How to Calculate Loan Payments: A Step-by-Step Guide

Gather the Key Loan Inputs: Principal, Rate, and Term

Before you can calculate a loan payment, you need three numbers: the principal (the amount you borrow), the annual interest rate, and the loan term (how long you have to repay). These are the inputs used by standard loan math.

For this guide, we’ll use a fixed-rate, fully amortizing loan—the type where your payment stays the same for the entire term and covers both interest and principal. Variable-rate, interest-only, and balloon loans work differently, so the formula here applies specifically to standard fixed-rate loans.

1. Principal: the total amount borrowed, not including fees or taxes.
2. Annual interest rate: the yearly cost of borrowing, expressed as a percentage (also called the nominal rate).
3. Loan term: the number of years you have to repay the loan.

Sources: OpenStax, Finance Formulas

Convert the Annual Rate to a Periodic Rate

The loan payment formula works with the interest rate that matches your payment frequency. If you make monthly payments, you need a monthly interest rate. To get it, divide the annual rate by 12.

For example, a 6% annual rate becomes 0.5% per month (6 ÷ 12 = 0.5). In the formula, you’ll use the decimal form, so 0.5% becomes 0.005.

• Monthly rate = annual rate ÷ 12
• Convert the percentage to a decimal by dividing by 100 (0.5% = 0.005).

Sources: Bankrate, MABTS

The Loan Payment Formula (PMT Formula)

The standard formula for a fixed-rate loan payment is: M = P × [r(1 + r)^n] / [(1 + r)^n – 1], where M is the monthly payment, P is the principal, r is the monthly interest rate (as a decimal), and n is the total number of monthly payments.

This is the same formula used for an ordinary annuity. It calculates the level payment that exactly repays the loan over the term, with each payment covering the interest that accrued plus a portion of the principal.

• M = monthly payment
• P = principal (loan amount)
• r = monthly interest rate (annual rate ÷ 12, expressed as a decimal)
• n = total number of monthly payments (loan term in years × 12)

Sources: Bankrate, Finance Formulas

Worked Example: Calculating a Payment Step by Step

Let’s calculate the monthly payment for a $20,000 auto loan with a 6% annual interest rate and a 5-year term. First, convert the inputs: the monthly rate is 0.06 ÷ 12 = 0.005, and the total number of payments is 5 × 12 = 60. Plugging these into the formula gives a monthly payment of $386.66.

• P = $20,000
• r = 0.005 (6% annual ÷ 12)
• n = 60 (5 years × 12 months)
• M = $20,000 × [0.005(1.005)^60] / [(1.005)^60 – 1] = $386.66

Sources: Bankrate

How to Set Up the Calculation in a Spreadsheet

Spreadsheet software like Excel or Google Sheets has a built-in PMT function that does the math for you. The syntax is =PMT(rate, nper, pv, [fv], [type]), where rate is the periodic interest rate, nper is the total number of payments, and pv is the present value, or the loan amount.

For monthly payments on a loan, you enter the annual rate divided by 12 for rate, and the loan term in years multiplied by 12 for nper. The fv and type arguments are optional; you can leave them blank for most loans.

• In an empty cell, type: =PMT(0.005, 60, 20000)
• The result is -386.66 (the negative sign indicates a cash outflow).
• You can also use cell references, e.g., =PMT(B1/12, B2*12, B3) if B1 holds the annual rate, B2 the term in years, and B3 the principal.
• To see a full amortization schedule, use the IPMT and PPMT functions to break down interest and principal portions.

Sources: MABTS

Common Errors in Manual Calculations and How to Avoid Them

The most common mistake is using the annual interest rate directly in the formula instead of converting it to a monthly rate. Another frequent error is forgetting to multiply the loan term by 12 to get the total number of payments. Both mistakes lead to incorrect results.

To avoid these, always state the periodic rate and the number of periods before you start. If your payment frequency is monthly, use the monthly rate and the number of months. Using the annual rate without converting it to the periodic rate will produce an incorrect payment; always convert the rate and term consistently.

• Always divide the annual rate by 12 for monthly payments.
• Always multiply the loan term in years by 12 to get the number of payments.
• Double-check that you’re using the decimal form of the rate (e.g., 0.005, not 0.5).
• If you’re working with a spreadsheet function, confirm that the rate and nper arguments are in the same time units.

Sources: Bankrate, Finance Formulas

Understanding Your Payment: Interest and Principal

Each loan payment has two parts: interest and principal. The interest portion is based on the remaining loan balance, so early in the loan you pay more interest and less principal; as the balance declines, the interest portion shrinks and more of your payment goes toward principal.

This is why an amortization schedule shows a decreasing interest amount over time. For mortgages, your total monthly payment often includes more than just principal and interest—it may also include property taxes and homeowners insurance, which are typically held in an escrow account. The loan payment formula in this guide only covers the principal and interest portion.

• Interest = remaining principal × monthly rate
• Principal portion = total payment – interest
• For a mortgage, the full monthly payment may also include taxes and insurance.

Sources: OpenStax, Consumer Financial Protection Bureau