How to Calculate Loan Payments: Principal, Interest, and Total Cost Explained

Before signing any loan agreement — whether it’s a car loan, a personal loan, or a student loan — you should know exactly what your monthly payment will be and how much you’ll pay in total over the life of the loan. The difference between the “sticker price” of a loan and its true cost is often thousands of dollars. Here’s how to calculate loan payments yourself, what each number means, and how to compare loans like a professional.

The Three Numbers That Matter

• Principal (P): The amount you borrow
• Interest rate (r): The annual rate, divided by 12 for monthly periods
• Number of payments (n): The loan term in months (e.g., 60 for a 5-year loan)

With these three numbers, you can compute your monthly payment using the standard amortization formula:

Monthly payment = P × [r(1+r)^n] / [(1+r)^n − 1]

This looks intimidating, but the math is straightforward. Let’s work through a real example.

Real Example: A $25,000 Car Loan

Suppose you’re financing a car with these terms:

• Principal: $25,000
• Annual interest rate: 6% (0.06 ÷ 12 = 0.005 monthly rate)
• Term: 60 months (5 years)

Plugging these into the formula gives a monthly payment of approximately $483.32. Over 60 months, you’ll pay $483.32 × 60 = $28,999.20 total. That means the interest cost is $28,999.20 − $25,000 = $3,999.20. The “$25,000 car” actually costs you almost $29,000.

How Interest Rate Affects Total Cost

Your interest rate is the single biggest factor in total loan cost. Compare these scenarios for the same $25,000 over 60 months:

APR	Monthly Payment	Total Paid	Total Interest
3%	$449.23	$26,953.80	$1,953.80
6%	$483.32	$28,999.20	$3,999.20
9%	$518.92	$31,135.20	$6,135.20
12%	$556.11	$33,366.60	$8,366.60

Moving from 6% to 12% APR more than doubles your total interest — from $3,999 to $8,367. That’s why shopping around for rates matters so much. A single percentage point on a large loan can save you hundreds of dollars.

Use our Loan Calculator to run these scenarios instantly and see how different terms and rates change your monthly payment.

Amortization: Why Early Payments Are Mostly Interest

In the early years of a loan, most of your payment goes toward interest, not the principal. For the $25,000 loan at 6%, your very first payment of $483.32 includes $125 of interest and only $358.32 toward principal. By the final payment, the split flips: almost the entire payment goes to principal. This is called amortization.

This matters for one practical reason: making extra payments early in the loan saves you far more interest than extra payments late in the loan, because early principal reductions shrink the balance on which future interest is calculated.

Frequently Asked Questions

What’s the difference between APR and the interest rate?

APR (Annual Percentage Rate) includes the interest rate plus fees and other loan costs, expressed as a yearly rate. It’s the more accurate number for comparing loans. A loan at 5.9% with $500 in fees can have a higher APR than a loan at 6.2% with no fees.

Does a longer loan term lower my monthly payment?

Yes, but it increases total interest. A $25,000 loan at 6% over 72 months (6 years) drops the monthly payment to about $414, but total interest rises to $4,817 — $818 more than the 60-month term.

How much can I save with extra payments?

Adding $50 per month to a $25,000, 6%, 60-month loan shortens the term by roughly 7 months and saves about $700 in interest. The earlier you start, the more you save.

Before you sign anything, run the numbers with our Loan Calculator and our Compound Interest Calculator to understand both the payment schedule and the opportunity cost of borrowing.