A feature filled WordPress interest calculator

Interest Calculations

Interest is calculated in a number of different ways. The four used by this plugin are: fixed, simple, compound and amortization.

Note: there are other ways of calculating interest and repayments. If the way you want to do it isn’t listed get in contact.

Variables Used

Principle = The amount you want to borrow

Interest = The percentage divided by 100. for example: 2% = 2/100

Term = the number of periods to which the Interest Rate applies. For example: 5 years or 28 days

Fixed Interest

This is the simplest one of all. The formula is:

Total to Pay = Principle x (1 + Interest)

For example: a loan of $100 with an interest rate of 5% returns:

$100 x (1 + 0.05) = $100 x 1.05 = $105

It doesn’t matter how long the loan is taken out for, the interest is always the same.

Try It Yourself:

I want to borrow:
for this long:
Monthly repayment is at interest
Total you will Pay:

Simple Interest

This interest calculation takes into account the time over which the loan is taken. The formula is:

Total to Pay = Principle x (1 + Interest x Term)

For example: a loan of $100 with an annual interest rate of 5% for 4 years returns:

$100 x (1 + 0.05 x 4) = $100 x 1.20 = $120

Try it yourself:

I want to borrow:
for this long:
Monthly repayment is at interest
Total you will Pay:

Compound Interest

This interest calculation uses the previous total to calculate the new total over the term of the loan.

Total to Pay = Principle x (1 + Interest)Term

For example: a loan of $100 with an annual interest rate of 5% for 4 years returns:

$100 x (1 + 0.05)4 = $100 x 1.054 = $121.55

Try it yourself:

I want to borrow:
for this long:
Monthly repayment is at interest
Total you will Pay:


This is used to calculated repayments where the amount of interest reduces as the loan is repaid.

Payment = rate(Present Value) / 1 – (1 + rate)-number of periods

This is further complicated by the difference between the USA and how the rest of the world calculate the rate. More about amortization repayments

Try it yourself (this example uses the US formula):

I want to borrow:
for this long:
Monthly repayment is at interest
Total you will Pay:


To calculate the repayments on fixed, simple and compound interest all you have to do is divide the total to pay by the number of repayments.

So for example, a 4 year Loan repaid monthly would have 48 payments.

Amortization works differently. More about amortization repayments

  1. Claudio Conigliaro on 22 Nov 2022:

    Hello and great job with this calculator!
    We want to use it but we need to use this formula:

    It’s possible to do it?

    It would be great.


    Reply to Claudio Conigliaro

    • Graham on 23 Nov 2022:

      Hi Claudio,

      Everything is possible. But if you just want to use that formula you are better off using one of the simpler versions I’ve created. I will send you an email with some examples.

      Reply to Graham

      • Claudio Conigliaro on 23 Nov 2022:

        Thank’s a lot Graham for your answer and I look forward to your email to finish our project.

        Reply to Claudio Conigliaro

  2. Perry on 07 Feb 2022:

    I want to set a slider for interest rate for annual figure selection (eg 12%), but calculate accrual monthly (1%). The default seems to apply the interest selection per month. How do I change this?

    Reply to Perry

    • Graham on 07 Feb 2022:

      Hi Perry,

      If you want to calculate monthly you need to set the rate to 1% and the divider to 12 (so the plugin knows it’s a monthly rate not APR).

      Reply to Graham

  3. Ondřej Šraitr on 09 Jun 2020:

    Hi Graham,
    I need to do only simple task with your plugin. Only one slider with loan ammount (CZK currency). In ‘total’ I need ((loan ammount/1000)*33).
    1000CZK is one loan step, every 1000CZK is valued by 33CZK. How can I achieve it, is it possible? As could be seeh here: https://www.pujckalevneji.cz/. Thank for your reply. BTW great plugin, thank you 😉

    Reply to Ondřej Šraitr

    • Graham on 09 Jun 2020:

      Hi Ondřej,
      Try using simple interest with a primary rate of 3.3%. There should be an option to hide the term slider.

      Reply to Graham

  4. Osanda Cooray on 04 Jun 2020:

    Hi Graham,

    Hope you are well, I wonder if we can have amortisation function enabled as below example. If there is a extra cost happy to pay. Does the pro version have this ?



    Reply to Osanda Cooray

    • Graham on 04 Jun 2020:

      Do you want the graphs and repayment schedule? Will you need the Loan type dropdown and if so what difference to the repayments does this make?

      This is a whole different calculator to the standard plugin. It’s not available in any version.

      Reply to Graham

  5. Stuart on 03 Dec 2019:

    Hello, I really like how your plugin looks with the slider styling!

    I need to create a loan calculator which can:

    1. Have a toggle to calculate payment in advance or in arrears

    2. Have an option for a residual payment as a percentage of the loan amount

    Can this be done?

    Reply to Stuart

    • Graham on 03 Dec 2019:

      Hi Stuart,
      1. This could be added
      2. This can be done
      Both of these would need to be paid for (as are all other customisations). I’ve sent you an email with costings.

      Reply to Graham

  6. Hein on 28 Nov 2019:

    I have the same request as Alan, please Graham

    “can you confirm if a 0% option is possible? I’m the free version, but before I buy the pro, can you confirm? Currently, it defaults doesn’t display any results when setting to 0 and then defaults to 2 at next save.”

    Reply to Hein

  7. Alan on 15 Nov 2019:

    can you confirm if a 0% option is possible? I’m the free version, but before i buy the pro, can you confirm? Currently it defaults doesn’t display any results when set to 0 and then defaults to 2 at next save.

    Reply to Alan

  8. Nick Banduch on 18 Oct 2019:

    Hi, can we do interest free using the Loan Payment plugin?

    Reply to Nick Banduch

  9. Bhanu Pratap on 26 May 2019:

    Hi Graham,
    I see, It appears you have fixed compound interest issue on this page. Please tell me how can I fix it in my website. It still gives £NaN output.

    Reply to Bhanu Pratap

  10. Bhanu Pratap on 23 May 2019:

    Hi Graham,

    I see Compound Interest is not giving the output. It gives £NaN. In my website also it gives £NaN.
    Please resolve.


    Reply to Bhanu Pratap

    • Graham on 23 May 2019:

      I saw this earlier today. I’m looking for a fix and will upload once it’s done.

      Reply to Graham

  11. Daco on 19 Jan 2019:

    How do you use this to calculate monthly interest rates.

    e.g. 10% per month ?

    Reply to Daco

    • Graham on 19 Jan 2019:

      Hi Daco,
      Make sure the period is set to months and uncheck the ‘Force APR’ box.

      Reply to Graham

  12. Phil on 22 Nov 2018:

    If I compare the Simple interest against the Excel PMT function https://exceljet.net/formula/calculate-payment-for-a-loan I get different results. My test is £2000 over 10 years at 5.9% APR. I get £26.50 a month compared to Excel of £22.10. Also the APR checkbox doesnt seem to make any difference!

    Reply to Phil

  13. David on 28 Sep 2018:


    I found how to do with filling repayments by 12 instead of 1.

    Thanks for your help,

    Best regards


    Reply to David

  14. David on 28 Sep 2018:

    Hello Graham,

    Thanks for your answer.
    Yes we have put a point and not a comma but we don’t have this result.
    What we need to put as values in your plugin to have 3 rates : 5,9%, 7,9% and 9,9% and to have the right calculation method?

    You can see examples of what we want as formula here and tell us if your locan calculator can do that:

    Reply to David

  15. David Pochet on 24 Sep 2018:

    We have installed your plugin on our website : http://www.acfservices.ch
    But there’s a problem with the calculation system. Indeed, we use this formula :

    For examples :
    The loans are > 10’000 CHF on 12 MONTHS : we have currently CHF899.17 per months so CHF10 790 for 12 months.
    But we should have 868.15CHF per month so 10’417.80 for 12 months.

    For 24’000 on 48 months we currently have : CHF539.50 per month so 25’896 CHF for 48 months
    We normally should have 581.70CHF per month so 27’921,6 CHF for 48 months.

    Can you help me ?

    Best regards,


    Reply to David Pochet

    • Graham on 24 Sep 2018:

      Hallo David,
      I can look at this for you but I’ve just compared this calculator with my calculator and get the same results on both 9ti within a few cents). Are you using a period or a comma for decimals? The plugin expects 6.9% not 6,9%

      Reply to Graham

  16. Gabriel on 11 Sep 2018:


    I’m trying to configure it to be correct in Brazil, we use the: French System of Amortization (Price Table)
    Is it possible to configure this way?
    I did a test, and see, 20,000 in 24 months from 1433 a month with interest of 3%
    Similarly in an Excel configured in the Price Table, gave the following:

    If you want to download the Excel file, I uploaded it to the library of our site: http://cidademotors.com.br/tabela_price_clubedospoupadores/

    Thank you!

    Reply to Gabriel

    • Graham on 11 Sep 2018:

      Hi Gabriel,
      If you can give me the formula used to calculate the repayments I can investigate why there is a difference. The plugin uses the standard formula (see here) and was taken from the Wikipedia Page.

      Reply to Graham

  17. Uchenna Igwe on 30 May 2018:

    Hello, I just started using your plugin but seems I am not able to properly configure it. The lender takes aflat rate of 20% per month on the loan. For eg, a loan of 100,000 will pay 20,000 per month. Somehow I can’t seem to configure it correctly. Kindly assist. Thanks

    Reply to Uchenna Igwe

    • Graham on 30 May 2018:

      Hi Uchenna,
      Have you set the period to months? If not you need to change to an annual interest rate of 240%. Send me a link to your calculator page so I can take a look.

      Reply to Graham

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