Monthly Loan Calculator With Interest: Formula & Example

A monthly loan calculator with interest estimates your fixed monthly payment and total interest from the amount borrowed, annual percentage rate (APR), and loan term. Enter those three figures to see the payment and the interest paid over the full term, using a standard fixed-rate amortizing-loan formula.

Formula

The calculator finds the level monthly payment needed to repay principal and interest over the entered term. It uses:

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

• P is the loan principal, in dollars.
• r is the monthly interest rate as a decimal: APR ÷ 12 ÷ 100, or APR ÷ 1,200.
• n is the number of monthly payments: loan term in years × 12.
• Total interest is monthly payment × number of payments − principal, in dollars.

The formula assumes a positive rate and equal payments made monthly. The displayed payment is rounded to cents. The total-interest result uses that displayed, cent-rounded payment multiplied by the number of months, then subtracts the original principal.

Worked Example

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

1. Convert the APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Convert the term to payments: 5 × 12 = 60 months.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
4. The monthly payment is $405.53 when rounded to cents.
5. Total interest is $405.53 × 60 − $20,000 = $4,331.80.

These figures are estimates based on the stated assumptions. Rounding each monthly payment to cents can make a small difference from a lender’s final payment schedule, especially if the last payment is adjusted.

Assumptions

• The interest rate stays fixed for the entire loan term.
• Payments are equal, made monthly, and treated as occurring at the end of each period.
• Interest compounds monthly using APR divided by 12.
• The calculation excludes fees, taxes, insurance, PMI, escrow, and other charges because they are not inputs.

How to Use the Monthly Payment Result

Enter the amount of principal being financed, not necessarily the purchase price. If a down payment or trade-in reduces the amount borrowed, use the resulting loan balance as the principal. Enter APR as a percentage: for example, enter 8 for 8%, not 0.08. Enter the term in years.

The first output is the estimated amount of each regular monthly payment for principal and interest. The second output is the interest accumulated across the full stated term, based on the rounded payment. It does not include the principal itself or costs not represented in the calculator.

A longer term generally spreads repayment over more monthly installments, which can lower the calculated payment while increasing the total interest in this fixed-rate model. A higher APR raises the interest portion and usually raises both the payment and total interest when principal and term stay unchanged. These are mathematical effects, not a judgment about whether a particular loan suits someone.

What the Estimate Leaves Out

Actual loan payments can differ because a loan may have fees financed into its balance, variable interest, a different day-count method, irregular payment dates, or a final installment adjusted for rounding. Some products also bundle other charges into a bill. This calculator models only principal and interest on a fixed-rate, equal-payment schedule; it does not estimate a complete monthly housing, vehicle, or personal-loan cost.

APR is an annualized rate expressed as a percentage, but this calculator converts it to a monthly rate by dividing by 12. A lender’s APR calculation or disclosure may reflect charges or conventions that this simple payment formula does not separately model. Use the rate and financed principal applicable to the specific offer, and check the payment schedule and disclosures for its exact treatment.

Common Calculation Mistakes

• Entering the wrong rate format: APR is entered as a percentage number, such as 8, rather than the decimal 0.08.
• Using the wrong principal: Enter the amount actually borrowed, including any charges financed into the balance if they are part of the loan—not an unadjusted purchase price when the financed amount differs.
• Confusing monthly and annual terms: This calculator asks for years and internally multiplies by 12. Do not enter 60 for a five-year loan.
• Treating interest as the entire borrowing cost: The total-interest result excludes fees and other expenses that are not inputs.
• Expecting every payment to be identical in practice: A lender may adjust the final installment or use a schedule that differs from the formula’s assumptions.

The calculator requires an APR above zero because its payment expression divides by the monthly rate. A zero-interest loan is a different calculation: principal divided by the number of payments. The displayed estimate also cannot account for a rate change or extra payments; those alter the repayment schedule and total interest.

Frequently Asked Questions

How do I calculate a monthly loan payment with interest?
Convert APR to a monthly decimal rate by dividing the percentage by 1,200, convert years to months, then use the level-payment formula shown above. The calculator performs these steps and rounds the payment to cents.

Does the monthly payment include interest?
Yes. Each modeled payment includes both interest accrued for that month and some repayment of principal. Early payments typically contain a larger interest share than later payments in a standard amortizing schedule.

Is total interest the same as the total amount repaid?
No. Total interest is the cost of interest over the modeled term. Total principal and interest repaid is the original principal plus total interest; this calculator displays the interest figure separately.

Why does a longer loan term change total interest?
A longer term means more monthly periods over which interest can accrue. With principal and APR held constant, the payment formula generally produces a lower monthly payment but more total interest over the longer repayment period.

Does this calculator include fees or insurance?
No. It calculates principal-and-interest payments only. Fees, insurance, taxes, escrow, and other charges are excluded unless reflected in the loan amount or entered separately, which this calculator does not allow.

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

Related

• Interest-Only Loan Calculator: Payment During and After the Interest-Only Period
• APR Calculator: The True Rate of a Loan With Fees
• Simple Interest Loan Calculator: I = P × r × t

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