Simple Interest Calculator

Calculate simple interest on your savings or loans. See how your principal earns interest over time with daily, monthly, and annual breakdowns.

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Enter your principal, interest rate, and time period, then click "Calculate" to see your simple interest results.

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Pro Tip

If you are earning simple interest on savings, consider switching to an account that compounds interest. Over 20 years, $10,000 at 5% simple interest earns $10,000, but with monthly compounding it earns $17,160 -- a 72% bigger return.

Try the Compound Interest Calculator →

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Understanding Simple Interest

Simple interest is the most straightforward method of calculating interest. It is computed only on the original principal amount throughout the entire duration of the loan or investment. Unlike compound interest, simple interest does not factor in previously accumulated interest.

Simple interest is commonly used for short-term loans, auto loans, personal loans, and some bonds. Because interest is not reinvested or compounded, the total interest grows linearly over time rather than exponentially.

The key difference between simple and compound interest becomes more pronounced over longer time periods. For a short-term loan of one or two years, the difference may be small. But over decades, compound interest can generate significantly more returns because each period's interest earns additional interest in subsequent periods.

For borrowers, simple interest is generally more favorable since you pay less total interest compared to compound interest. For investors and savers, compound interest is preferred because your returns grow faster. Understanding which type applies to your financial products helps you make better decisions.

Simple Interest Formula

I = P × r × t

Where:

I = Total interest earned or paid

P = Principal (initial amount)

r = Annual interest rate (as a decimal)

t = Time period in years

Example

$10,000 at 5% annual interest for 5 years:

  • • Interest = $10,000 x 0.05 x 5
  • • Interest = $2,500
  • • Total amount = $10,000 + $2,500 = $12,500
  • • Annual interest = $10,000 x 0.05 = $500/year
  • • Monthly interest = $500 / 12 = $41.67/month
  • • Daily interest = $500 / 365 = $1.37/day

Frequently Asked Questions

What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows linearly. Compound interest is calculated on the principal plus all previously accumulated interest, so it grows exponentially. For example, $10,000 at 5% simple interest earns $500/year every year. With compound interest, Year 1 earns $500, but Year 2 earns $525 (5% of $10,500), and so on.
When is simple interest used in real life?
Simple interest is commonly used for auto loans, short-term personal loans, some bonds, student loans during the grace period, and Treasury bills. It is also used in some promissory notes and certain types of mortgages for interest-only periods.
Is simple interest better for borrowers or investors?
Simple interest is generally better for borrowers because you pay less total interest over the life of the loan. For investors and savers, compound interest is preferred because your money grows faster as interest earns additional interest over time.
How does the time period affect simple interest?
With simple interest, doubling the time period exactly doubles the interest earned. The relationship is perfectly linear. For example, if you earn $500 in interest over 1 year, you will earn $2,500 over 5 years and $5,000 over 10 years, assuming the same principal and rate.
Can simple interest be calculated for periods shorter than a year?
Yes. For periods shorter than a year, you can express the time as a fraction. For example, 6 months = 0.5 years, 90 days = 90/365 years. The formula remains the same: I = P x r x t, where t is the fraction of the year.