Method

I = P × r × t, with r per year and t in years (months ÷ 12, days ÷ 365); A = P + I.

## Assumptions

- Interest is charged on the principal only; it never earns interest itself.
- A time in months is months ÷ 12 years; a time in days is days ÷ 365 years.
- The rate is a yearly rate and does not change.

## Worked examples

1. principal = $5,000.00, rate = 3.5%, years = 18, timeUnit = months gives interest = $262.50. Source: OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications.
2. principal = $10,000.00, rate = 5%, years = 3, timeUnit = years gives interest = $1,500.00, total = $11,500.00. Source: OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications.
3. principal = $10,000.00, rate = 6%, years = 90, timeUnit = days gives interest = $147.95, total = $10,147.95. Source: OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications.
4. interest = $500.00, rate = 5%, years = 2, timeUnit = years gives principal = $5,000.00, total = $5,500.00. Source: OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications.
5. principal = $10,000.00, interest = $1,000.00, years = 2, timeUnit = years gives rate = 5%. Source: OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications.
6. principal = $10,000.00, rate = 5%, interest = $1,500.00, timeUnit = years gives years = 3, total = $11,500.00. Source: OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications.

## FAQ

### What is Simple Interest?

Simple interest is interest that is calculated only on the initial principal amount borrowed or invested. Unlike compound interest, simple interest does not accumulate on previously earned interest. The formula for simple interest is: I = P × r × t, where I is the interest, P is the principal, r is the annual interest rate, and t is the time in years.

### How is Simple Interest Calculated?

Simple interest is calculated using the formula: I = P × r × t. For example, if you invest $10,000 at 5% annual interest for 3 years, the calculation would be: $10,000 × 0.05 × 3 = $1,500. The total amount after 3 years would be $10,000 + $1,500 = $11,500.

### What's the Difference Between Simple and Compound Interest?

Simple interest is calculated only on the original principal amount, while compound interest is calculated on both the principal and any accumulated interest. Compound interest typically results in higher returns over time because you earn interest on your interest. Simple interest is often used for short-term loans or investments, while compound interest is more common for long-term investments.

### When is Simple Interest Used?

Simple interest is commonly used for short-term loans, car loans, personal loans, and some types of bonds. It's also used in some savings accounts and certificates of deposit (CDs) that pay simple interest rather than compound interest. Simple interest is preferred when you want predictable, linear growth without the complexity of compounding.

### Can I Calculate Simple Interest for Different Time Periods?

Yes, you can calculate simple interest for any time period. The key is to convert the time period to years. For example, for 6 months, use 0.5 years; for 18 months, use 1.5 years; for 90 days, use 90/365 = 0.247 years. Our calculator handles years, months, and days automatically, using a 365-day year for days.

### How Do I Find the Principal Amount?

If you know the interest earned, rate, and time, you can find the principal using the formula: P = I / (r × t). For example, if you earned $500 in interest at 5% for 2 years, the principal would be: $500 / (0.05 × 2) = $5,000.

### How Do I Find the Interest Rate?

If you know the principal, interest earned, and time, you can find the interest rate using the formula: r = I / (P × t). For example, if you invested $10,000 and earned $1,000 in interest over 2 years, the rate would be: $1,000 / ($10,000 × 2) = 0.05 or 5%.

### What are the Advantages of Simple Interest?

Simple interest offers predictable, linear growth that's easy to understand and calculate. It's transparent - you know exactly how much interest you'll earn or pay. Simple interest is often better for borrowers on short-term loans since they don't pay interest on accumulated interest. It's also useful for financial planning when you need consistent, predictable returns.

### What are the Disadvantages of Simple Interest?

Simple interest typically provides lower returns compared to compound interest over long periods. As an investor, you miss out on the potential for exponential growth that compound interest offers. Simple interest doesn't account for the time value of money as effectively as compound interest, which can be a disadvantage for long-term investments.

### Is Simple Interest Better for Borrowers or Lenders?

Simple interest is generally better for borrowers because they pay less total interest over time compared to compound interest loans. However, for lenders or investors, simple interest typically provides lower returns. Most modern financial products use compound interest because it better reflects the true cost of borrowing and the potential returns on investment.

## Sources

- Simple interest I = P × r × t: S. A. Broverman, Mathematics of Investment and Credit, section 1.3.
- Consumer Financial Protection Bureau, What is the difference between a simple interest loan and a precomputed interest loan? https://www.consumerfinance.gov/ask-cfpb/
- OpenStax, Prealgebra 2e, §6.4 Solve Simple Interest Applications (I = Prt). https://openstax.org/books/prealgebra-2e/pages/6-4-solve-simple-interest-applications