Maybe this will help.
30 Year Mortgage Amount: $500,000
Interest Rate: 4.00%
12 Monthly Payments: $2,410.91 ($28,930.92 yearly)
Total of Payments: $867,927.60
Total Interest Paid: $367,927.60
Now change that to bi-weekly.
26 bi-weekly payments of $1,112.73 ($28,930.98 yearly)
Total of Payments: $851,238.45
Total Interest Paid: $351,238.45
Total Savings: $16,889.17
Total number of payments: 765 (~ 29.42 years so you pay off the mortgage about 6 months early with the same annual payment)
Now change the bi-weekly payment to half your monthly payment.
26 bi-weekly payments of $1205.46 ($31,341.96 annually, comes out to 13 monthly payments a year instead of 12)
Total of Payments: $798,014.52
Total Interest Paid: $298,014.52
Savings over monthly: $69,913.08
Total Number of Payments: 662 (~ 25.46 years so you payoff the mortgage about 4.5 years early, but you are paying more each year)