Creation of custom calculators is very easy with TotalSheets. As an example look how easily you can create and share your work online:
You will need a sheet with 4 rows and 2 columns. Fill principal amount (B1), terms (B2) and interest rate (B3). The fourth row will show the results of the calculation. Use function PMT and in cell B4 insert:
You can add anything more to make it more attractive. Don’t forget to make the cells B1 to B3 unprotected, otherwise others will not be able to edit the values.
If you want to share it, go to Publish in Main menu. You can add this calculator to your page with iframe element as demonstrated below.