The basic formula for your monthly payment
Your monthly mortgage payment is calculated using a standard formula that takes three pieces of information: the loan amount you borrowed, the interest rate, and how many months you have to pay it back. The formula is:
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
In this formula, M is your monthly payment, P is the principal (the amount borrowed), r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments (years multiplied by 12). This is called an amortizing loan calculation, and it's what banks use to determine what you owe each month.
The formula assumes you make equal payments every month for the full term and that your interest rate stays the same. If you have an adjustable-rate mortgage, your rate changes on a schedule, so your payment will change too — but each payment period still uses this same calculation.
Key Takeaways
- Your monthly payment depends on three numbers: how much you borrowed, your interest rate, and how many months you have to repay it.
- The standard mortgage formula multiplies the principal by a ratio that accounts for interest compounding over time.
- You can calculate this by hand, use a mortgage calculator online, or ask your lender for an amortization schedule that shows every payment.
- A higher interest rate or shorter loan term raises your monthly payment; a lower rate or longer term lowers it.
- Your actual payment may include property taxes, insurance, and mortgage insurance, which are not part of the basic formula.
Working through a real example
Let's say you borrow $300,000 at 6.5% annual interest over 30 years. First, convert the annual rate to a monthly rate: 6.5% ÷ 12 = 0.542% per month, or 0.00542 as a decimal. The number of payments is 30 years × 12 months = 360 payments.
Plugging into the formula: M = 300,000 × [0.00542(1.00542)^360] / [(1.00542)^360 − 1]. The (1.00542)^360 part is where the compounding happens — it shows how much $1 grows over 360 months at that rate. That number comes out to about 6.898. So M = 300,000 × [0.00542 × 6.898] / [6.898 − 1] = 300,000 × [0.0374] / [5.898] = 300,000 × 0.00634 = $1,902 per month.
This $1,902 is principal and interest only. Your actual monthly payment to the lender will be higher if it includes property tax, homeowners insurance, or mortgage insurance (PMI). Those are added on top of the base payment.
Why the formula matters: principal versus interest
Early in your loan, most of your payment goes toward interest. As time passes, more of each payment goes toward principal. This is why the formula matters — it's the only way to know how much of your money is actually reducing what you owe.
In the $300,000 example above, your first payment of $1,902 includes about $1,625 in interest and only $277 in principal. By payment 300, you're paying about $50 in interest and $1,852 in principal. The total stays the same, but the split changes every month.
Your lender is required to give you an amortization schedule — a table showing exactly how much principal and interest you pay each month. This schedule is built using the same formula and shows you the full picture of your loan.
How interest rate changes affect your payment
A small change in interest rate creates a large change in your monthly payment. Using the same $300,000 loan over 30 years, a rate of 5.5% gives you a payment of about $1,703. At 7.5%, it jumps to $2,098. That's a difference of nearly $400 per month, or $144,000 over the life of the loan.
This is why shopping for the best rate matters so much. Even a 0.5% difference in rate can save or cost you tens of thousands of dollars. The formula shows this clearly: the interest rate is multiplied through the entire calculation, so it has an outsized effect on your final payment.
How loan term affects your payment
A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same loan amount and rate, because you're paying back the money faster. Using the $300,000 at 6.5% example: a 15-year term gives you a payment of about $2,596 per month, versus $1,902 for 30 years.
The tradeoff is that you pay far less interest over the life of the loan. On the 30-year mortgage, you pay about $384,000 in total interest. On the 15-year mortgage, you pay about $167,000. The shorter term costs you $694 more per month but saves you $217,000 in interest.
The formula shows why: when n (the number of payments) is smaller, the compounding effect is smaller, so the ratio that multiplies your principal is lower. Fewer months means less time for interest to accumulate.
Using calculators and amortization schedules instead of doing it by hand
Most people don't calculate this by hand. Online mortgage calculators let you enter your loan amount, rate, and term, and they do the formula work instantly. You can also find calculators on your lender's website, on real estate sites, and on financial sites like Bankrate or NerdWallet.
Your lender is required to provide you with a Loan Estimate within three business days of your application. This document includes your monthly payment, broken down into principal, interest, taxes, insurance, and any other fees. It also includes an amortization schedule or a reference to where you can find one.
If you want to see how different rates or terms would change your payment before you apply, a calculator is the fastest way. If you want to see the exact breakdown of every payment over 30 years, ask your lender for the full amortization schedule.
What the formula does not include
The basic mortgage formula calculates principal and interest only. Your actual monthly payment to your lender often includes other costs: property tax, homeowners insurance, and mortgage insurance (if you put down less than 20%). These are sometimes bundled into a single payment called PITI (Principal, Interest, Taxes, Insurance).
Property tax varies by location and is set by your county or municipality. Homeowners insurance is set by your insurance company and depends on your home's value and location. Mortgage insurance is set by your lender and depends on your down payment and credit score. None of these are part of the formula, but all of them affect what you actually pay each month.
Your Loan Estimate will show you the estimated total payment including all of these. The formula gives you the foundation, but the full picture is wider.
Frequently Asked Questions
Can I use this formula for loans other than mortgages?
Yes. This formula works for any fixed-rate, amortizing loan: car loans, personal loans, student loans, and home equity loans all use the same calculation. The only difference is the numbers you plug in — a car loan might be $25,000 at 5% over 60 months, but the formula is identical.
What if my interest rate is adjustable?
The formula still applies, but only for the period when your rate is fixed. When your rate adjusts, your lender recalculates your payment using the new rate and the remaining balance and term. Your payment will change on the adjustment date. Your loan documents will tell you when and how often your rate can change.
Why is my actual payment different from what the formula gives me?
The formula calculates principal and interest only. Your actual payment includes property tax, insurance, and possibly mortgage insurance, which are added on top. Your Loan Estimate breaks down all of these separately so you can see what each one costs.
Does making extra payments change how the formula works?
The formula itself doesn't change, but extra payments reduce your principal faster, which means less interest accrues over time and you pay off the loan sooner. Your lender will recalculate your remaining balance and term if you make a large extra payment, but the monthly payment formula stays the same for the regular payments you owe.
Can I use the formula to compare loans from different lenders?
Yes. If two lenders offer different rates or terms, plug each one into the formula (or use a calculator) to see what your monthly payment would be. This lets you compare the true cost of borrowing from each lender, not just the interest rate they quote.