15 Year Fixed Calculator

What a fixed rate locks in

A fixed rate is a promise about time as much as about money. The lender sets one rate for the whole 180 month term, and the payment built from it is identical on payment 1 and payment 180. On a $300,000 loan at 6.60 percent, principal and interest sit at $2,629.84 every month for 15 years, whatever happens to rates in the meantime.

The lock happens in two stages. A short rate lock covers the weeks between offer and closing, usually 30 to 60 days, and protects the quote while the property is inspected and the paperwork moves. The loan itself then carries the fixed rate for the full term. The second stage is the one this calculator models.

One boundary is worth stating plainly. The fixed rate controls principal and interest only. Property tax, homeowners insurance and any association dues are collected on top and can rise at any time, so a monthly housing cost can grow even when the loan payment cannot. The figures here cover the loan, which is the part the rate controls.

The payment formula behind the calculator

Every fixed payment calculator runs the same amortization formula. It needs the loan amount, the annual rate and the number of payments, which for a 15 year loan is 180.

Walk through the $300,000 example at 6.60 percent. The monthly rate is the annual rate divided by 12: 0.066 / 12 = 0.0055. Interest on the full balance in month one is 300,000 x 0.0055 = 1,650. The rest of the work is a growth factor: 1.0055 raised to the 180th power equals 2.6839, and dividing that by one less than itself gives a multiplier of 1.59384. The payment is 1,650 x 1.59384 = 2,629.84.

Totals follow from the payment. 2,629.84 x 180 = 473,372 in payments, and 473,372 - 300,000 = 173,372 of that is interest.

What matters for a fixed loan is what the formula does not contain. There is no index, no reset date and no margin. The rate you enter is the rate for all 180 months, so the payment is fully determined the day the loan closes.

The payment at rates near your quote

Freddie Mac's weekly survey put the 15 year fixed average at 6.60 percent and the 30 year at 7.24 percent for the week ending October 29, 2026, as published in the Primary Mortgage Market Survey. Individual quotes scatter around that average with credit score, points, down payment and property type, so the useful question is what a quarter point is worth on your own balance.

On $300,000 each 0.25 percent step is about $41 a month: 2,629.84 - 2,588.65 = 41.19. Held for the term, that step is 41.19 x 180 = 7,414 of extra interest. Two lenders a quarter point apart are therefore $7,414 apart on the same house, which is why comparing loan estimates line by line beats comparing headline rates.

The rate table inside the calculator rebuilds those rows from whatever amount and rate you type, so the spread around your own quote stays one click away.

15 year fixed against a 5/1 ARM

A 5/1 adjustable rate mortgage holds its starting rate for five years, then adjusts once a year for the remaining ten. The adjusted rate is an index plus a margin written into the contract, limited by caps that are commonly 2 percent at the first change, 1 percent at later changes and 5 percent over the life of the loan. The Consumer Financial Protection Bureau walks through how those pieces combine.

The appeal of an ARM is a lower starting rate. Suppose a 5/1 ARM quotes 6.10 percent while the 15 year fixed sits at 6.60 percent. On $300,000 the ARM payment is 2,547.81 against 2,629.84, a difference of 2,629.84 - 2,547.81 = 82.03 a month. Kept for the full five year fixed period, that is 82.03 x 60 = 4,922 in the borrower's pocket.

Month 61 decides the bet. If the index has fallen, the ARM can reset below the fixed rate and the saving continues. If it resets to 7.60 percent, the payment becomes 2,798.11, which is 2,798.11 - 2,629.84 = 168.27 more than the fixed loan, every month, for ten years. The fixed loan has no month 61, and that certainty is what the half point premium buys.

An ARM tends to win for a borrower who sells or refinances inside five years, and it tends to lose for one who stays put through a rising rate cycle.

15 year fixed against a 30 year fixed

The same survey that puts the 15 year at 6.60 percent puts the 30 year at 7.24 percent, a spread of 7.24 - 6.60 = 0.64 points. Lenders price the shorter loan lower because the balance falls faster and the money is exposed to rate changes for half as long. Historical weekly data sits in the St. Louis Fed series if you want to see how that spread has moved.

The trade is visible in two numbers. The 30 year payment is 2,629.84 - 2,044.49 = 585.35 lower each month, which is real room in a budget. The interest line runs the other way: 436,018 - 173,372 = 262,646 more over the life of the loan, and the loan lasts twice as long.

Twelve months of that payment gap is 585.35 x 12 = 7,024. A borrower who takes the 30 year and invests the difference needs a return above the loan rate, after tax, for 30 years to come out ahead. That is a portfolio decision rather than an arithmetic one, and the calculator cannot make it.

How the payment splits over 15 years

Interest is charged on the balance that is still owed, so the split inside a level payment shifts every month. In month one of the $300,000 example the interest is 1,650 and the principal is 2,629.84 - 1,650 = 979.84. By payment 55, early in year 5, principal finally exceeds interest.

Across the first 12 payments the loan absorbs 19,438 of interest against 12,120 of principal, and 19,438 + 12,120 = 31,558 matches 2,629.84 x 12 = 31,558. By the final year the shape has reversed: roughly 1,100 of interest and 30,458 of principal.

Five years in, the balance is 230,572, so 300,000 - 230,572 = 69,428 of principal is gone. Ten years in, 134,087 remains. That curve is why extra money paid early in the term is worth more than the same money paid late: every dollar of principal removed also removes all the interest it would have earned for the remaining payments.

What can still change after the rate is locked

Four things move on a fixed loan even though the rate does not.

Escrow items move first. Property tax follows assessments, insurance follows premiums, and association dues follow the board. A payment that includes escrow can change every year without the note rate changing at all.

Refinancing replaces the loan rather than editing it. If rates fall far enough in year 3, the remaining 261,109 can be refinanced into a new fixed loan, and the new payment follows the new rate and the new term. Closing costs usually run 2 to 5 percent of the balance, and a refinance restarts amortization, so the early payments are interest heavy all over again.

Selling ends the loan at the payoff balance. After 10 years that balance is 134,087 on the original 300,000, and the sale proceeds clear it before any cash reaches the seller.

Mortgage insurance is the fourth item. It usually appears when the down payment is under 20 percent, and it drops off once the balance falls below the threshold in the loan contract or the borrower asks for removal under the rules for the loan type.

What this calculator leaves out

The payment shown is principal and interest on the loan. It does not include property tax, homeowners insurance, flood insurance, association dues or mortgage insurance, all of which add to a monthly housing cost.

Points and lender fees are not part of the payment either. A point costs 1 percent of the loan amount and buys a lower rate, which changes the payment in the table without changing the balance. Fees also drive the gap between the note rate and the APR printed on a loan estimate.

The default rate is a national weekly average published every Thursday, not a quote for a specific borrower. Credit score, down payment, occupancy, property type and lock length all move the rate a lender will offer.

Extra payments are not modeled here. The 15 year loan calculator handles extra payments and early payoff on any 180 month loan.

Treat the output as an estimate of loan cost, not as a loan offer or financial advice, and check the numbers against the loan estimate your lender is required to provide.