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01 Loan payments

Interest Loan Amortization Calculator: Payment and Interest

Calculator No. 01-8106 · Updated · Formula and assumptions shown below

Monthly payment $405.53
Total interest
$4,331.67
= Monthly payment × loan term × 12 − loan amount
Loan amount
$20,000
Input
Interest rate (APR)
8 %
Input
You enter
Loan amount ($), Interest rate (APR) (%), Loan term (years)
You get
Monthly payment ($), Total interest ($)
Rounding
2 dp

Estimate for education, not financial advice. The result follows from the numbers you enter and the formula shown. It is not a quote or an offer, and it does not include fees, taxes or insurance unless the calculator asks for them. Ask your lender for the exact figures.

An interest loan amortization calculator estimates the fixed monthly payment and total interest for a fully amortizing loan. Enter the loan amount, annual percentage rate (APR), and term in years to see the payment and the interest paid across the scheduled payments.

Formula

The calculator uses the standard fixed-payment formula for a loan repaid with equal monthly payments. The payment before rounding is:

Payment = P × r / [1 − (1 + r)−n]

Here, P is the starting principal in dollars, r is the monthly interest rate as a decimal, and n is the number of monthly payments. Convert the APR from a percentage to a monthly decimal using r = APR / 1200; convert years to payments using n = years × 12. The displayed monthly payment is rounded to cents. Total interest is calculated as the rounded payment times the number of payments, minus principal: Total interest = payment × n − P.

For the calculator’s default inputs, the monthly rate is 0.08 / 12 = 0.0066667 and the number of payments is 5 × 12 = 60. Interest is applied to the remaining balance each month, while each scheduled payment covers interest due and reduces principal.

Worked Example

Using the default loan amount of $20,000, APR of 8%, and term of 5 years:

  1. Convert APR to a monthly rate: r = 8 / 1200 = 0.0066667.
  2. Convert the term to monthly payments: n = 5 × 12 = 60.
  3. Substitute into the payment formula: 20,000 × 0.0066667 / [1 − (1.0066667)−60] ≈ $405.53 per month, rounded to cents.
  4. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The calculator’s results for these defaults are a $405.53 monthly payment and $4,331.80 in total interest. Small differences can arise if another calculator rounds intermediate values differently.

Assumptions

  • The interest rate remains fixed for the full term.
  • Payments are equal, made monthly, and begin one month after the loan starts.
  • Interest is modelled monthly at APR divided by 12.
  • The estimate excludes fees, taxes, insurance, PMI, and escrow; none of these are calculator inputs.

How to Read an Amortization Estimate

The monthly payment is the scheduled amount for principal and interest under the assumptions above. Total interest is the sum of those rounded payments less the original principal. It is not the total amount paid: add the principal to total interest to get the scheduled repayment total. For the example, that amount is $24,331.80.

An amortization schedule shows how each payment is divided. Early in repayment, the outstanding balance is larger, so monthly interest generally takes up a larger share. As the balance falls, less interest accrues and more of a level payment goes toward principal. The payment itself stays level in this model; its internal split changes over time.

This result is useful for understanding the basic cost and payment pattern of a fixed-rate loan, or for checking how changes in principal, APR, or term affect the formula’s estimate. It is not a complete household budget or a decision about whether to borrow. Use the actual loan amount, APR, and term supplied for the specific offer; the default values are only an example.

When This Calculator Fits—and Its Limits

Use this calculation when a loan is expected to have a fixed annual rate, equal monthly payments, and a payoff at the end of the stated term. It can provide a baseline for common installment-loan arithmetic, including a mortgage, auto loan, student loan, or personal loan, but it does not model product-specific terms.

It will not reproduce a variable-rate loan, an interest-only period, a balloon payment, irregular payment dates, or a loan with changing payment amounts. It also does not calculate a payoff quote on a particular date. A lender may calculate interest using its own day-count conventions and payment-processing dates, and fees or financed costs can change the amount being repaid. The lender’s repayment schedule is needed for those details.

Common Calculation Mistakes

  • Entering an APR as a decimal: Enter 8 for 8%, not 0.08. The formula divides the percentage by 1,200 to obtain a monthly decimal.
  • Confusing APR and monthly interest: APR is entered as an annual percentage. Do not enter a monthly rate in the APR field.
  • Using years as the number of payments: A five-year monthly loan has 60 payments, not five.
  • Comparing unlike totals: Total interest excludes principal. Scheduled payments also exclude items such as fees and escrow that may appear in a real bill.
  • Expecting exact lender figures: Rounding, payment dates, fees, and contract terms can cause a lender’s figures to differ from a simplified estimate.

Extra payments are not included. If a borrower pays extra toward principal, the balance and later interest may change, but the result depends on when the payment is received and how the loan applies it. This calculator assumes only the regular level payments in the stated term.

Frequently Asked Questions

What does an interest loan amortization calculator show?
It estimates a fixed monthly principal-and-interest payment and the total interest over the full term from the amount borrowed, APR, and duration.

Is total interest the same as total repayment?
No. Total interest is the cost above principal under the model. Add the original loan amount to total interest to find the scheduled repayment total.

Does the calculator include fees or insurance?
No. Fees, taxes, insurance, PMI, and escrow are excluded. Check the lender’s disclosure and payment schedule for any amounts that apply.

Why can my lender’s payment differ?
The loan may have different inputs, fees included in the balance, payment timing, rounding, or terms not represented by a fixed-rate monthly model.

Does making extra payments change total interest?
Extra principal payments can change the balance and interest path, but this calculator does not model them. The effect depends on the loan’s terms and how extra payments are applied.

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

Related

Formula used
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

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard loan formula for this calculation, fixed rate and monthly compounding unless stated, rounded as each result states. Not financial advice; no lender, rate or product is recommended. See formulas and assumptions, the disclaimer or report an error.

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