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Simple Interest Calculator

Work out simple interest and the final amount from a principal, an annual rate, and a length of time in years or months.

Details

Calculated in your browser as you type. Nothing is uploaded.

Time unit

The rate is annual, so months are counted as a fraction of a year.

Result
1,150.00
Final amount
150.00
Interest earned
1,000.00
Principal
Breakdown
Principal
1,000.00
Annual rate
5%
Time
3 years
Interest (P × r × t)
150.00
Final amount
1,150.00

How to calculate simple interest

  1. Enter the principal

    Type the starting amount the interest is charged or earned on.

  2. Set the annual rate

    Enter the yearly interest rate as a percentage, for example 5 for 5%.

  3. Choose the time

    Enter how long the money is invested or borrowed and switch the unit between years and months.

  4. Read the result

    The interest, the final amount, and a line-by-line breakdown update the moment you type.

Why use this tool

Straight simple interest math

Interest equals principal times annual rate times time, with no compounding, so the result matches how flat interest is quoted.

Years or months

Switch the time unit with one tap; months are converted against the annual rate automatically, so a six month term counts as half a year.

Interest and final amount split

The interest earned and the final amount are shown separately, so you always see what was added on top of the principal.

Line-by-line breakdown

A receipt style breakdown shows the principal, rate, time, interest, and final amount, so the number is never a black box.

Guards empty and negative input

Blank fields prompt gently and negative values are flagged instead of producing a nonsense figure.

Runs entirely in your browser

Everything is calculated on your device as you type; nothing is uploaded and there is no signup.

About this tool

Simple interest is interest charged only on the original principal, never on interest already added. The formula is principal times rate times time: put in 1,000 at 5% for 3 years and you earn 150, fifty each year, for a final amount of 1,150. Because nothing compounds, the interest is the same in the last year as in the first, which makes simple interest easy to check by hand and common on short term and fixed instalment lending. This calculator applies that formula to a principal, an annual rate, and a length of time, then reports the interest, the final amount, and a full breakdown as you type.

Time can be entered in years or months. The rate is always annual, so choosing months divides the term by twelve before multiplying: 5% for 6 months earns half of one year of interest. Enter the principal, the rate as a plain percentage, and the term, then read the interest and the final amount from the stat cards, or copy the one line summary. Blank fields and negative numbers are handled rather than turned into a broken result.

Simple interest describes many car loans, personal loans, and some bonds, but savings accounts and most mortgages compound, meaning interest earns further interest over time. For that, the compound interest calculator projects year by year growth, and the loan calculator works through amortised repayments. Everything here runs in your browser, and nothing is sent to a server.

Frequently asked questions

What formula does the simple interest calculator use?
Interest equals principal times the annual rate times the time in years. The final amount adds that interest back to the principal. There is no compounding, so the interest earned is the same in every period.
How are months handled?
The rate you enter is annual, so a term set in months is divided by twelve before the interest is worked out. A 6 month term counts as half a year, and 18 months counts as one and a half years.
How is simple interest different from compound interest?
Simple interest is charged only on the original principal, so it grows in a straight line. Compound interest is charged on the principal plus any interest already added, so it grows faster over time. Use the compound interest calculator when interest earns further interest.
What happens if I leave a field blank or enter a negative number?
A blank principal, rate, or time shows a short prompt instead of a result. A negative value in any field is flagged, since a simple interest amount, rate, or term below zero has no meaning here.
Which currency does it use?
None. The calculator works in plain numbers, so the answer is in whatever currency you had in mind. Values are formatted with thousands separators and two decimals but no symbol.
Is my data uploaded anywhere?
No. Everything runs in your browser as you type; nothing is sent to a server, stored, or logged.

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