This is how to use the Texas Instrument BA II Plus to calculate the monthly payment for a 30 year fixed rate mortgage with equal monthly payment and total loan amount is $100,000 at annual rate of 5%. https://datafireball.com/2018/10/28/f... If you are interested in the why, feel free to visit my blog and I explained the monthly payment calculation in three different ways. Last but not least, pmt = P * r / (1-1/(1+r)^n)