Interest and Payment Calculator for Fixed-Rate Loans

This interest and payment calculator estimates a fixed-rate loan’s monthly payment and total interest from the loan amount, annual percentage rate (APR), and term. Enter those three figures to see the estimated payment and the interest paid over the full scheduled term.

Formula

The calculator uses the standard amortizing-loan payment formula. It assumes equal monthly payments and a fixed annual rate converted to a monthly rate:

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

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

The displayed payment is rounded to the nearest cent. Total interest is calculated as the rounded monthly payment multiplied by the number of payments, minus the original principal: Total interest = monthly payment × n − P. This means the result reflects the displayed payment rather than an unrounded theoretical payment.

Worked Example

Using the calculator’s default values—$20,000 borrowed, 8% APR, and a 5-year term:

1. Convert APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Find the number of monthly payments: 5 × 12 = 60.
3. Apply the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] ≈ $405.53 per month, rounded to cents.
4. Calculate scheduled payments: $405.53 × 60 = $24,331.80.
5. Subtract the $20,000 principal: $24,331.80 − $20,000 = $4,331.80 in total interest.

The calculator’s headline result is the estimated monthly payment: $405.53. Its total-interest result is $4,331.80.

Assumptions

• The interest rate is fixed for the full loan term.
• Payments are equal, made monthly, and begin on the schedule used by the lender; the calculation treats interest as accruing monthly.
• Monthly compounding is represented by APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, late charges, and any lender-specific adjustments are not included.

When to Use the Calculator

Use this estimate to understand how principal, rate, and term affect a level-payment loan. It can help you check a quoted payment, compare hypothetical loan structures using figures supplied by a lender, or estimate the interest component of scheduled payments. It does not determine whether you qualify, what rate you may receive, or whether a loan fits your circumstances.

To make a useful estimate, enter the amount actually financed, not necessarily the item’s purchase price. If a lender adds eligible charges to the amount borrowed, use the resulting financed principal only when it is included in the loan disclosure. Enter the APR as a percentage—for example, type 8 for 8%, not 0.08. The term is in years; a 30-month term can be entered as 2.5 years.

How to Read the Results

The monthly payment is the scheduled principal-and-interest amount under the formula’s assumptions. Early in an amortizing loan, a larger share of each payment generally goes toward interest because the unpaid balance is larger. As principal is repaid, the interest portion falls. The calculator shows total interest over the full stated term, not the interest due in a particular month.

A longer term generally produces a smaller required payment for the same principal and rate, but it also means more scheduled payments. With the same fixed rate and principal, that commonly increases the sum of interest paid over the full schedule. A higher APR increases the payment and total interest when the other inputs remain unchanged. These are mathematical effects, not a recommendation about which term or loan to use.

Common Errors and Limits

• Confusing APR with a monthly rate: enter the annual percentage as a number such as 8; the formula converts it to a monthly rate.
• Entering the wrong term unit: this calculator accepts years, then multiplies by 12. Do not enter months as though they were years.
• Using the purchase price instead of the loan amount: down payments and amounts financed can make these figures different.
• Treating the estimate as a full bill: housing or vehicle payments may include charges absent from this calculation, and actual loan disclosures can use different timing or rounding conventions.
• Assuming extra payments are included: the calculation models only the regular level payment. It does not model extra principal, skipped or late payments, variable rates, or early payoff.

Because each displayed monthly payment is rounded to cents, the calculated total interest can differ slightly from a schedule that carries more decimal places internally or adjusts the final payment. For exact figures, use the lender’s payment schedule and disclosure. The formula is not suitable for interest-only loans, irregular payment schedules, or loans whose rate changes unless those features are separately modelled.

Frequently Asked Questions

How do I calculate monthly loan interest and payment?
Convert the APR to a monthly decimal rate by dividing it by 1,200, convert the term to months, and use the amortizing-payment formula shown above. The payment covers both principal and interest under the stated assumptions.

Is total interest the same as the total amount repaid?
No. Total interest is the amount above the borrowed principal. The total of scheduled payments equals principal plus interest in this estimate; fees and other charges are excluded.

Does this calculator include fees or insurance?
No. It calculates a principal-and-interest payment and estimated interest only. Entering a financed amount that includes a charge changes the principal input, but the calculator does not identify or calculate that charge separately.

Why might my lender’s payment be different?
The lender may use a different financed amount, payment date, day-count convention, rounding method, or included charge. Compare the inputs and assumptions with the official loan disclosure.

Can I use this for a variable-rate or interest-only loan?
Not as a complete estimate. The formula assumes a fixed rate and equal fully amortizing payments. A changing rate or interest-only period requires a calculation that models those specific terms.

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

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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