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01 Loan payments

What Is My Interest? Loan Interest Calculator

Calculator No. 01-2167 · Updated · Formula and assumptions shown below

Monthly payment $405.53
Total interest
$4,331.80
= 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.

This calculator estimates the monthly payment and total interest on a fixed-rate, level-payment loan. Enter the loan amount, annual interest rate (APR), and term in years to see the estimated payment and the interest paid over the full term.

Formula

For a fixed-rate loan repaid in equal monthly installments, the payment is calculated with the standard amortizing-loan formula:

PMT = P × r / [1 − (1 + r)−n]

  • PMT is the monthly payment in dollars, rounded to the nearest cent.
  • P is the starting loan principal in dollars.
  • r is the monthly interest rate as a decimal: annual APR ÷ 1,200. For an 8% APR, r = 8 ÷ 1,200.
  • n is the number of monthly payments: loan term in years × 12.

Total interest is estimated as PMT × n − P. Because the payment is rounded to cents before the total is calculated, the estimate reflects repeated payments of that rounded amount. The formula assumes the regular payment is made monthly and includes both principal and interest.

Worked Example

Using the calculator’s default inputs: a $20,000 loan, 8% APR, and a 5-year term.

  1. Convert the annual APR to a monthly decimal rate: r = 8 ÷ 1,200 = 0.0066666667.
  2. Find the number of payments: n = 5 × 12 = 60.
  3. Substitute in the payment formula: PMT = 20,000 × 0.0066666667 ÷ [1 − (1 + 0.0066666667)−60].
  4. Round the monthly payment to cents: PMT = $405.53.
  5. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The calculator therefore returns an estimated monthly payment of $405.53 and estimated total interest of $4,331.80. The final payment in an actual amortization schedule may differ slightly because of rounding or the lender’s payment and accrual conventions.

Assumptions

  • The interest rate is fixed for the whole term.
  • Payments are equal, made monthly, and treated as occurring at the end of each period.
  • Interest is modelled monthly at APR ÷ 12.
  • The calculation does not include fees, taxes, insurance, PMI, escrow, or other charges; none of these are calculator inputs.
  • The displayed payment is rounded to the nearest cent, and total interest uses that rounded payment for every month.

What “My Interest” Means

In a loan question, “my interest” commonly means the interest cost associated with a particular loan: the amount charged for borrowing, or the rate used to determine that charge. The two are related but are not interchangeable. The APR entered here is a percentage rate per year; total interest is a dollar amount estimated across the repayment term.

Interest is generally calculated on the unpaid principal. Early in a typical amortizing loan, the balance is larger, so more of a level payment goes toward interest. As the principal is paid down, the interest portion usually falls and the principal portion rises. For example, an $800 payment is not automatically $800 of principal reduction: the interest due for that period is applied first under the loan’s terms, and the remainder reduces principal.

“My interest” can also mean an ownership share or a personal concern, as in “a financial interest in a business” or “I have an interest in learning.” Those are separate meanings of the word and are not calculated by this loan tool.

How to Use and Read the Estimate

Enter the principal you expect to borrow, the APR supplied for the loan, and the repayment term in years. The calculator uses those inputs to estimate a level monthly payment and the total interest over the scheduled number of payments. Use the lender’s actual quoted figures when available; a changed APR or term changes the result.

  • Monthly payment is the calculated recurring principal-and-interest amount, not necessarily the full amount due on a bill.
  • Total interest is the estimated interest component over the term, not the total amount repaid. Estimated principal plus interest is the loan amount plus the displayed interest.
  • APR is entered as a percentage, such as 8 for 8%, not as 0.08. The formula converts it to a monthly rate.

The result is a mathematical estimate, not a bill or offer. A lender’s quoted payment can differ if its APR, payment dates, fees, final-payment adjustment, or calculation conventions differ from the assumptions here.

Common Mistakes and Limits

A frequent error is entering a monthly rate as though it were an annual APR, or entering 0.08 when the field expects 8 for 8%. Another is treating the term as months: this calculator asks for years and converts them to monthly payments. Check that the loan amount, rate, and term all describe the same loan scenario.

The total-interest result is not a general-purpose measure of every borrowing cost. It excludes charges that are not entered, and it assumes a fixed rate and regular on-time payments. It does not model variable rates, skipped or late payments, extra principal payments, a changing balance, or a payoff before the scheduled end. For those cases, the balance and interest path would need to be recalculated using the actual payment history and loan terms.

Rounding also matters. The calculator rounds the monthly payment to cents and multiplies that rounded amount by the number of payments, as specified by its model. A real account may adjust the final payment to clear the remaining balance, so its total can differ by a small amount.

Frequently Asked Questions

What does “my interest” mean on a loan?
It may refer to the interest rate charged or the total dollar cost of borrowing. In this calculator, APR is the rate input and total interest is the estimated dollar output.

How do I calculate the interest on my loan?
For a fixed-rate loan with equal monthly payments, first calculate the payment from the principal, monthly rate, and number of payments. Then subtract the original principal from the sum of the rounded monthly payments. This calculator performs those steps.

Is APR the same as total interest?
No. APR is a rate expressed as a percentage per year; total interest is a dollar amount over a specified repayment period. The APR alone is not enough to determine the total without the loan amount and term.

Why does the lender’s payment differ from this estimate?
The lender may use different quoted terms, fees, payment dates, rounding, or final-payment treatment. This model assumes a fixed rate, monthly payments, and no additional charges.

Does the monthly payment include taxes and insurance?
No. The calculated payment covers modeled principal and interest only. Taxes, insurance, PMI, escrow, and fees are not included.

This result is an estimate; 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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 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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