Monthly Loan Calculator With Amortization Schedule

This monthly loan calculator estimates your fixed monthly payment and total interest, then explains how each payment changes the balance over time. Enter the loan amount, annual percentage rate (APR), and term in years to see the payment and interest estimate.

Formula

The calculator uses the standard level-payment formula for a fixed-rate, fully amortizing loan. It calculates a monthly payment intended to repay principal and interest in equal installments over the selected term:

Payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the original loan principal, in dollars.
• r is the monthly interest rate as a decimal: annual APR ÷ 100 ÷ 12, or APR ÷ 1,200 when APR is entered as a percentage.
• n is the number of monthly payments: years × 12.

Total interest is calculated as the monthly payment multiplied by the number of payments, minus the original principal. In the calculator, the monthly payment is rounded to cents before the total is calculated. That makes the displayed total consistent with paying the displayed monthly amount for every scheduled month; a lender’s final payment can differ slightly because of its exact rounding and payment dates.

Worked Example

Using the calculator’s default inputs—$20,000 principal, 8% APR, and a five-year term:

1. Convert APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents.
4. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The schedule assumes 60 equal monthly payments. The final amount charged by a lender may vary by a small amount if its calculations use more precise daily accrual or different rounding.

Assumptions

• The interest rate is fixed for the entire term.
• Payments are made monthly, at the end of each period, with the first payment one month after borrowing.
• Interest compounds monthly at APR ÷ 12.
• The estimate excludes fees, taxes, insurance, PMI, and escrow because they are not calculator inputs.

How Amortization Changes the Balance

Amortization is the gradual repayment of a loan through scheduled payments. Each payment is divided between interest accrued since the prior payment and a reduction in principal. Early in a standard fixed-rate loan, the outstanding balance is relatively large, so the interest portion is larger. As principal falls, the interest charged each month generally falls too, leaving more of the same payment to reduce principal.

For example, at the start of the default example, one month’s interest is approximately $20,000 × 0.0066667, or $133.33. From a $405.53 payment, about $272.20 therefore goes toward principal, leaving roughly $19,727.80 outstanding. The next month’s interest is calculated on that lower balance. A full amortization schedule repeats this calculation payment by payment and can show the date, opening balance, payment, interest, principal reduction, and closing balance.

The calculator provides the monthly payment and overall interest, not a row-by-row schedule. A schedule is useful when you want to inspect the balance at a particular point or understand how much of a payment goes to interest versus principal. Actual servicing statements are the reference for a real loan’s posted balance.

When to Use the Estimate

This calculator is useful for understanding the basic payment mechanics of a fixed-rate installment loan, such as a personal, auto, or mortgage loan. It can help you compare hypothetical combinations of amount, APR, and term on equal mathematical assumptions. It does not determine whether a lender will approve an application or what terms a lender will offer.

Enter the amount actually being borrowed, rather than a purchase price if a down payment or other financing changes the principal. Enter the APR as a percentage per year—for example, type 8 for 8%, not 0.08. Use the term in years. The calculator accepts a minimum APR of 0.01% because the displayed formula divides by the monthly rate; it is not designed to calculate a zero-interest loan.

For a realistic estimate, obtain the loan’s actual principal, APR, term, and payment details from the lender’s disclosures. If a quoted payment includes financed fees, those may affect the principal or APR; do not assume the calculator’s result includes them.

Common Misunderstandings and Limits

• Payment is not the same as total borrowing cost. The monthly payment covers scheduled principal and interest in this model. It does not include excluded charges such as taxes, insurance, or escrow.
• APR and the periodic rate are different formats. APR is entered as an annual percentage; the formula converts it to a monthly rate by dividing by 1,200.
• A longer term changes both payment and interest. Holding the principal and rate constant, spreading repayment over more months generally lowers the scheduled payment but gives interest more time to accrue.
• Extra payments are not included. Additional principal payments can change the payoff timeline and interest, depending on how they are applied. This calculator assumes only the scheduled level payment.
• Variable rates do not fit a fixed-rate estimate. If the rate changes, later interest and payments may differ from this calculation.

Rounding matters: this calculator rounds the monthly payment to cents, then multiplies that amount by the number of payments to estimate total interest. A lender may use unrounded figures internally, calculate interest using actual days, or adjust the last payment. The formula also assumes regular monthly periods and no skipped, late, or irregular payments.

Frequently Asked Questions

How do I calculate a monthly loan payment?
Convert the annual APR to a monthly decimal rate by dividing by 1,200, convert the term to months, and use the fixed-payment formula shown above. The calculator performs those steps and rounds the payment to cents.

What does an amortization schedule show?
It lists each scheduled payment and separates it into interest and principal, while tracking the remaining balance. The balance generally declines as principal is repaid.

Does the monthly payment include taxes and insurance?
No. This estimate includes principal and interest only. Taxes, insurance, PMI, escrow, and fees are excluded unless separately included in the loan amount or payment figures you obtain from the lender.

Why does a longer loan term usually cost more interest?
With the same principal and rate, a longer schedule keeps a balance outstanding for more months. The scheduled payment may be lower, but interest can accrue over a longer period.

Can I use this for a variable-rate loan?
Only as a fixed-rate illustration for the APR entered. It cannot predict future payments when the rate can change.

This result is an estimate; the lender’s disclosure gives the exact figures.

Related

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2