Personal Loan Installment Calculator and Payment Formula

A personal loan installment calculator estimates your fixed monthly payment and total interest from the amount borrowed, annual percentage rate (APR), and repayment term. Enter those three figures to see the estimated installment and the cost of scheduled payments over the full term.

Formula

The calculator uses the standard amortizing-loan payment formula. It assumes the same payment is made once a month, with interest accruing at a constant monthly rate:

Monthly payment = round[P × r ÷ (1 − (1 + r)⁻ⁿ), 2]

• P is the loan principal, 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.
• round(…, 2) rounds the monthly payment to the nearest cent.

Total interest is calculated as monthly payment × n − P. Because the formula uses a payment rounded to cents, the total is based on that displayed payment. An actual final payment can differ slightly if the lender adjusts it to clear a small remaining balance.

Worked Example

Using the calculator’s defaults: a $20,000 loan, 8% APR, and a five-year term.

1. Convert the APR to a monthly decimal rate: r = 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: n = 5 × 12 = 60 months.
3. Substitute the values: payment = round[20,000 × 0.0066667 ÷ (1 − (1.0066667)⁻⁶⁰), 2].
4. The estimated monthly installment is $405.53.
5. Total scheduled payments are $405.53 × 60 = $24,331.80. Subtract the $20,000 principal to get $4,331.80 in estimated interest.

Assumptions

• The interest rate stays fixed throughout the term.
• Payments are equal, made monthly, and begin on the schedule assumed by the formula.
• Interest compounds monthly at APR ÷ 12.
• The calculation excludes origination or other fees, taxes, insurance, and any charges or payment adjustments not entered as inputs.

How to Use the Estimate

Enter the principal you expect to borrow, the APR supplied for the loan, and the repayment term. APR is entered as a percentage per year—for example, enter 8 for 8%, not 0.08. The term is entered in years, so a 30-month term is 2.5 years. The calculator accepts terms from approximately one month to 50 years, though the lender’s available terms may be different. Use the figures in the lender’s offer or disclosure rather than assuming an advertised or illustrative rate applies to you.

The first result is the scheduled monthly payment under these assumptions. The second is the interest implied by making that rounded payment for every month in the selected term. Neither result includes costs that are not represented in the inputs, so a payment shown here may not match the amount a lender collects.

What Changes the Payment?

• Principal: A larger amount borrowed increases the payment and, other things held constant, increases total interest.
• APR: A higher entered APR increases the periodic interest charge and generally raises both the installment and total interest.
• Term: Spreading repayment over more months generally lowers each installment, but leaves the balance accruing interest for longer. The total interest can therefore rise.
• Fees and payment structure: Charges added to the balance, fees paid separately, variable rates, or non-level installments can make the real schedule differ from this estimate.

These relationships describe the formula, not whether a particular loan is suitable. The calculator does not determine eligibility, predict a lender’s offer, or establish what a person can afford.

Common Misunderstandings and Limits

A personal loan installment is not simply the principal divided by the number of months. Each payment covers interest accrued on the outstanding balance as well as part of the principal. Early in a level-payment schedule, the interest portion is larger; as the balance declines, more of each payment goes toward principal.

APR and the rate used in the formula should not be confused with a monthly percentage. The formula converts the annual percentage to a monthly rate by dividing by 12 and by 100. It models a fixed APR as a nominal annual rate divided into monthly periods; loan disclosures may account for fees and timing in ways this simplified calculation does not.

Rounding can also create small differences. This calculator rounds the installment to cents, then multiplies that amount by the number of payments to estimate total interest. A lender’s amortization schedule may use unrounded internal calculations and adjust the final installment. For an exact payoff amount or current balance, request the applicable figure from the lender.

Frequently Asked Questions

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

Does the payment include personal loan fees?
No. The calculator has no fee input. Fees may be paid separately or handled in another way, so check the lender’s disclosure for how charges affect the amount received, balance, and payment.

Why is my lender’s payment different from this result?
The lender may use a different APR or term, include fees, apply different payment dates or rounding, or calculate a final installment adjustment. Compare the same principal, APR, and schedule, and use the disclosed repayment figures for the exact terms.

Does a longer loan term reduce total interest?
Not necessarily. A longer term often spreads repayment into smaller monthly installments, but interest accrues over more months. With the same principal and APR, the total interest can be higher.

Can I use this calculator for a variable-rate loan?
It can show a hypothetical payment if the entered rate remained unchanged, but it cannot model future rate changes. Actual payments or total interest may change under a variable-rate schedule.

Is the total interest the same as the total amount repaid?
No. Total interest is the amount above principal in this estimate. Add the original principal to estimated interest to get total scheduled payments.

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

Related

• Lack of Recent Installment Loan Information?
• Installment Loan vs Revolving Credit: How They Differ
• Auto Loan Calculator: Is It an Installment Loan?

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2