How to Calculate Loan Payments

The Loan Payment Formula

Whether you are financing a car, paying off student loans, or taking out a personal loan, the monthly payment is calculated using the same standard formula for an amortizing loan:

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

Where:

• M = monthly payment
• P = principal (the amount borrowed)
• r = monthly interest rate (annual rate ÷ 12)
• n = total number of payments (loan term in years × 12)

For example, a $20,000 auto loan at 6% annual interest for 5 years: r = 0.06 ÷ 12 = 0.005, n = 60 months. The monthly payment comes out to approximately $386.66. Over the life of the loan, you would pay a total of $23,199.36 — meaning $3,199.36 goes to interest alone.

How Amortization Works

Amortization is the process of spreading a loan into a series of fixed payments over time. Each payment covers both interest and principal, but the ratio between them shifts dramatically over the life of the loan.

In the early months, most of your payment goes toward interest because the outstanding balance is still high. As you pay down the principal, the interest portion shrinks and more of each payment goes toward reducing the actual debt. This is why the first few years of a 30-year mortgage feel like slow progress — you are primarily paying interest.

An amortization schedule is a table that shows the exact breakdown of principal vs. interest for every single payment. Reviewing this schedule helps you understand the true cost of a loan and the impact of making extra payments early.

Fixed vs. Variable Interest Rates

Understanding the type of interest rate on your loan is crucial for financial planning:

• Fixed rate: The interest rate stays the same for the entire loan term. Your monthly payment is predictable and never changes. This is the safest option for borrowers who want budget certainty, and it is standard for most auto loans and conventional mortgages.
• Variable (adjustable) rate: The interest rate is tied to a benchmark index (like the prime rate or SOFR) and can change periodically — often annually after an initial fixed period. Payments may start lower than fixed-rate loans, but they can increase significantly if rates rise. ARM (Adjustable Rate Mortgage) products like 5/1 or 7/1 ARMs offer a fixed rate for the first 5 or 7 years before adjusting.

Variable rates carry more risk but can save money if you plan to pay off the loan or refinance before the adjustable period begins.

How Loan Term Affects Total Cost

The length of your loan has a dramatic impact on both your monthly payment and total interest paid:

• A shorter term means higher monthly payments but significantly less total interest. A $200,000 mortgage at 7% over 15 years costs about $1,798/month with $123,600 in total interest.
• The same loan over 30 years drops the payment to $1,331/month but balloons total interest to $279,000 — more than double.

Always compare the total cost of the loan, not just the monthly payment. A lower monthly payment can feel easier to manage, but it often costs tens of thousands more over the loan’s lifetime.

Extra Payments Strategy

Making extra payments is one of the most powerful strategies for saving money on a loan. Even small additional amounts can make a significant difference because extra payments go directly toward the principal, reducing the balance that accrues interest.

• One extra payment per year: On a 30-year mortgage, making one additional monthly payment per year can shave off roughly 4–5 years and save tens of thousands in interest.
• Biweekly payments: Paying half your monthly amount every two weeks results in 26 half-payments (13 full payments) per year instead of 12, automatically creating one extra payment annually.
• Rounding up: Simply rounding your payment up to the next hundred dollars accelerates payoff with minimal budget impact.

Before making extra payments, confirm with your lender that there are no prepayment penalties and that extra amounts are applied to principal, not future interest.

Refinancing Considerations

Refinancing replaces your existing loan with a new one, ideally at a lower interest rate or better terms. It makes sense when:

• Interest rates have dropped at least 0.75–1% below your current rate
• Your credit score has improved significantly since the original loan
• You want to switch from a variable rate to a fixed rate for stability
• You want to shorten your loan term to pay off debt faster

However, refinancing comes with closing costs (typically 2–5% of the loan amount). Calculate your break-even point — the number of months it takes for your monthly savings to recoup the closing costs. If you plan to keep the loan past the break-even point, refinancing is likely worthwhile.

Related Calculators

Run the numbers on your specific situation with these tools:

• Mortgage Calculator — Calculate monthly mortgage payments with taxes and insurance
• Compound Interest Calculator — See how interest compounds over time
• Simple Interest Calculator — Calculate basic interest on a principal amount