If the principal portion, $381.27 (green cell) of payment 6 is prepaid along with payment 5 on March 15th 2007 the interest savings is the interest portion of payment 6 which is $1,739.07. In other words, by prepaying payment #6 principal one month in advance you save payments #6 interest portion.

This is easily verified by subtracting the SPREADSHEET interest of the second amortization schedule, $334,361.09 (that shows the prepayment made) from the SPREADSHEET interest of $336,101.16 from the regular amortization schedule.

The result,

1.. Interest savings of $1,739.07

2.. Reduced amortization period by one month.

3.. Cut out payment line #6 in the initial amortization schedule and the resulting amortization schedule is MATHEMATICALLY accurate and intact. The dates wont be correct and you will need to subtract one from all the payment numbers.

A little extra effort but you will not have to recalculate or print the amortization schedule or pay someone to do it for you.