Free Simple Interest Calculator
Solve simple interest for interest, principal, rate or time, with day-count choices, a compound comparison, a schedule and CSV export in any currency.
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How to calculate simple interest and solve for principal, rate or time
Solve simple interest (I = P × R × T) for interest, principal, rate or time, with a choice of day-count rules for dates.
- Choose what to solve for: interest, principal, annual rate or time.
- Pick a currency and enter the known values. Rates are annual percentages.
- Give time in years, months, days, or as start and end dates. For days and dates, choose Actual/365, Actual/360 or 30/360.
- Read the result, the total (principal + interest), and the comparison line showing what compound interest would give on the same terms.
- Switch the schedule between yearly and monthly rows, then copy the summary or download the schedule as CSV.
When simple interest is the right model
Simple interest applies whenever interest is calculated on the original amount only and is not added back to the balance to earn more interest. Short-term business loans, many car title and personal loans quoted on a flat basis, treasury bills and certificates held for a fixed term, interest on late invoices, and back-of-the-envelope checks of a quoted interest charge all use it.
Solving for the other variables is just as common. A lender quotes a total interest charge and you want the annual rate it implies. You know how much interest you need to earn and want the deposit size. Or you want to know how long a deposit must sit to earn a given amount. The Solve for menu handles all four cases with the same formula rearranged.
Worked example: 181 days at 6% with three day counts
Suppose 10,000 is borrowed at 6% a year from 1 January to 1 July 2026. Choose Interest, enter 10,000 and 6%, and give the time as start and end dates. Actual/365 counts 181 actual days over a 365-day year: 10,000 × 0.06 × 181 ÷ 365 = 297.53. Actual/360 counts the same 181 days over a 360-day year: 301.67. The 30/360 bond basis treats every month as 30 days, giving 180 days and exactly 300.00. Same loan, a difference of more than 4 between the highest and lowest result.
Solving the other way: if a lender charges 240 on 4,000 for six months, set Solve for to Annual rate, enter 240, 4,000 and 6 months, and the implied rate is 12% a year. If you want to earn 1,000 on 8,000 at 5%, Solve for Time gives 2.5 years.
For the comparison line, take 10,000 at 5% for 3 years. Simple interest is 1,500, a total of 11,500. Compounded monthly the total would be 11,614.72, and compounded yearly 11,576.25, so compounding adds 76 to 115 over three years. Over ten years the gap widens sharply: monthly compounding reaches 16,470.09 against 15,000 with simple interest.
How the calculator works
Every result comes from I = P × r × t, with r as a decimal and t in years. Principal = I ÷ (r × t), rate = I ÷ (P × t) and time = I ÷ (P × r). Months are converted as months ÷ 12. A count of days is divided by 365 for Actual/365 and by 360 for Actual/360 or 30/360. With dates, Actual conventions use the real number of days between them, while 30/360 uses the bond-basis rule: a start day of 31 becomes 30, and an end day of 31 becomes 30 when the start day is 30 or 31.
The schedule spreads interest evenly across each year or month, with a shorter final row when the term is not a whole number of periods. The compound column uses P × (1 + r/n)^(n × t) with the compounding frequency you pick, or P × e^(r × t) for continuous compounding.
Common mistakes to avoid
Entering a monthly rate as if it were annual is the classic error: 1% a month is 12% a year in simple interest terms. Another is mixing time units, such as using 18 for 18 months while the time unit is set to years. The line under the time input shows the exact year fraction being used, so check it.
Finally, a flat-rate loan quote is not the same as the annual percentage rate on a reducing balance. Because you repay principal over time, the true cost of a flat-rate instalment loan is higher than the quoted simple rate suggests; use the Loan Calculator to compare.
Useful tips and limitations
- Your loan or deposit agreement says which day-count convention applies. Actual/360 gives slightly more interest than Actual/365 for the same dates.
- If interest is added to the balance and then earns interest, you need the Compound Interest Calculator instead.
- Use the Download CSV button to keep the schedule alongside a contract or invoice.
- To check a quoted interest charge, choose Annual rate as the unknown and enter the charge, principal and term.
Common questions
What is the simple interest formula?
I = P × r × t, where P is the principal, r the annual rate as a decimal and t the time in years. 10,000 at 5% for 3 years gives 10,000 × 0.05 × 3 = 1,500.
How is this different from the Compound Interest Calculator?
Simple interest never earns interest on earlier interest. This page solves for any of the four variables and handles day counts; the Compound Interest Calculator projects growth with contributions and compounding.
Which day count should I use?
Use the one in your contract. Actual/365 is common for many personal loans and deposits, Actual/360 for money-market and many commercial loans, and 30/360 for many bonds.
What is the difference between Actual/360 and Actual/365?
Both count the real days in the period. Actual/360 divides by 360, so the same dates produce about 1.4% more interest than Actual/365, which divides by 365.
Can the Simple Interest Calculator handle a negative rate?
No. Rates, principal and time must be zero or more, and the value you solve for must be well defined, so a zero rate cannot be used when solving for principal or time.
Does simple interest ever beat compound interest?
For a borrower, yes: on the same rate and term simple interest costs less. For a saver, compound interest always earns at least as much once the term is longer than one compounding period.
Everything you enter is calculated in your browser. Nothing is uploaded and no account is needed. Copy or download your results before leaving the page.