Simple Interest Calculator
Use our simple interest calculator to calculate the interest earned or charged on a principal amount over a specific period.
Modify the values and click Calculate to see simple interest and the total amount.
Related Calculators
How to Use the Simple Interest Calculator
Using the calculator takes only a few seconds.
- 1
Enter the principal amount
The principal is the original amount of money being borrowed, invested, or deposited. For example, Principal = $10,000.
- 2
Enter the interest rate
Enter the annual interest rate as a percentage. For example, Interest Rate = 5%. The calculator converts the percentage into the decimal value required for the calculation.
- 3
Enter the time
Enter how long the money will earn or be charged interest. You can use days, months, or years.
- 4
Calculate
Select Calculate to see the simple interest and total amount. For $10,000 at 5% for 3 years, simple interest is $1,500 and the total amount is $11,500.
Simple Interest Formula
The formula for calculating simple interest is:
Simple interest
- I
- Simple interest
- P
- Principal
- r
- Interest rate expressed as a decimal
- t
- Time in years
To calculate the total amount:
Total amount
You can also write the total amount formula as:
Combined formula
Simple Interest Formula Using a Percentage
If you enter the interest rate directly as a percentage, you can use:
Percentage form
For example, Principal = $5,000, Rate = 6%, Time = 4 years:
Simple Interest = ($5,000 × 6 × 4) ÷ 100 = $1,200
Total amount: $5,000 + $1,200 = $6,200.
Simple Interest Example
Suppose you invest $8,000 at a simple annual interest rate of 7% for 3 years.
I = $8,000 × 0.07 × 3 = $1,680. The total amount is $8,000 + $1,680 = $9,680.
| Value | Amount |
|---|---|
| Principal | $8,000 |
| Interest Rate | 7% |
| Time | 3 years |
| Simple Interest | $1,680 |
| Total Amount | $9,680 |
The calculator performs these calculations instantly when you enter your values.
What Is Simple Interest?
Simple interest is interest calculated using the original principal amount. Unlike compound interest, previously accumulated interest is not added to the principal when calculating future simple interest.
For example, suppose you have a principal of $10,000 and a simple interest rate of 5% per year. Your simple interest for one year is $500. If the principal, rate, and calculation method remain unchanged:
| Year | Interest for the Year | Total Interest |
|---|---|---|
| 1 | $500 | $500 |
| 2 | $500 | $1,000 |
| 3 | $500 | $1,500 |
| 4 | $500 | $2,000 |
The interest increases at a constant rate because it is based on the original principal.
How to Calculate Simple Interest Step by Step
Let's calculate simple interest on $12,000 at 4.5% for 5 years.
- 1
Identify the principal
P = $12,000
- 2
Convert the rate to decimal form
4.5% ÷ 100 = 0.045, so r = 0.045.
- 3
Enter the time
t = 5 years
- 4
Multiply the values
I = $12,000 × 0.045 × 5 = $2,700
- 5
Find the total amount
A = $12,000 + $2,700 = $14,700
Simple Interest = $2,700. Total Amount = $14,700.
Simple Interest Calculator for Months
If your interest rate is annual but your time period is given in months, convert months into years:
Months to years
For $10,000 at 6% for 6 months: 6 ÷ 12 = 0.5 years. Then I = $10,000 × 0.06 × 0.5 = $300. Total amount: $10,300.
With this calculator, choose Months as the time unit and the conversion is done for you.
Simple Interest Calculator for Days
For a basic calculation using a 365-day year:
Days to years
For $20,000 at 5% for 90 days: 90 ÷ 365 ≈ 0.2466 years. Then I = $20,000 × 0.05 × 0.2466 ≈ $246.58.
How to Calculate Simple Interest for 1 Year
For exactly one year, the formula becomes I = P × r.
For $25,000 at 8%: I = $25,000 × 0.08 = $2,000. The total amount after one year is $27,000.
How to Find Principal Using Simple Interest
If you already know the interest, rate, and time, start with I = Prt and rearrange:
Solve for principal
Example: simple interest $1,500, rate 5%, time 3 years. P = $1,500 ÷ (0.05 × 3) = $10,000.
How to Find the Simple Interest Rate
Solve for rate
Example: principal $10,000, interest $2,000, time 4 years. r = $2,000 ÷ ($10,000 × 4) = 0.05 = 5% per year.
How to Find Time Using Simple Interest
Solve for time
Example: principal $5,000, interest $1,500, rate 6%. t = $1,500 ÷ ($5,000 × 0.06) = 5 years.
Simple Interest vs. Compound Interest
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Based on | Original principal | Principal plus accumulated interest |
| Interest on previous interest | No | Yes |
| Growth pattern | Linear | Compounding |
| Basic formula | I = Prt | Uses a compounding formula |
| Calculation | Usually simpler | Depends on compounding frequency |
For $10,000 at 5% for 3 years, simple interest is $1,500 and the total is $11,500. Compound interest can be different because previously earned interest may also earn interest.
If your calculation involves compounding, use our Compound Interest Calculator instead.
When Is Simple Interest Used?
Simple interest calculations can be useful when you need to estimate interest based directly on a principal amount, rate, and time period.
- Basic loan calculations
- Estimate interest charged on a principal over a stated period.
- Short-term borrowing
- Useful when interest is based directly on the original amount.
- Educational finance problems
- Work through I = Prt examples without compounding.
- Simple investment estimates
- Compare interest earned on deposits or investments that use simple interest.
Simple Interest and APR Are Not the Same Thing
An interest rate and an annual percentage rate, or APR, should not automatically be treated as identical. A simple interest calculation uses the interest rate entered into the calculator. APR may reflect additional borrowing costs depending on the financial product.
If you are comparing real loans, review the lender's stated interest rate, APR, fees, and repayment terms instead of relying only on a simple-interest estimate.
Why Use a Simple Interest Calculator?
You can calculate simple interest manually, but an online calculator makes it easier to compare different scenarios. You can quickly test different principal amounts, rates, and time periods. Changing one input instantly shows how it affects the amount of interest.
How This Calculator Works
RealCalculators calculates simple interest using I = P × r × t. The total amount is calculated using A = P + I, where P is principal, r is the annual interest rate as a decimal, t is time in years, I is simple interest, and A is the total amount.
When months or days are selected, the calculator converts the selected period into the equivalent fraction of a year before applying the formula.
Last reviewed: August 2026
Related tools: Compound Interest Calculator, Savings Calculator, and Loan Calculator.
Frequently Asked Questions
A simple interest calculator calculates the interest on a principal amount using an interest rate and time period without compounding previously accumulated interest. It commonly uses I = P × r × t.
The simple interest formula is I = P × r × t, where P is principal, r is the interest rate in decimal form, t is time, and I is simple interest.
Multiply the principal by the interest rate expressed as a decimal and then multiply by the time. For example, $10,000 × 0.05 × 2 = $1,000.
Use Simple Interest = (Principal × Rate × Time) ÷ 100. For example, ($5,000 × 6 × 3) ÷ 100 = $900.
The answer depends on the time period. For one year: $10,000 × 5% × 1 = $500. For three years: $10,000 × 5% × 3 = $1,500.
Using simple interest: $5,000 × 0.06 × 2 = $600. The total amount is $5,600.
If the rate is annual, convert months into a fraction of a year using Months ÷ 12. For example, 9 months equals 0.75 years. Then use 0.75 as the time value in I = Prt.
For a simplified calculation using a 365-day year, Time = Days ÷ 365. Then use I = Prt. Some financial products use different day-count conventions.
The principal is the original amount of money used in the interest calculation. If you borrow $15,000, the starting principal is $15,000.
The total amount is the principal plus the interest: A = P + I. For example, $10,000 + $1,500 = $11,500.
In the standard simple-interest formula, interest is calculated using the original principal rather than adding accumulated interest to the principal for future calculations.
No. Standard simple interest does not compound previously accumulated interest. If interest is added to the balance and future interest is calculated on that larger balance, use a compound-interest calculation instead.
Simple interest uses the original principal when calculating interest. Compound interest can include previously accumulated interest in the balance used for future interest calculations, so results can grow differently over time.
Yes. If interest, rate, and time are known, rearrange the formula as P = I ÷ (rt). For example, $1,500 ÷ (0.05 × 3) = $10,000.
If principal, interest, and time are known, r = I ÷ (Pt). Convert the decimal to a percentage by multiplying by 100. For example, $2,000 ÷ ($10,000 × 4) = 5%.
If principal, interest, and rate are known, t = I ÷ (Pr). For example, $1,500 ÷ ($5,000 × 0.06) = 5 years.
Neither method is automatically better in every situation. They are different ways of calculating interest. Which calculation applies depends on the terms of the loan, investment, or financial product you are evaluating.
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