calculate-loan

01 Loan payments

Installment Loan Payment Calculator and Formula

Calculator No. 01-5015 · 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 installment loan payment calculator estimates the fixed monthly payment and total interest for a loan repaid in equal monthly installments. Enter the amount borrowed, annual percentage rate (APR), and term in years to see the estimated payment and interest; it does not include fees or other charges.

Formula

For a fixed-rate loan with equal monthly payments, the payment is calculated using the standard amortizing-loan formula:

Payment = P × r ÷ [1 − (1 + r)−n]

  • P is the principal, or amount borrowed, in dollars.
  • r is the monthly interest rate as a decimal: APR ÷ 1,200. The APR input is expressed as a percentage per year.
  • n is the number of monthly payments: loan term in years × 12.

Total interest is estimated as monthly payment × n − P. The calculator displays the payment to cents and uses that displayed payment to calculate total interest. Small differences from a lender’s figures can occur because a real payment schedule may use different rounding, dates, or fee treatment.

Worked Example

Using the calculator’s default inputs—$20,000 principal, 8% APR, and a 5-year term—the loan has 60 monthly payments. The monthly rate is 8 ÷ 1,200 = 0.0066667, or about 0.6667%.

Substitute those values into the payment formula:

Payment = 20,000 × 0.0066667 ÷ [1 − (1.0066667)−60]

The result is approximately $405.53 per month when rounded to cents. Using the rounded payment, estimated total interest is $405.53 × 60 − $20,000 = $4,331.80. The corresponding total of scheduled payments is $24,331.80.

Assumptions

  • The interest rate is fixed for the entire term.
  • Payments are equal, made monthly, and begin one month after the loan is issued.
  • Interest is modelled monthly at APR ÷ 12.
  • Fees, taxes, insurance, escrow, and other charges are not included unless they are reflected in the loan amount or rate entered.

When to Use the Calculator

This estimate can help you understand how the principal, rate, and term affect a level-payment installment loan. Installment loans are repaid through scheduled payments over a set term; examples can include personal, vehicle, and education loans, though actual contract terms differ. The calculator is useful for checking the arithmetic in a quoted payment or comparing hypothetical combinations of inputs. It does not determine whether an application will be approved or whether a payment fits a particular budget.

Use the figures from the lender’s disclosure or loan offer when available. In particular, enter the amount actually financed, not necessarily the item’s advertised price or the amount you originally planned to borrow. If charges are added to the balance, they can affect the payment even when they are not listed as a separate monthly cost.

How to Read the Results

The monthly payment is the level amount implied by the inputs under the assumptions above. In an amortizing loan, each scheduled payment covers interest accrued for the period and reduces principal. Early in the term, the interest portion is generally larger because the outstanding balance is larger; as principal falls, the interest portion falls. The payment remains level in this simplified fixed-rate model.

Total interest is the estimated sum of interest over all scheduled payments. It is not the total amount repaid: add the original principal to total interest to get the estimated payment total. A longer term often lowers the required monthly payment for the same principal and rate, while increasing the number of periods over which interest accrues. A higher rate generally raises both the payment and total interest, all else equal.

Common Errors and Limits

  • Entering a monthly rate as APR: The input is annual APR as a percentage, such as 8 for 8%, not 0.08 and not the monthly rate.
  • Confusing term units: Enter years. A five-year term is 5, which the calculator converts to 60 months.
  • Using the wrong principal: The financed balance may differ from the cash price because of a down payment, trade-in, or financed charges. Use the actual balance being amortized.
  • Expecting an exact statement amount: The formula assumes regular monthly periods and a fixed rate. Payment dates, daily interest calculations, rounding, fees, and contract-specific rules can change actual amounts.

This calculator does not model variable rates, skipped or irregular payments, extra principal payments, late charges, balloon payments, or a final adjusted payment. It also treats the entered APR as the rate used in the payment formula; APR disclosures can incorporate costs differently from a simple interest rate. To model a loan with any of these features, use the lender’s detailed schedule or a calculator that includes the relevant inputs.

Frequently Asked Questions

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

Does the monthly payment include interest?
Yes. Under the standard installment formula, each payment includes that month’s interest and a principal portion. The proportions change as the outstanding balance declines.

Why is my lender’s payment different?
The lender may use a different financed amount, payment start date, rate convention, rounding method, or include charges that this estimate omits. Check the loan disclosure and payment schedule for the contract-specific calculation.

Does a longer loan term reduce total interest?
Not necessarily. For the same principal and rate, extending the term generally spreads repayment across more months and can increase total interest, even though the scheduled monthly payment is often lower.

Can I use this for a variable-rate loan?
This calculation assumes a fixed rate. A variable-rate loan can change its payment or repayment schedule when its rate changes, so this estimate cannot predict those future changes.

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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