How Mortgage Amortization Works: The Math Behind Every Payment

Amortization in one sentence

Amortization is the scheduled, monthly conversion of debt into ownership. Every fixed-rate mortgage payment is split — by formula — between interest charged on the current balance and principal that reduces that balance. Over 360 months on a 30-year loan, the split shifts from mostly-interest to mostly-principal. That's the whole game.

The formula

The standard amortization formula computes a monthly payment that, paid every month for the loan's term, will reduce the balance to exactly zero at the end. For a loan of L, monthly rate r, and n total payments:

Payment = L × (r × (1+r)ⁿ) / ((1+r)ⁿ − 1)

On a $320,000 loan at 6.75% over 30 years: r = 0.0675/12 = 0.005625, n = 360. Plugging in gives a monthly payment of $2,075. That's the same payment every month for 30 years, unless you refinance.

Why early payments are mostly interest

Each month the lender charges interest on the current balance. In month one of the $320,000 loan example, the interest is 320,000 × 0.005625 = $1,800. Your full payment is $2,075. So principal that month is 2,075 − 1,800 = $275, and your new balance is $319,725.

Month two repeats the math: 319,725 × 0.005625 = $1,798.83 of interest, leaving $276.17 for principal. The principal share grows imperceptibly each month, but it's growing. By month 60 you're paying about $400 of principal. By month 240, principal exceeds interest in a single payment. By month 360, the payment is almost entirely principal.

This is not a trick or a 'front-load.' It's the unavoidable mechanical consequence of charging interest on a balance — and a starting balance is the largest balance you'll ever have on the loan.

The crossover month

There's a specific month on every amortization schedule where principal finally exceeds interest in a single payment. On a 30-year loan at 6.75%, the crossover is around month 240 (year 20). At lower rates the crossover comes earlier; at higher rates it comes later. On a 15-year loan, the crossover comes much earlier — around year 7.

Why does this matter? Because the years before the crossover are years of low equity build. If you're planning to move within 5 years and you stay on schedule, you'll have built very little ownership. That's a real consideration in the rent-vs-buy decision.

How extra payments compress the schedule

An extra $1 of principal paid in month one eliminates not just $1 of debt — it eliminates 359 future months of interest charged on that $1. The earlier the extra payment, the more interest it skips. This is why prepayment in the first decade of a mortgage is far more powerful than prepayment in the last decade.

On the same $320,000 loan at 6.75%, sending an extra $200/month of principal shortens the loan from 360 months to roughly 300 — five years saved — and saves about $90,000 of total interest. Sending an extra $500/month cuts the loan to about 220 months, saving nearly $170,000.

The mechanic is the same as the rule of thumb 'pay one extra full payment per year.' That single payment, paid at tax-refund season for example, knocks roughly 4 years off a 30-year loan because each successive month's interest is charged on a smaller balance.

Biweekly schedules — same trick, different costume

A biweekly mortgage splits your monthly payment in half and pays it every two weeks. Because there are 26 biweekly periods per year, you end up making 13 monthly payments instead of 12. That extra payment is what compresses the schedule.

Some servicers charge for biweekly enrollment ($200–$400 setup). The math doesn't require any servicer involvement — you can replicate it for free by sending 1/12 of one extra payment every month with instructions to apply to principal. Same result, no fee.

Tools to visualize amortization

Our mortgage payment calculator includes a full amortization schedule, a year-by-year bar chart showing principal vs. interest, and a timeline slider that aggregates totals for any window of payments. The slider answers questions like 'how much of my payment in years 5–15 is actually interest?' — useful for refinance decisions and lifecycle planning.