Personal Loan Amortization Calculator and Formula

A personal loan amortization calculator estimates the fixed monthly payment and total interest for a loan when you enter its amount, annual percentage rate (APR), and term. It also shows how the payment formula spreads repayment over time; the estimate assumes regular monthly payments and does not include lender-specific fees.

Formula

The calculator uses the standard fixed-payment amortization formula. It first finds the monthly payment, then subtracts the original principal from all scheduled payments to estimate total interest:

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

• P is the loan principal, or amount borrowed, in dollars.
• r is the monthly interest rate as a decimal: APR ÷ 1,200. For an 8% APR, r = 0.08 ÷ 12.
• n is the number of monthly payments: loan term in years × 12.

The result is rounded to cents for display. Total interest is calculated as the displayed monthly payment multiplied by the number of months, minus the principal. The formula assumes a positive APR, which is why the rate input begins above zero.

Worked Example

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

1. Convert the APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰].
4. The monthly payment is approximately $405.53.
5. Using the rounded payment, scheduled payments total $405.53 × 60 = $24,331.80. Estimated total interest is $24,331.80 − $20,000 = $4,331.80.

Actual payment schedules can differ by a few cents because of rounding conventions or payment dates. The example follows the calculator’s displayed, cent-rounded payment.

Assumptions

• The interest rate stays fixed for the full term.
• Payments are equal, made monthly, and paid at the end of each monthly period.
• Interest compounds monthly using APR ÷ 12.
• The calculation excludes origination or other fees, taxes, insurance, and any optional products or charges. It does not adjust the principal for amounts withheld from loan proceeds.

How Amortization Works

Amortization is the gradual repayment of a loan through scheduled payments. Each payment covers interest accrued for the period and reduces principal by whatever remains. Early in a level-payment loan, the balance is larger, so more of the payment goes to interest. As principal falls, the interest portion generally shrinks and more of each later payment reduces principal.

For a particular month, an illustrative schedule calculates interest as the starting balance multiplied by the monthly rate. Subtract that interest from the payment to estimate the principal repaid, then subtract the principal repaid from the starting balance. Repeating those steps produces an amortization schedule. The calculator gives the payment and total interest, not a month-by-month table.

Amortization does not mean that the borrower pays the same amount of interest every month. The scheduled payment can remain level while its interest and principal portions change. The total interest figure is the sum across the scheduled term under the stated assumptions.

How to Use and Read the Estimate

• Enter the amount borrowed as principal, not the total of all future payments.
• Use the APR supplied for the loan quote, expressed as a yearly percentage. Do not enter 0.08 for 8%; enter 8.
• Enter the term in years. A term of five years means 60 monthly payments.
• Read the monthly payment as the modelled scheduled amount, and total interest as the interest implied by those payments over the full term.

For a real comparison or payment plan, obtain the exact APR, repayment term, payment due dates, fees, and payment amount from the lender’s disclosure. APR and the rate used to accrue interest can be presented differently in loan documents; this calculator uses APR divided by 12 as its monthly rate and does not separately model fees.

What Changes the Payment and Total Cost

For the same principal and APR, a longer term generally produces a smaller scheduled monthly payment but more payments over time, which can increase total interest. A shorter term generally means larger monthly payments and fewer periods in which interest accrues. Increasing the principal or APR increases the payment under this formula. These are mathematical effects, not a recommendation about which term or loan to choose.

Additional payments can change the schedule if they are applied to principal, but this calculator assumes only the required level payment. The timing and treatment of extra money depend on the loan terms. A late payment, skipped payment, variable rate, or fee charged during repayment can also make actual balances and costs differ from this estimate.

Common Calculation Problems and Limits

A frequent mistake is treating the APR as a monthly rate. Because the formula converts a percentage APR to a monthly decimal, it divides by 1,200—not 12. Another is confusing the amount borrowed with the total repayment: principal is the starting balance, while total scheduled payments include principal and interest.

Rounding can matter. This tool rounds the monthly payment to cents and uses that displayed payment to estimate total interest. A lender may calculate each period using more decimal places and then adjust the final payment, so its final total can differ slightly. The formula also assumes that the full loan principal is disbursed and that payments arrive on the assumed monthly schedule. It cannot determine approval, the rate a lender will offer, exact fees, or the lender’s payment-processing rules.

Use this estimate to understand the arithmetic, then check the lender’s own repayment schedule for the contractual figures. The result is an estimate, and the lender’s disclosure gives the exact figures.

Frequently Asked Questions

What does amortization mean on a personal loan?
It means the loan is repaid through scheduled payments that cover interest and reduce the outstanding principal over time.

How do I calculate a personal loan payment?
Convert the APR to a monthly decimal by dividing by 1,200, convert the term to months, and apply the fixed-payment formula shown above.

Does a longer loan term lower the monthly payment?
For the same principal and APR, extending the term generally lowers the formula’s monthly payment, but it also increases the number of payments and may increase total interest.

Is total interest the same as APR?
No. APR is an annualized rate expressed as a percentage. Total interest is a dollar amount estimated across the full repayment term.

Why does my lender’s payment differ from the calculator?
The lender may use different fee treatment, payment dates, rounding, disbursement amounts, or interest calculations. Compare the calculator’s assumptions with the loan disclosure.

Related

• Loan Amortization Schedule: What It Shows
• Loan Amortization Journal Entry: Examples and Formula
• Loan Amortization on the Balance Sheet Explained

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