Enter principal amount $
Enter rate (as 6.5 for 6.5%) %
Enter term in years
Enter additional fixed monthly payment (optional) $
Enter payment beginning month year as 1/2015 (optional)
Enter additional variable payments
in the form of payment number and amount.
Use only numbers 0-9,.,- characters (optional) 
Example: 1 1 250 2 3000 5 20000

to specify:
payment # 1 of $250,
payment # 2 of $3000 etc

Example: 2 1-12 250 13-24 300
to specify:

Payments #1 through # 12 of $250 and

payments # 13 through # 24 of $300 etc
Check here to include a complete schedule of payments