Simple Interest Calculator
Work out flat, non-compounding interest on a loan or deposit in one step. Enter the principal, an annual rate and a time in years, and the tool returns the interest earned and the final amount owed or received. Because simple interest is charged only on the original principal, the interest is the same every year and grows in a straight line, and time can be fractional such as 2.5 years.
How to use the simple interest calculator
- Enter the Principal amount you invest or borrow (0 up to 1,000,000).
- Set the Annual interest rate as a percentage, adjustable in tenth-point steps.
- Set the Time period in years, in half-year steps up to 50 years.
- Choose your currency from the selector in the top right to reformat the figures.
- Read the Principal, Interest earned and Total amount cards that update instantly.
Examples
1,000 at 10% for 2 years
Principal 1,000, Annual rate 10%, Time 2 yr
Interest earned $200.00, Total amount $1,200.00
5,000 at 6% for 3.5 years
Principal 5,000, Annual rate 6%, Time 3.5 yr
Interest earned $1,050.00, Total amount $6,050.00
Frequently asked questions
What is the simple interest formula?
Simple interest is I equals P times R times T, where P is the principal, R is the annual rate as a decimal and T is the time in years. This tool uses percent input, so it computes principal times rate percent times years divided by 100. The total amount is the principal plus that interest.
How is simple interest different from compound interest?
Simple interest is charged only on the original principal, so the yearly interest is flat and never changes. Compound interest is charged on the principal plus any interest already added, so it snowballs and outpaces simple interest over time. For that behaviour, use the compound interest calculator.
When is simple interest actually used?
It is common on short-term and some car loans, certain personal loans, treasury bills and many fixed deposits where interest is paid out to you rather than reinvested. Retail savings and credit cards, by contrast, almost always compound.
Can I use fractional years or months?
Yes. Enter the time as a decimal, for example 0.5 for six months or 3.5 for three and a half years. Interest scales linearly with time, so half the years means exactly half the interest, and the slider moves in half-year steps.
How are the results rounded?
The interest and total amount are each rounded to two decimal places for display, matching normal money formatting. The underlying calculation is done at full precision first, then rounded once at the end.
How do I find the rate, principal or time from the interest?
Rearrange the formula. Rate equals interest divided by principal times time, principal equals interest divided by rate times time, and time equals interest divided by principal times rate. Plug in the two values you know to solve for the third.
Does the currency selector convert money?
No. It only changes the currency symbol and number formatting on screen. The principal, interest and total keep the same numeric values regardless of the currency you pick, since no exchange rate is applied.
What happens with a zero rate or zero time?
Interest becomes zero and the total amount equals the principal, because either factor being zero makes the product zero. The tool accepts these values and simply shows no interest earned.
Is my data private?
Yes. Every calculation runs locally in your browser, so the numbers you type are never sent to or stored on a server.
Learn more
- Simple vs compound interest, with examples
Why flat simple interest grows in a straight line while compound interest curves upward, plus worked numbers and where each one shows up in real life.
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