How to Calculate Mortgage Interest Per Month: The Formula, First-Payment Breakdown & Amortization

To calculate mortgage interest per month, multiply your current loan balance by your annual rate, then divide by 12. The formula — monthly interest = balance × (rate ÷ 12) — shows exactly how much of each payment goes to the bank. Run your own numbers in our free mortgage calculator; worked examples and the amortization logic are below.

The Monthly Mortgage Interest Formula

One multiplication and one division: monthly interest = current balance × (annual interest rate ÷ 12). Take a $400,000 loan at 6.5%: first convert the annual rate to a monthly rate (0.065 ÷ 12 = 0.0054167), then multiply by the balance: $400,000 × 0.0054167 = $2,166.67 of interest in month one. That is it — no amortization table required for any single month. The subtle part is “current balance”: the formula always uses what you owe right now, not the original loan amount. After your first payment shaves $361.61 off the principal, month two's interest is $399,638.39 × 0.0054167 = $2,164.71 — two dollars less, because you owe two dollars' worth less of balance. This is the entire engine of amortization: same payment every month, but the interest slice shrinks as the balance falls and the principal slice grows to fill the gap.

How Much of My Mortgage Payment Goes to Interest?

The interest formula tells you the bank's cut; the rest of your payment attacks the principal. On that $400,000 loan at 6.5% for 30 years, the monthly payment is $2,528.27. Month one's split: $2,166.67 interest + $361.61 principal = $2,528.28 (a penny of rounding). So 86% of your first payment is interest — the number that shocks every new homeowner. By month 60 the split is $2,030.93 interest and $497.34 principal; by month 180 (year 15) it is $1,577.27 interest and $951.01 principal; the final payment is $13.62 interest and $2,514.65 principal. Same $2,528.27 every month, completely different composition. Two practical uses: first, your mortgage interest deduction at tax time is the sum of twelve of these monthly figures — your lender's year-end statement does this math for you. Second, comparing loans: a 7% rate on $380,000 costs $2,108.33 in first-month interest versus $2,166.67 on the 6.5% $400,000 loan — the rate and the balance fight each other, and only the formula shows the winner. See how the full monthly payment is calculated for the payment side of this split.

Why Early Payments Are Almost All Interest

New borrowers always ask whether the bank “front-loads” interest as some kind of trick. It does not — the math just works out that way. Interest each month equals the rate times the entire remaining balance, and the balance is biggest at the start. Nothing is front-loaded; you are simply paying rent on a large debt that slowly gets smaller. The flip side is where the real money hides: because early balances are huge, early extra payments destroy more future interest than late ones. An extra $200 against principal in month 6 of our example loan saves roughly $1,050 in lifetime interest; the same $200 in year 20 saves about $330. That asymmetry is why every pay-off-early strategy hammers the first years. Over the full 30 years, our example borrower pays $510,178 in interest — more than the $400,000 house itself. Seeing that number is usually the moment people get serious about extra payments, refinancing, or a shorter term.

Monthly Interest vs APR: Do Not Confuse Them

Your loan documents show two rates: the note rate (6.5% in our example) and the APR (perhaps 6.72%). The monthly-interest formula uses the note rate — the APR folds in closing costs and fees to show the true yearly cost, but the bank accrues your monthly interest off the note rate. Mixing them up overstates each month's interest slightly. One more nuance: most US mortgages accrue interest monthly as shown here, but the fine print of some loans accrues daily (balance × rate ÷ 365 × days in month). Daily accrual makes February's interest a touch lower and 31-day months a touch higher — a difference of a few dollars a month, worth knowing but not worth losing sleep over. Adjustable-rate mortgages add a third wrinkle: when the rate resets, rerun the formula with the new rate and the then-current balance. The formula never changes; only its inputs do.

Three Levers That Shrink the Interest Number

Every dollar of monthly interest comes from balance × rate, so you only have three levers. 1. Shrink the balance faster — extra principal payments, even $100 a month, compound against every future month's interest. 2. Shrink the rate — refinancing from 6.5% to 5.75% on our example loan cuts first-month interest from $2,166.67 to $1,916.67, saving $250 every single month. 3. Shrink the time — a 15-year term at the same rate raises the payment but slashes total interest from $510,178 to about $199,000, because the balance collapses years sooner. The honest way to compare any two options is total interest paid, not the monthly payment — a lower payment stretched over more years almost always costs more. Run both scenarios side by side before you sign anything; five minutes with a calculator beats five years of regret. A fourth tactic deserves mention: biweekly payments. Paying half your mortgage every two weeks makes 26 half-payments a year \u2014 the equivalent of 13 full monthly payments instead of 12. That one extra payment a year goes entirely to principal, quietly shortening a 30-year loan by roughly four years and saving on the order of $60,000 in interest on our example loan, with no refinancing paperwork and no lifestyle change beyond the payment rhythm.

What Happens to Monthly Interest When You Refinance or Pay Extra

Because monthly interest is just balance × rate ÷ 12, any move that shrinks the balance or the rate shrinks next month's interest immediately. Pay an extra $5,000 toward principal on our $400,000 example loan and the balance drops to about $394,638 — so next month's interest falls from $2,164.71 to $2,137.62, saving $27 that month and a little more every month after, because the balance stays permanently lower. Refinancing works the same lever from the other side: dropping from 6.5% to 5.75% on the same $400,000 balance cuts month-one interest from $2,166.67 to $1,916.67 — $250 less, every month, before you have paid a cent of extra principal. This is also how to sanity-check a refinance offer: multiply the rate difference by your balance and divide by 12. A 0.75-point drop on $400,000 saves 0.0075 × 400,000 ÷ 12 = $250/month. If the closing costs are $5,000, the breakeven is 20 months — stay longer than that and the refinance wins. The formula turns every sales pitch into arithmetic you can verify.

Model Your Own Loan in Seconds

The formula fits on an index card, but real decisions need the full schedule — all 360 months, the running balance, the crossover point where principal finally exceeds interest (month 233 in our example, early in year 20). Our free mortgage calculator builds it instantly from your balance, rate, and term. It is one of 130+ free online tools at ToolNest. House-hunting across borders? Our square feet to square meters converter translates listing areas. Related guides: calculating your monthly mortgage payment, paying off a mortgage early, and how percentages work.