Loan Interest Estimate Calculator and Formula

This calculator estimates the monthly payment and total interest for a fixed-rate, fully amortizing loan. Enter the amount borrowed, annual percentage rate (APR), and term in years; it returns the monthly payment and the interest paid over the full term, using a payment rounded to cents.

Formula

The calculator first finds a level monthly payment, then subtracts the amount borrowed from the sum of the rounded monthly payments to estimate total interest.

• Monthly payment: M = P × r / [1 − (1 + r)⁻ⁿ]
• Total interest: I = M × n − P

Here, P is the loan principal in dollars, r is the monthly interest rate as a decimal (APR ÷ 12 ÷ 100), n is the number of monthly payments (years × 12), M is the monthly payment in dollars, and I is estimated total interest in dollars. The displayed payment is rounded to the nearest cent before total interest is calculated. The APR input is a percentage per year, so an 8% APR becomes a monthly rate of 0.08 ÷ 12, or about 0.006667.

Worked Example

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

1. Convert the APR to a monthly decimal rate: r = 8 ÷ 1200 = 0.0066667.
2. Convert the term to monthly payments: n = 5 × 12 = 60.
3. Apply the payment formula: M = 20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰], which rounds to $405.53 per month.
4. Multiply the rounded payment by the number of payments: $405.53 × 60 = $24,331.80.
5. Subtract the principal: $24,331.80 − $20,000 = $4,331.80 estimated total interest.

Assumptions

• The interest rate stays fixed for the entire term.
• Payments are equal monthly payments, made at the end of each monthly period.
• Interest is modelled monthly at APR ÷ 12; this is a standard estimate and may not match a lender’s exact daily accrual or payment-date method.
• The calculation excludes fees, taxes, insurance, PMI, escrow, and any other charges or optional products.

When to Use a Loan Interest Estimate

Use this calculation to understand the interest component of a straightforward installment loan when you have a principal amount, an APR, and a fixed repayment term. It can help separate the scheduled payment from the cost of borrowing over time. The result is a mathematical estimate, not a lender quote, approval decision, or complete measure of every possible borrowing cost.

The APR and term must describe the same loan scenario you want to estimate. If you have a lender’s written disclosure, use its stated rate and repayment period as inputs. If the rate can change, the term includes a final irregular payment, or payments occur more or less often than monthly, this fixed monthly model will not fully represent the contract.

How to Read the Results

The monthly payment is the scheduled amount the formula calculates for principal and interest. In an amortizing loan, each payment covers interest accrued for the period and reduces principal; early payments generally contain a larger interest share than later ones. The calculator does not show that changing allocation month by month. Total interest is the sum of the rounded scheduled payments minus the original principal, assuming every payment is made as modelled.

For example, the default result means 60 payments of $405.53, or $24,331.80 in total scheduled payments, of which $4,331.80 is estimated interest. It does not mean the borrower will necessarily pay exactly that amount under a real contract: payment dates, rounding conventions, fees, extra payments, and other terms can change the figures.

What Changes the Interest Estimate?

• Principal: A larger amount borrowed generally produces more interest when the rate and term are unchanged.
• APR: A higher input rate increases the modeled payment and interest cost. Enter the rate supplied for the specific loan; do not treat the example default as a market rate.
• Term: A longer term spreads repayment over more months. It can lower the calculated monthly payment while increasing total interest, all else equal.
• Payment behavior: Extra payments, missed payments, or a payoff before the scheduled end change the outstanding balance and actual interest.

These factors interact. A payment amount alone does not reveal the total cost, and a total-interest estimate does not show whether a payment fits a household budget. This calculator does not assess affordability or predict whether a lender will approve an application.

Limits and Common Mistakes

A common input error is entering an APR as a decimal when the calculator expects a percentage: enter 8 for 8%, not 0.08. Another is mixing years and months. This form asks for years and converts the term to months internally. Check that the amount is the principal actually being financed, rather than a purchase price before a down payment or a total that includes charges treated differently by the lender.

Rounding also matters. The calculator rounds the monthly payment to cents and uses that displayed payment to estimate total interest. A lender may calculate interest using unrounded amounts internally, so a small difference is possible. The formula also assumes a fixed rate and regular, equal monthly payments. It is not suitable as an exact payoff quote for a variable-rate loan, an interest-only period, a balloon payment, irregular payment dates, or a loan with fees added to the balance.

APR is an annualized measure disclosed for a particular credit arrangement; this calculator uses it as a nominal annual rate divided by 12. It does not separately model how a lender treats financed fees or other APR components. For the applicable rate, fees, payment schedule, and payoff amount, consult the lender’s disclosure or servicing information rather than inferring contract terms from this estimate.

Frequently Asked Questions

How do I estimate interest on a loan?
For a fixed-rate loan with equal monthly payments, calculate the monthly payment from the principal, monthly rate, and number of payments. Multiply the rounded payment by the number of payments and subtract the principal to estimate total interest.

Does this show total interest or the monthly interest charge?
The interest result is the estimated total over the full scheduled term. The monthly payment includes both principal and interest; the calculator does not break each payment into its monthly interest and principal portions.

Why does a longer loan term affect total interest?
A longer term means more monthly periods in which interest can accrue. It may reduce the payment calculated for each month while increasing the sum of interest across the full schedule, assuming the same principal and rate.

Is APR the same as the interest rate used in the formula?
Not necessarily in every lender’s calculation. This estimate converts the APR input to a monthly rate by dividing by 12. The actual agreement may account for charges, accrual conventions, or timing in ways this simple model does not reproduce.

Can I use this for a variable-rate loan?
Only as a snapshot under the assumption that the entered rate remains fixed throughout the term. If the rate changes, future payments and total interest can change, so this result is not a full variable-rate projection.

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

Related

• Education Loan Interest Rate Calculator and Costs
• Directors’ Loan Interest Rate 2025/26 Explained
• Compound Interest Loan Calculator: Payments and Cost

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