plainlySimple interest calculator
Start with the principal.
See the interest, the total, and the working.
Simple interest
$100.00
At 5% per year for 2 years.
- Original principal
- $1,000.00
- Interest per full year
- $50.00
- Principal + interest
- $1,100.00
$1,000.00 × (5 ÷ 100) × 2 = $100.00 interest.
Interest stays on the original principal. Amounts are rounded to two decimal places; no fees, payments, or compounding are included.
Free to use. No account. Your numbers stay in this tab.
One principal.
A straight calculation.
Enter the starting amount, the annual percentage rate, and how long it stays at that rate. Select Calculate interest to see the interest separately from the original principal, along with their combined total. Use the working line to check the calculation or copy it into your notes.
The simple interest formula
Interest = principal × (annual rate ÷ 100) × time in years
Total = principal + interest
Simple interest uses the original principal throughout the period. For $1,000 at 5% for 2 years: $1,000 × 0.05 × 2 = $100 interest. Add the principal to get $1,100 total. This follows the formula in OpenStax’s simple interest chapter.
Years, months, and days
For months, divide the duration by 12: 18 months is 1.5 years. For days, choose the year basis explicitly. With 90 days, the time factor is 90 ÷ 365 or 90 ÷ 360. The calculator uses the number of days you enter; it does not count dates or adjust months and leap years.
The 360-day option changes only the denominator. It does not implement a 30/360 calendar convention. Match the basis specified in your exercise or agreement before comparing answers.
See where
the difference comes from.
| Principal | Rate/year | Duration | Interest |
|---|---|---|---|
| $1,000 | 5% | 2 years | $100.00 |
| $2,000 | 6% | 18 months | $180.00 |
| $1,000 | 10% | 90 days / 365 | $24.66 |
| $1,000 | 10% | 90 days / 360 | $25.00 |
Simple interest versus compound interest
With simple interest, interest does not earn additional interest. With compounding, previously earned interest can also earn interest. On $1,000 at 5% for 2 years, annual compounding gives $102.50 interest rather than the $100 simple-interest result. The distinction is explained by the Consumer Financial Protection Bureau.
When a loan’s actual cost can differ
This is a fixed-principal arithmetic calculator. It assumes a constant rate and no repayments during the entered duration. Real installment loans may calculate interest on a balance that changes after each payment; fees and payment timing can also affect the amount owed. See the CFPB’s explanation of simple-interest auto loans. This result is not a payment schedule, APR calculation, or quote from a lender.
Check a percentage in another context
To compare an old and new amount, use the percentage change calculator. For course marks with different weights, use the weighted grade calculator.
Good questions.
Straight answers.
Do I enter 5 or 0.05 for a 5% rate?
Enter 5. The calculator divides the annual percentage by 100 for the formula. Entering 0.05 means 0.05%, not 5%.
Does the rate need to be annual?
Yes. Convert a simple monthly or daily rate to an annual simple rate before entering it. The time unit selector changes the duration, not the rate’s meaning.
Can I use it for savings and borrowing?
You can use the same arithmetic for either when interest applies to an unchanged principal at a constant annual rate. Check whether the actual product compounds interest or reduces principal through payments.
What if the rate, principal, or time is zero?
Interest is zero. The combined total is the principal. Negative values and incomplete entries are rejected so they do not silently become a different calculation.
Does changing currency convert the amount?
No. It changes the currency used to label the result. No exchange rates are applied.
How are results rounded?
Calculations use browser number precision and display amounts to two decimal places. Positive interest smaller than one cent is labeled as less than one cent. Rounded displayed amounts may differ by one cent when added together.
Are my financial details uploaded or saved?
No. Your entries are calculated in this browser tab without uploading them or saving them to website storage. Reloading restores the example.
Keep the useful numbers close.
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