30 Year Mortgage Amortization Schedule Calculator

The 86.3% Problem: Where Year-One Payments Actually Go

Run a $350,000 loan at 6.75% for 30 years through the standard amortization formula — M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1] — and the fixed payment comes out to $2,270.09 a month. In the first twelve payments, $23,511.00 of that money goes to interest and just $3,730.12 reduces the loan balance. That's 86.3% interest, 13.7% principal, on the exact payment you'll be making from your very first month in the house. Zoom in further and it's even more lopsided: payment number one alone is $1,968.75 interest against only $301.34 principal — 86.7% interest on that single check.

This isn't a quirk of this particular rate or balance; it's how every fixed-rate, fully-amortizing loan works by design. Your lender doesn't split the payment evenly across interest and principal — each month's interest is simply your current balance times the monthly rate (annual rate ÷ 12), and whatever's left of the fixed payment becomes principal. Since the balance is at its highest point on day one, the interest charge is also at its highest point on day one. Every dollar of principal you pay shrinks the balance slightly, which shrinks next month's interest charge slightly, which frees up slightly more of the fixed payment for principal — a slow-motion snowball that takes decades to fully unwind on a 30-year term.

How a 30-Year Schedule Front-Loads Interest — and Why

"Front-loading" is the standard term for this pattern: a disproportionate share of total interest is concentrated in the early years of a long amortization schedule. On the $350,000/6.75% example above, the interest share of the payment doesn't drop below 80% until roughly year six, and it takes until month 238 — 19.8 years into the loan — before the principal portion of a single payment finally exceeds the interest portion for the first time. Put another way: for the first two-thirds of a 30-year mortgage, more than half of every payment is still interest.

The mechanism is pure math, not a lender trick: a longer term means more total payments spread over which to recover the same principal, so each individual payment can be smaller — but that smaller payment also means the balance declines more slowly in the early years, which keeps the interest charge elevated for longer. A 15-year loan collapses this same curve into half the time, which is exactly why its payment is larger but its total interest is dramatically smaller, as the comparison below shows. This front-loading is also the entire reason extra principal payments made in year one or two save so much more interest than the same dollar amount paid in year twenty-five — a dollar of extra principal early avoids interest charges across nearly three more decades of remaining balance, not just a few years.

30-Year vs. 15-Year: The Real Tradeoff on the Same $350,000 Loan

To compare terms fairly, both scenarios below use the same $350,000 loan amount, but each uses the rate a lender would realistically quote for that term: 6.75% for 30 years, and 6.25% for 15 years, since lenders typically price shorter, lower-risk terms a bit below longer ones.

The 15-year loan costs $730.89 more per month, but it saves $277,057.20 in interest over the life of the loan — largely because it both charges a lower rate and forces a much faster principal payoff, so the front-loading problem described above resolves in half the time. The right choice comes down to whether your budget can absorb that extra $730.89 a month: if it can, the 15-year loan is a straightforwardly cheaper way to borrow the same $350,000; if it can't, the 30-year term is what makes the purchase possible at all. For the full month-by-month picture of the shorter-term option, see the full 15-year breakdown, built on these same matched assumptions.

How Extra Payments Change a 30-Year Schedule Specifically

Because 30-year interest is so front-loaded, extra principal payments made early have an outsized effect on this loan type in particular. Staying with the same $350,000 loan at 6.75%: adding just $200 a month in extra principal payments shortens the term from 30 years to 23.8 years — 75 months, or over six years, saved — and cuts total interest from $467,233.60 down to $352,254.93, a savings of $114,978.67. Raise the extra payment to $300 a month and the loan pays off in 21.6 years (101 months saved), with total interest falling to $315,534.73 — a savings of $151,698.87 for roughly the cost of a car payment added on top of the original mortgage payment.

The reason extra payments punch above their weight on a 30-year schedule specifically is the same front-loading dynamic from the section above: because so much of the original schedule's interest is concentrated in the first 10-15 years, any extra dollar applied to principal during that window eliminates interest charges across nearly the entire remaining term, not just a few months of it. The same $200/month applied to a 15-year loan still helps, but there's simply less front-loaded interest left to eliminate. You can model your own extra-payment amount, a one-time lump sum, or biweekly payments against your real numbers on our extra payments calculator, or see how a lump sum specifically lowers your payment without shortening your term on the refinance amortization calculator if rates have moved since you took out your loan.

Methodology: How Every Number on This Page Was Calculated

Every figure above comes directly from the standard fixed-rate amortization formula, M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1], where P is the $350,000 principal, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments (360 for the 30-year loan, 180 for the 15-year loan). From that fixed payment, we generated a full month-by-month schedule — not an approximation — by calculating each month's interest as the current balance times the monthly rate, applying the remainder of the payment to principal, and repeating for every month of the term. That's the same method used to produce the amortization table on a lender's closing disclosure, so the totals here should match what you'd get from your own loan documents for the same inputs.

The 6.75% and 6.25% rates used in the comparison table are illustrative assumptions chosen to reflect the realistic gap between 30-year and 15-year pricing, not a live rate quote. For current national average rates, Freddie Mac publishes a weekly Primary Mortgage Market Survey covering both 30-year and 15-year fixed-rate conventional loans — check it before plugging your own rate into the calculator above, since actual market rates shift week to week and vary by lender, credit profile, and down payment. For the complete formula-by-formula breakdown behind this calculator, see our mortgage amortization calculator home page.