Loan Repayment Payment Calculator: Monthly Cost & Interest

A loan repayment payment calculator estimates the fixed monthly payment needed to repay a loan, plus the total interest paid over the term. Enter the amount borrowed, annual percentage rate (APR), and term in years to see an estimate based on equal monthly payments.

Formula

The calculator uses the standard amortizing-loan payment formula. It first calculates a level monthly payment, rounded to the nearest cent, then subtracts the original principal from the sum of those monthly payments to estimate total interest.

Monthly payment: M = round[P × r ÷ (1 − (1 + r)⁻ⁿ), 2]. Total interest: I = M × n − P.

• P is the loan principal, in dollars.
• APR is the annual percentage rate entered as a percentage, not a decimal.
• r is the monthly rate, calculated as APR ÷ 1,200.
• n is the number of monthly payments, calculated as loan term in years × 12.
• M is the monthly payment in dollars, rounded to cents.
• I is estimated total interest in dollars, using the rounded monthly payment.

The APR is divided by 1,200 because a percentage must be converted to a decimal and then divided into 12 monthly periods. For example, an 8% APR becomes a monthly rate of 0.0066667. The formula assumes the same payment amount throughout the loan and a rate that does not change.

Worked Example

Using the calculator’s default inputs: principal P = $20,000, APR = 8%, and term = 5 years.

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

The estimate treats each of the 60 payments as $405.53. A lender’s actual schedule can differ slightly if its payment dates, rounding practices, or final payment amount differ.

Assumptions

• The interest rate stays fixed for the full term.
• Payments are equal, made monthly, and applied at the end of each monthly period.
• Interest compounds monthly using APR divided by 12.
• The calculation excludes fees, taxes, insurance, PMI, escrow, late charges, and optional add-ons.

When to Use the Calculator

Use this estimate to understand how principal, APR, and repayment time relate to a scheduled payment. It can help you compare hypothetical loan inputs on the same mathematical basis, or check a quoted payment against a simple fixed-rate calculation. It does not determine whether you qualify, what a lender will charge, or what payment fits your circumstances.

Try changing one input at a time to see how the calculation responds. With the other inputs held constant, borrowing more increases the payment; a higher APR increases the payment and interest; and a longer term generally lowers the scheduled monthly payment while increasing the number of months interest can accrue. These are formula relationships, not a recommendation about loan length or borrowing.

Enter the loan amount that will actually accrue interest. If a fee is added to the principal, the amount financed may differ from the amount received. For an accurate estimate, check the lender’s documents for the amount financed, APR, term, payment frequency, and any charges that affect the balance.

Reading the Results and Common Mistakes

The monthly payment is the estimated regular installment, not the total cost of borrowing. Total interest is the amount above the original principal implied by the rounded payment and the number of scheduled months. The sum of principal and interest is therefore the principal plus the displayed total interest; it does not include excluded charges.

• Entering APR as a decimal: Enter 8 for 8%, not 0.08. The calculator converts the percentage internally.
• Confusing term units: The term field is in years. A 60-month term is 5 years, not 60 years.
• Using the wrong principal: A requested loan amount may not match the final amount financed if charges are included or funds are withheld.
• Expecting exact lender figures: Payments are rounded to cents. A lender may adjust the final installment or use different dates and calculation conventions.
• Assuming every loan is fixed-rate: Variable-rate loans can have changing payments and cannot be represented accurately by one unchanged APR for the full term.

APR is an input, not a guarantee that every cost is included in this estimate. Check the loan disclosure to understand how the lender defines the rate and calculates the payment. Use the actual quoted figures rather than a general or assumed rate.

Limits and Other Repayment Situations

This formula describes a fully amortizing loan: each scheduled payment covers the period’s interest, and the balance is reduced so that it reaches approximately zero after the stated number of payments. It is not designed for interest-only periods, balloon payments, irregular installments, changing rates, skipped-payment arrangements, or a balance with separate fees. Those structures need additional inputs and a different repayment schedule.

Extra payments also change the schedule. If additional money is applied to principal, the payoff time and total interest may fall, but the result depends on when payments are made and how the servicer applies them. This calculator does not model extra payments or confirm how a particular lender handles them.

A related misconception is that a smaller monthly payment automatically means a lower total cost. A longer term can reduce the scheduled payment but create more payment periods, so total interest may rise. The calculator shows both figures to make that distinction visible; it does not assess whether either option is suitable for a reader.

Frequently Asked Questions

How do I calculate a monthly loan payment?
Convert APR to a monthly decimal rate by dividing it by 1,200, convert years to months by multiplying by 12, and apply the fixed-payment formula shown above. Round the result to cents.

Does the payment include interest and principal?
Yes. For a standard amortizing loan, each scheduled payment includes interest for that period and an amount that reduces principal. The proportions change over time as the balance declines.

Why does a longer loan term lower the monthly payment?
The principal is divided across more scheduled payments. However, interest may accrue over more months, so the total interest can be higher even when the monthly payment is lower.

Why does my lender’s payment differ from this estimate?
The quoted loan may include fees in the financed balance, use a different rate or payment date, have a variable rate, or apply a different rounding or final-payment method. Check the lender’s disclosure for the calculation details.

Can I use this for a variable-rate loan?
Only as a snapshot under the entered rate. The formula assumes a fixed rate for the entire term and does not forecast future rate changes or resulting payment adjustments.

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

Related

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• Loan Payment Calculator: How Compounding Interest Works
• Loan Calculator Using Payment Amount: Find Loan Size

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