How to Calculate Total Interest Paid on a Mortgage (Formula + Examples)

To calculate total interest paid on a mortgage, find your monthly payment with the amortization formula, multiply by the number of payments, then subtract the loan amount. On a $400,000 loan at 6.5% over 30 years, the payment is $2,528.27, total paid is $910,177, and total interest is $510,177. Model your own loan in our free mortgage calculator; the full math is below.

The total-interest formula

Three steps, one formula. First compute the monthly payment with the amortization formula: M = P × r(1+r)ⁿ ÷ ((1+r)ⁿ − 1), where P is the loan amount, r is the monthly rate (annual rate ÷ 12), and n is the number of payments. Then total paid = M × n. Then total interest = total paid − P. That is the entire mortgage total interest formula — everything else on this page is that formula wearing different numbers. Note what the formula assumes: a fixed rate, fixed payment, and no extra payments or fees. Adjustable-rate loans and extra payments change the total, which is why the worked scenarios below matter more than the raw algebra.

Worked example: $400,000 at 6.5% for 30 years

Run the numbers once, slowly. P = $400,000, annual rate 6.5%, so r = 0.065 ÷ 12 = 0.00541667, n = 360 payments. (1+r)ⁿ = 1.00541667³⁶⁰ ≈ 6.9923. Monthly payment M = 400,000 × 0.00541667 × 6.9923 ÷ 5.9923 = $2,528.27. Total paid = 2,528.27 × 360 = $910,177. Total interest = 910,177 − 400,000 = $510,177. Read that again: on a $400,000 loan, you pay $510,177 in interest — 127.5% of the amount you borrowed. You buy the house once and pay for it 1.28 times over in interest. This is not a trick or a fee; it is what three decades of compounding at 6.5% costs. The number shocks first-time buyers, which is exactly why running it before you sign matters.

Total interest on a 30-year vs 15-year mortgage

Term length is the biggest lever you control. Take the same $400,000 at 6.5% but over 15 years (n = 180): the payment rises to $3,483.87, total paid is $627,097, and total interest is $227,097. Compared with the 30-year’s $510,177, the 15-year saves $283,080 in interest and ends the loan 15 years sooner. The trade-off is cash flow: the payment is $955.60 higher every month, which not every budget absorbs. A middle path exists — take the 30-year for flexibility but pay it like a 15-year when cash allows; extra payments attack principal directly, and our guide to paying off a mortgage early shows exactly how much interest each extra dollar kills.

How your interest rate changes the total

One percentage point moves the lifetime total by roughly $95,000 on a $400,000 loan. At 5.5% for 30 years: payment $2,271.16, total interest $417,618. At 6.5%: $510,177 (our base case). At 7.5%: payment $2,796.97, total interest $606,909. The jump from 6.5% to 7.5% costs an extra $96,732 — nearly a hundred thousand dollars for one point of rate. Two implications. First, rate shopping is the highest-paid hour in home buying: three lender quotes can easily differ by 0.25%, worth ~$24,000 here. Second, buying down the rate with discount points can pay off fast — paying $8,000 to drop from 7.0% to 6.5% saves roughly $48,000 in interest, a 6-to-1 return if you keep the loan past the ~3-year breakeven. If you have not computed your base payment yet, start with calculating your monthly mortgage payment.

Down payment: less principal means less total interest

Every dollar of down payment is a dollar you never pay interest on — for 30 years. Borrow $320,000 instead of $400,000 (20% down) at 6.5%: payment $2,022.62, total interest $408,143, a saving of $102,034 versus the $400,000 loan. The 20% threshold carries a bonus: no private mortgage insurance, which saves another $150–$300 a month until you hit 20% equity anyway. Smaller down payments work in reverse — 10% down on a $400,000 price means a $360,000 loan and about $459,000 in total interest, plus PMI. The math favors the biggest down payment you can make while keeping a 3–6 month emergency fund intact; raiding your safety net to save interest is a bad trade.

How much interest will I pay over the life of my loan?

The question everyone actually asks is personal: my balance, my rate, my term. The recipe: take your current principal balance (not the original loan amount — use what you owe today), your annual rate ÷ 12, and your remaining payments, then run the three-step formula from the top. Example: $350,000 remaining at 6.0% with 300 payments left. r = 0.005, (1.005)³⁰⁰ ≈ 4.4677, M = 350,000 × 0.005 × 4.4677 ÷ 3.4677 = $2,254.58. Total paid = $676,374, total interest = $326,374. Doing this by hand once teaches the mechanics; doing it for every what-if scenario is what our free mortgage calculator is for — it shows the full amortization schedule so you can see exactly which years the interest piles up.

Why lenders collect most interest early

Total interest is just the sum of 360 monthly interest charges, and those charges are front-loaded. Each month’s interest equals current balance × (rate ÷ 12) — when the balance is highest, the interest slice is biggest. On the $400,000 example, month one’s $2,528.27 payment contains $2,166.67 of interest and only $361.60 of principal: 86% interest. By year 15 the split is roughly 50/50; in the final year it is almost all principal. This is why extra payments early in the loan are devastating to total interest — a dollar of extra principal in year one skips interest charges for 29 years — and why refinancing “restarts the clock” on the front-loaded schedule. Our monthly mortgage interest guide breaks down the payment-by-payment mechanics.

Total interest with extra payments: the $200/month example

The formula assumes you never pay extra — but extra payments are the fastest way to shrink total interest, because every extra dollar skips interest for all remaining years. On the $400,000, 6.5%, 30-year example, adding just $200 a month toward principal ends the loan in 293 months instead of 360 and drops total interest from $510,177 to about $399,383 — a saving of $110,794 for $58,600 of extra payments, roughly a 2-to-1 return. An extra $500 a month saves about $204,591 and finishes the loan in 19.4 years. The leverage is highest in the early years, when the balance — and therefore each month’s interest charge — is largest. One critical detail: tell your servicer in writing to apply extra funds to principal, not to future payments, or the money sits idle instead of killing interest. Our extra-payments guide models all four strategies with full scenarios.

Get your exact number in 30 seconds

The formula fits on an index card, but your loan deserves exact numbers: plug your balance, rate, and term into our free mortgage calculator and read the lifetime interest off the amortization schedule — then test the what-ifs (15-year term, extra $200 a month, a point lower rate) in seconds. Comparing listings across borders? Our square feet to square meters converter translates listing areas, and the ToolNest homepage lists every free tool. Related reading: calculating your monthly payment, monthly interest breakdown, and paying it off early.