Helping potential home buyers calculate their mortgage payments online is a great value added service. If you want to add the word "payment" to your online calculator you have a couple popular options. You can use the keyword "payment calculator," and it turns out that adding the word mortgage to this keyword can actually increase monthly impressions. The keyword "mortgage payment calculator" is not as popular in as many states as the keyword "payment calculator," so keep that in mind. The upside is that there are more people searching for the keyword "mortgage payment calculator" on a monthly basis. It also appears to be dramatically increasing in popularity and is more closely tied to the term "mortgage."
We used Google Trends and Google Adwords to obtain this data