How amortization actually works

A fixed-rate loan has the same total payment every month, but the split between principal and interest inside that payment shifts constantly. Early on, most of the payment is interest. Later, most of it is principal. Here's why.

Interest is charged on what you still owe

Each month's interest charge is the outstanding balance times the periodic rate. In month one of a 30-year mortgage, you owe almost the entire loan, so the interest portion is large and the principal portion — what's left of the fixed payment — is small.

The crossover point

As the balance shrinks, the interest charge shrinks with it, so more of each fixed payment goes to principal. On a 30-year loan at a typical rate, the payment doesn't become "mostly principal" until somewhere around year 15–20 — meaning for roughly the first half of the loan, you're paying down interest faster than the loan balance.

Why extra payments early matter more

An extra payment applied directly to principal early in the loan removes balance that would otherwise have generated interest charges for years. The same extra payment made in year 25 removes balance that was only going to generate a few more years of interest anyway. That's why extra principal payments are most powerful in the first third of a loan's life.

Refinancing resets the clock

Refinancing into a new 30-year term restarts amortization from the beginning — you're back to a mostly-interest payment split, even if your new rate is lower. That doesn't make refinancing a bad idea, but it's worth comparing total interest over the full remaining timeline, not just the new lower monthly payment.