Your mortgage interest rate determines how much you pay your lender each month for the privilege of borrowing money to buy a home. It’s expressed as a yearly percentage, but it’s applied to your remaining loan balance every month, which means the actual dollar amount of interest you pay changes over time. Understanding this mechanic helps you see where your money goes each month, why some borrowers pay tens of thousands more than others over the life of a loan, and what you can do about it.
How Interest Is Calculated Each Month
Your lender takes your annual interest rate, divides it by 12, and multiplies that monthly rate by your remaining loan balance. That’s the interest portion of your payment for that month. If you have a 6% annual rate, your monthly rate is 0.5%. On a $300,000 balance, that means $1,500 in interest for that month alone.
Your total monthly payment stays the same for a fixed-rate loan, but the split between interest and principal (the actual loan amount you’re paying down) shifts over time. In the early years, most of your payment covers interest because the outstanding balance is still large. As the balance shrinks, the interest charge drops and more of each payment chips away at the principal. This structure is called amortization.
On a 30-year fixed mortgage, the shift is dramatic. In the first few years, you might see 70% or more of your payment going to interest. By the final years, the ratio flips almost entirely toward principal. This is why making extra principal payments early in a loan’s life can save you a significant amount of interest over time: you’re reducing the balance that interest is calculated on for every remaining month.
What Determines Your Rate
Two broad categories shape the interest rate you’re offered: factors in the broader economy and factors specific to you as a borrower.
On the economic side, mortgage rates are influenced by yields on government bonds, decisions by the Federal Reserve, and investor demand for mortgage-backed securities. The Fed doesn’t set mortgage rates directly, but its benchmark interest rate creates a floor that ripples through the lending market. When the Fed raises or holds rates, mortgage rates tend to follow the same direction, though not in lockstep.
On the personal side, your credit score is one of the biggest levers. According to Curinos data from March 2026, a borrower with a 620 FICO score would see an average 30-year fixed rate around 7.14%, while a borrower with a 780 score or higher could get roughly 6.25% on the same type of loan. That gap of nearly a full percentage point translates to tens of thousands of dollars over 30 years. On a $350,000 mortgage, the difference between 6.25% and 7.14% works out to roughly $230 more per month, or about $83,000 in extra interest over the full loan term.
Beyond credit score, lenders also weigh your down payment size, your debt-to-income ratio (how much of your monthly income goes to debt payments), the loan amount, and the type of property you’re buying. A larger down payment generally earns a lower rate because it reduces the lender’s risk.
Fixed Rates vs. Adjustable Rates
A fixed-rate mortgage locks in the same interest rate for the entire loan term, whether that’s 15 or 30 years. Your monthly principal and interest payment never changes. This is the most popular choice because it’s predictable.
An adjustable-rate mortgage (ARM) starts with a lower introductory rate for a set period, often 5 or 7 years, then resets periodically based on market conditions. ARMs are described with two numbers, like “5/6,” meaning the rate is fixed for the first 5 years and then adjusts every 6 months after that.
When the introductory period ends, your new rate is calculated by adding two numbers together: an index and a margin. The index is a benchmark interest rate that fluctuates with the broader market. The margin is a fixed number of percentage points set by your lender at closing, and it never changes. So if the index sits at 4% and your margin is 2.5%, your adjusted rate would be 6.5%. ARMs include rate caps that limit how much the rate can increase at each adjustment and over the life of the loan, which provides some protection against dramatic jumps.
The tradeoff is straightforward. ARMs offer lower initial payments, which can be attractive if you plan to sell or refinance before the adjustable period kicks in. Fixed-rate loans cost more upfront but eliminate the risk of rising payments down the road.
Interest Rate vs. APR
When you shop for a mortgage, you’ll see two percentages: the interest rate and the APR (annual percentage rate). The interest rate is purely the cost of borrowing the money. The APR folds in additional costs like discount points, mortgage broker fees, and certain closing charges, giving you a fuller picture of what the loan actually costs per year.
The APR will almost always be higher than the interest rate. If two lenders offer the same interest rate but one has a noticeably higher APR, that lender is charging more in fees. Comparing APRs side by side is one of the fastest ways to evaluate competing loan offers, because it levels the playing field between a loan with a low rate but high fees and one with a slightly higher rate but minimal fees.
How Loan Term Affects Interest Cost
Shorter loan terms come with lower interest rates. A 15-year fixed mortgage typically carries a rate about 0.5 to 1 percentage point below a 30-year fixed loan. Using the March 2026 Curinos data, a borrower with a 700 credit score would see roughly 6.63% on a 30-year term compared to 5.71% on a 15-year term.
The savings compound in two ways. You’re paying a lower rate, and you’re paying it for half as many years. On a $350,000 loan, a 30-year term at 6.63% results in roughly $455,000 in total interest. The same loan at 5.71% over 15 years costs about $163,000 in interest. The tradeoff is a significantly higher monthly payment, since you’re compressing the same principal into half the time.
Ways to Lower Your Interest Cost
Your interest rate isn’t entirely out of your hands. Improving your credit score before you apply is one of the most effective moves. Paying down existing debt, correcting errors on your credit report, and avoiding new credit applications in the months before your mortgage application can all help push your score into a better tier.
A larger down payment reduces the loan amount and signals lower risk to lenders, which can earn you a better rate. Putting down at least 20% also eliminates the need for private mortgage insurance, which adds to your monthly cost even though it doesn’t reduce your rate.
You can also buy discount points at closing. Each point typically costs 1% of your loan amount and lowers your rate by a fraction of a percentage point. This makes sense if you plan to keep the loan long enough for the monthly savings to exceed the upfront cost. On a $350,000 loan, one point costs $3,500. If it drops your rate enough to save $50 per month, you’d break even in about 70 months, or just under six years.
Finally, shopping multiple lenders matters more than most borrowers realize. Rates and fees vary meaningfully from one lender to the next, and getting quotes from at least three or four lenders gives you leverage to negotiate. Multiple mortgage inquiries within a short window, typically 14 to 45 days, count as a single inquiry on your credit report, so rate shopping won’t hurt your score.