How Mortgage Payments Work (Amortization Explained)

The amortization formula

Each payment covers the month's interest first, then chips away at the principal. Early on, most of the payment is interest; over time the balance falls and more goes to principal. The monthly amount is fixed, but its split shifts.

The formula for a fixed-rate mortgage payment is: M = P × [r(1+r)^n] / [(1+r)^n − 1], where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of payments (years × 12).

On a $300,000 loan at 7% annual for 30 years: r = 0.07/12 = 0.005833, n = 360. Plugging in: M = 300,000 × [0.005833 × (1.005833)^360] / [(1.005833)^360 − 1] = $1,995.91/month. Our mortgage calculator does this instantly for any loan amount, rate, and term.

In month 1, the interest is $300,000 × 0.005833 = $1,750. The remaining $245.91 goes to principal. Your new balance is $299,754.09. In month 2, interest is $299,754.09 × 0.005833 = $1,748.57, and $247.34 goes to principal. The principal portion grows slightly each month — a snowball effect.

Rate and term drive the total

Two levers change what you pay overall: the interest rate and the loan term. Even small changes in either can mean tens of thousands of dollars over the life of the loan.

• Interest rate — at 7% on $300,000/30yr, total interest over 30 years is $418,607. At 5%, it's $279,767. A 2% difference = $138,840 less interest. At 3%, it drops to $155,332 — less than half the 7% cost.
• Term — a 15-year loan at 7% costs $2,696/month (higher), but total interest is only $185,337. A 30-year at the same rate costs $1,996/month (lower) but $418,607 in interest — $233,270 more for the privilege of smaller monthly payments.

The amortization schedule — why equity is slow at first

After 5 years on the $300,000 loan at 7%/30yr, you've paid $119,755 in total payments — but only $15,311 has gone to principal. Your balance is $284,689. After 10 years, the balance is $263,922. After 20 years, $177,558. Only in the final years does the principal portion dominate.

This is why many homeowners feel like they're not building equity — because for the first decade, they largely aren't. The interest front-loading means that refinancing or selling early in the loan term means you've paid mostly interest, not principal.

Making extra principal payments can dramatically shorten the loan. An extra $200/month on the $300,000/7%/30yr loan pays it off in about 22 years instead of 30, saving roughly $120,000 in interest. The earlier you start, the bigger the impact — because each extra dollar of principal reduces interest on every future payment.

15-year vs 30-year — the real tradeoff

A 15-year mortgage has higher monthly payments but dramatically less total interest. On a $300,000 loan at 7%, the 15-year costs $2,696/month vs $1,996 for the 30-year — a $700/month difference. But total interest is $185,337 vs $418,607 — you save $233,270 over the life of the loan.

The question is whether you can invest that $700/month difference and earn more than the mortgage rate. Historically, the stock market has returned 7–10% annually over long periods. If your mortgage is at 7% and you can invest at 8%+, the 30-year plus investing may come out ahead. But that requires discipline — actually investing the difference, not spending it.

A middle path: take the 30-year for flexibility (lower required payment) and make extra principal payments when you can. You get the safety of a low minimum payment and the interest savings of a shorter term, on your schedule.

Refinancing — when it makes sense

Refinancing replaces your current mortgage with a new one at a different rate. The math: if you can drop your rate by 1% or more, and you'll stay in the home long enough to recoup the closing costs (typically 2–5% of the loan), refinancing usually saves money.

On a $300,000 loan, dropping from 7% to 5.5% reduces the payment from $1,996 to $1,703 — $293/month savings. If closing costs are $6,000, the break-even is about 20 months. If you'll stay longer than that, refinance pays off.

But refinancing resets the amortization clock. If you're 10 years into a 30-year loan and refinance to another 30-year, you're paying interest for 40 years total — which can eat the rate savings. A refinance calculator can show you the exact break-even and lifetime savings for your situation.

See it for your loan

Enter a price, down payment, rate, and term to see the monthly payment, total interest, and full amortization schedule. The mortgage calculator uses the same formula lenders use — no estimates, no fudge factors.