Most fixed-rate loans -- mortgages, car loans, personal loans -- use the same repayment structure: the same payment amount every month, for a fixed number of months, until the balance hits zero. That fixed payment hides something that surprises a lot of people the first time they look at an amortization table: in the early months, almost none of your payment goes toward the actual amount you borrowed.
Why early payments are mostly interest
Interest is charged on whatever balance is still outstanding. Early in the loan, the balance is close to the full amount you borrowed, so the interest portion of that month's payment is large. As the balance shrinks, the interest portion shrinks with it, and a larger share of each fixed payment goes toward principal instead.
On a 30-year mortgage, it is common for the first several years of payments to be more than half interest. The split only crosses over to mostly-principal somewhere past the halfway mark of the loan term, not the halfway mark of the balance.
What this means for extra payments
Because interest is calculated on the outstanding balance, any extra payment you make toward principal reduces the balance that future interest gets calculated on -- for every remaining month of the loan, not just the next one. That is why even small, consistent extra payments early in a loan term can cut off a disproportionate amount of total interest and shorten the payoff date by more than the extra amount alone would suggest.
The effect is strongest early in the loan (more remaining months to benefit from a lower balance) and weakest near the end (the balance is already small, so there is less interest left to save).
Seeing your own numbers
The amortization calculator builds the full month-by-month schedule for any loan amount, rate, and term, so you can see exactly where the interest/principal split is for your situation right now, not just the average. If you want to see the effect of paying extra, the mortgage extra payment calculator runs the same schedule with an additional recurring or one-time payment applied, so you can compare the payoff date and total interest side by side.