Why extra payments work
Each month, interest is charged on your remaining balance. Any amount above your required payment goes straight to principal, so every later month's interest is a little smaller and more of each payment goes to principal. The effect builds over time.
Worked example
A new $300,000 loan at 6.5% for 30 years costs $1,896.20 a month. Adding $200 a month pays it off in 23 years 1 month, which is 6 years 11 months sooner, and saves about $103,449 in interest.
Tip: tell your servicer the extra amount is for principal. Thinking of investing it instead? Try the pay off or invest calculator.