How this is calculated
The simulator runs your loan month by month: interest accrues on the balance, your payment lands, and any extras — the monthly amount, the once-a-year bonus, the lump sums in the years you chose — go straight to principal.
new balance = balance × (1 + rate/12) − payment − extras due that month
Why extras punch above their weight
A dollar of principal paid today stops accruing interest for every remaining month of the loan. Early extras are therefore worth far more than late ones — a $10,000 lump sum in year 2 typically saves several times what the same sum saves in year 20.
Extra payments or refinance?
Extra payments shorten the loan at your current rate; refinancing replaces the rate itself. If rates have dropped meaningfully since you closed, run the refinance calculator — and note that the two combine well: refinance to a lower rate, keep paying your old payment, and the loan collapses years early.
Notes
Assumes a fixed rate and no prepayment penalty (rare on US mortgages, but check). Escrowed taxes and insurance continue regardless — this models the loan itself.