The basic formula: principal, rate, and time
To figure out what you'll pay each month on a loan or savings account with interest, you need three pieces of information: the amount you borrowed or deposited (called the principal), the interest rate, and how long you're borrowing or saving for. Banks use a standard formula to turn those three numbers into a monthly payment.
The formula looks like this: M = P × [r(1 + r)^n] / [(1 + r)^n − 1]. In plain terms: M is your monthly payment, P is the principal, r is the monthly interest rate (the annual rate divided by 12), and n is the total number of months you'll be paying.
You don't have to do this math by hand. Every bank, lender, and loan servicer has a calculator built into their website. But understanding what the numbers mean helps you spot mistakes and compare offers side by side.
Key Takeaways
- Your monthly payment depends on three things: how much you borrowed, the interest rate, and how many months you have to pay it back.
- A higher interest rate or shorter payoff period raises your monthly payment; a longer payoff period lowers it but costs more in total interest.
- You can use a loan calculator on any lender's website, or a spreadsheet like Excel with the PMT function, to find your exact monthly payment.
- The first payments you make go mostly toward interest; later payments go more toward paying down what you actually borrowed.
Why the monthly payment changes based on how long you borrow
The length of your loan—called the term—has a big effect on what you pay each month. A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same house at the same interest rate, because you're paying back the money faster. But over the full 30 years, you'll pay much more in total interest, even though each monthly payment is smaller.
This is why lenders always show you two numbers: the monthly payment and the total amount you'll pay over the life of the loan. A $200,000 mortgage at 6% interest costs about $1,199 per month over 30 years—but you'll pay roughly $431,676 total. The same loan over 15 years costs about $1,844 per month, but only $331,900 total. The shorter term saves you about $100,000 in interest, but your monthly budget has to handle the higher payment.
How to use a loan calculator
Every major bank and lender has a free calculator on their website. To use one, you enter three numbers: the loan amount, the annual interest rate, and the number of months (or years) you want to borrow for. The calculator then shows you the monthly payment and usually the total interest you'll pay.
If you're comparing loans from different lenders, use their calculators side by side with the same numbers. This shows you exactly how much the interest rate difference costs you each month. A difference of even 0.5% in interest rate can mean $50 to $100 per month on a $200,000 loan.
You can also use a spreadsheet. In Microsoft Excel or Google Sheets, the function is =PMT(rate, nper, pv). The rate is your monthly interest rate (annual rate divided by 12), nper is the number of months, and pv is the loan amount as a negative number. For example, a $10,000 loan at 5% annual interest over 60 months would be =PMT(0.05/12, 60, -10000), which gives you $188.71 per month.
Why your first payments are mostly interest
When you make your first payment, most of it goes to interest, not to paying down what you borrowed. This surprises many people. On a $200,000 mortgage at 6% interest, your first payment might be $1,199, but only about $200 of that actually reduces what you owe. The other $999 goes to the lender as interest.
As time goes on, this flips. By payment 300 on a 360-payment mortgage, most of your payment goes toward principal and very little toward interest. This is why paying extra toward principal early in the loan saves you the most money—you're reducing the amount that interest gets calculated on for the rest of the loan.
How interest rate changes affect your payment
A small change in interest rate creates a big change in your monthly payment. On a $300,000 home loan over 30 years, the difference between 5% and 6% interest is about $180 per month. Between 6% and 7%, it's another $200 per month. Over 30 years, that 1% difference costs you roughly $72,000 more.
This is why shopping around for the best interest rate matters so much. Spending an hour comparing rates from three or four lenders can save you tens of thousands of dollars over the life of a loan. Many lenders will show you a rate estimate without a hard credit check, so you can compare without damaging your credit score.
Understanding amortization: how your balance shrinks over time
An amortization schedule is a table that shows you, for each payment, how much goes to interest and how much goes to principal, and what your remaining balance is. Most lenders will give you this schedule when you take out a loan, or you can ask for it.
Early in the schedule, the principal portion is tiny. On a $200,000 mortgage at 6%, your first payment might show $999 to interest and $200 to principal. By payment 180 (halfway through a 30-year loan), that flips to about $500 to interest and $700 to principal. By the final payment, almost all of it goes to principal because there's almost no balance left to charge interest on.
You can use this schedule to see what happens if you pay extra. If you add $100 to your principal payment each month, you'll see the balance drop faster and the total interest shrink. Some lenders let you make extra payments without penalty, which is worth asking about.
The difference between fixed and variable interest rates
A fixed interest rate stays the same for the entire loan, so your monthly payment never changes. This makes budgeting simple: you know exactly what you'll pay every month for the next 15 or 30 years.
A variable interest rate (sometimes called adjustable) starts at one rate and then changes based on market conditions. Your monthly payment might be $1,000 for the first five years, then jump to $1,200 when the rate adjusts. Variable rates are sometimes lower at the start, which tempts borrowers, but they're riskier because you don't know what your payment will be later. Most people are better off with a fixed rate unless they plan to sell or refinance before the rate adjusts.
Frequently Asked Questions
What's the difference between APR and interest rate?
The interest rate is just the cost of borrowing. The APR (annual percentage rate) includes the interest rate plus other costs the lender charges, like origination fees. When comparing loans, always compare APRs, not just interest rates, because APR shows you the true cost.
Can I pay off a loan early without a penalty?
Most mortgages and personal loans have no prepayment penalty, meaning you can pay extra toward principal whenever you want. Some loans, especially older mortgages or certain car loans, do have penalties. Check your loan documents or ask your lender before you start making extra payments.
Why does my actual monthly payment not match the calculator?
Loan payments often include more than just principal and interest. They may include property taxes, homeowners insurance, and mortgage insurance (PMI), all bundled into one payment. Ask your lender for a breakdown of what's included in your payment. The calculator usually shows only principal and interest unless you tell it otherwise.
If I make one extra payment per year, how much do I save?
One extra payment per year typically shortens a 30-year mortgage by about four to five years and saves you roughly 10% of the total interest. The exact savings depend on your interest rate and loan amount. Use your lender's amortization schedule or a calculator to see the specific impact on your loan.
What happens if interest rates drop after I lock in my rate?
You're stuck with your rate unless you refinance, which means taking out a new loan to pay off the old one. Refinancing costs money in fees and closing costs, so it only makes sense if the new rate is low enough to save you more than those fees cost. Generally, a 0.5% to 1% drop in rates makes refinancing worth considering.