This simple interest calculator calculates interest between any two dates. Per simple interest is "interest payable only on the principal". Interest is never earned or collected on previous interest.

Because this calculator is date sensitive, it is a suitable tool for calculating simple interest owed on any debt when the debtor has not made payments or from a point in time when the balance is known. More details below the calculator



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Simple Interest

Simple interest is the interest calculation method that is least beneficial to savers and the most beneficial to borrowers. But note, if payments on a debt are paid as frequently as the compounding and the payment covers the interest due, then even if the terms of the loan call for compounding, there will be no impact on the total amount paid because at no point will there be any unpaid interest.

When the terms of a debt call for a simple interest calculation, if a payment does not cover the interest due, the unpaid interest must be tracked separately from the unpaid principal balance (also known as the US Rule). We believe that our Time Value of Money Calculator is the only online financial calculator that gives users this option and creates a schedule that shows the unpaid interest balance.


102 Comments on “Simple Interest Calculator”

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  • This is exactly what I needed to calculate interest for an account that is past due. I work in the legal field and needed to find out what would be owed to date on a previous judgement.

    • Sonia Shields says:

      This is perfect – Exactly what I needed to calculate interest on my monthly invoices. I used it on my job. Thanks

  • Walter Hanssen says:

    This works if you are receiving one lump payment but it does not if you have multiple or more than one payment.

    Do you have a calculator for that?

    • Yes, I’m sure I do.

      But I’m not 100% sure I’m clear on what you mean. You use the word "payment." Is this a loan cash flow, and the balance is approaching 0? If so, then try this loan calculator.

      Or is this a savings or investment cash flow and the balance is growing? Then try this savings calculator.

      You can also try this time value of money calculator. This one is the most flexible. It works with both loans and investments. Scroll down the page for 25 tutorials.

  • Works great! Do you have a calculator for calculating simple interest when monies are borrowed at different times? (i.e. one loan but two advances of money…and yes, I know I could just calculate them separately but was wondering if you had a program for doing it in one step)

    • Yes.

      Please see this calculator.

      Basically what you’ll do is enter the two loans in the first two rows with their dates.

      Then in the 3rd row, set it for payment and the amount as "Unknown."

      Scroll down the page to the tutorials. If you want the payment to reflect just the interest amount, that can be done too by setting it as an interest only series.

      If you have any questions just ask.

  • Personal reasons. I invested some capital in equity so cash-flow is on the low side. Whenever I’m late on payements for whatever expenses, I componsate my crediteur in order to meet eachother halfway. This site is very helpfull for quick calculations.

  • For a loan, I need to calculate the change in 1) EMI if the repayment duration remains same or 2) early completion date of a loan repayment if EMI is to remain fixed , after paying off a portion of the loan principle. Can you please suggest a way/calculator for the same. Thanks

  • Do you have a simple interest calculator that uses a national average interest? I need to calculate interest earned on funds held for legal proceedings but the interest rate is not given, so looking for some national average to use.

    • That’s not so much a question about the ability of a calculator, but rather what research you need to do to come up with the rate. Once you have the rate, you can plug it into this calculator.

      Isn’t there any guidance on the interest rate? There is no just one national average. The rate for a 10-year treasury bond (currently @ about 2.5%) is a lot different than the rate for a 30-year fixed rate mortgage (perhaps around 4.5%). Those would both be national averages.

  • Nice work to offer, Karl.
    Do you have a calculator that can take a loan amortization schedule and allow for non regularly scheduled periodic extra principal payments by a mortgage holder on a shorter term carry back? I’d like to have a running amortization schedule that gets revised to track the subsequent P & I balances each time a payment is made for record keeping and 1099-int’s?
    I found a one time extra payment loan calculator but need multiple entries…
    Much appreciated!
    C L C

    • Thank you. I appreciate that.

      For what you need to do, please see the Ultimate Financial Calculator. Scroll down the page to the tutorial section and look at #25. This calculator will let users record payments as they are made on any date, for any amount. So it supports extra payments as well. The balance can be calculated as of any date too – that is, between payment due dates. (I would suggest also reading tutorial #1 for a quick overview.)

    • Hello, is back up and running. I’m sorry for the problems you experienced. An automatically applied update went wrong and it eventually brought down the entier site on Tuesday. Unfortunately, I could not figure out the problem and get it fixed until late Tuesday (eastern time USA). Thank you for reporting the problem.

  • This calculator (as many other online calculators) does not cover the issue with a leap-year
    For period from 01/01/2016 until 01/01/2017 I should put days in year to 366 to obtain the correct result.
    But for period from 01/01/2016 until 01/01/2018 obviously neither 366 nor 365 do not provide the correct result
    If you would like completely support for 366-day-year, then I could suggest
    – Remove drop-down list “days in year”: a common year has 365 days and a leap-year has 366 days, for what you provide value 360, 364, etc?
    – You could not summarize days for the whole period:
    o if period contents whole year you take in consideration “annual interest rate” identical for a common year and for a leap-year
    o if period is started or is finished in the middle of year then you convert this part of year to days and calculate interest earned based on number of days in this year (is it a leap year or a common year)
    o you summarize the interest earned for all parts of period
    – You could easily distinguish a leap–year (see;
    function leapYear(year)
    return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);

    • Thank you for your comment. Are you in the US?

      First, in the US, to my knowledge, there are no laws governing the interest calculation. It’s a matter of convention. In the US, we do have a 360 day year. It may even be more commonly used than a 365/366 day year.

      It is true, as you have stated, that this calculator does not support a 366 day year. How would you handle the divisor if someone want to calculate interest between Dec. 1, 2015 and Feb. 1, 2016? 31 days in either year, and none of the days span Feb. 29th, though one-half of the days are in a leap year.

      My calculations are not wrong. They may just not support a convention that you need/want. That does not mean that I can’t add support for it however.

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