Personal Loan Interest Calculator and Payment Formula

A personal loan interest calculator estimates your monthly payment and total interest from the loan amount, annual percentage rate (APR), and repayment term. Enter the lender’s figures to see the estimated cost of a fixed-rate loan paid monthly; the result is an arithmetic estimate, not a loan offer.

Formula

The calculator uses the standard fixed-payment loan formula. It treats APR as a nominal annual rate divided into 12 monthly periods:

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

P is the amount borrowed in dollars, APR is the annual percentage rate entered as a percent, r is the monthly rate (APR ÷ 1,200), n is the number of monthly payments (years × 12), M is the monthly payment in dollars, and I is total interest in dollars. The calculator displays the payment to the nearest cent and uses that rounded payment to estimate total interest.

Worked Example

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

1. Convert the APR to a monthly rate: 8 ÷ 1,200 = 0.0066667, or about 0.66667% per month.
2. Convert the term to payment periods: 5 × 12 = 60 months.
3. Substitute into the payment formula: 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents.
4. Calculate total interest using the displayed payment: $405.53 × 60 − $20,000 = $4,331.80.

The estimate is therefore a monthly payment of $405.53 and total interest of $4,331.80.

Assumptions

• The interest rate stays fixed throughout repayment.
• Payments are equal, made monthly, and begin one month after the loan is issued.
• Interest is modelled monthly at APR ÷ 12.
• Fees, taxes, insurance, PMI, and escrow are not included. If a lender charges an upfront or financed fee, the amount borrowed or repayment cost may differ.

How to Use the Estimate

Enter the principal you expect to borrow, the APR quoted for the loan, and the repayment term in years. APR should be entered as a percentage: enter 8 for 8%, not 0.08. The calculator returns the scheduled monthly payment and the interest implied by making that payment for every month of the term.

Use the figures from a lender’s written disclosure when available. A preliminary rate, a rate shown before an application, and the final APR may not be the same. The calculator does not determine what rate a person might receive, whether an application will be approved, or how much a lender will offer.

Reading Payment and Interest

The monthly payment combines repayment of principal with interest. Early in a level-payment loan, a larger share of each payment generally goes toward interest; as the balance falls, more goes toward principal. Total interest is the estimated sum of payments above the original principal, under the assumptions shown.

For example, extending a term while holding the amount and rate constant generally lowers the scheduled monthly payment but creates more payment periods. That can increase total interest. A shorter term generally has the opposite mathematical effect on the scheduled payment and total interest. This comparison does not account for a lender changing the APR or fees between loan options.

Limits and Common Mistakes

• Confusing APR with a monthly rate: APR is entered as an annual percentage; the formula divides it by 1,200 to get the monthly rate.
• Using the wrong principal: If a fee is added to the balance, the amount actually financed may exceed the cash received. Use the financed principal to model its payment impact.
• Assuming all borrowing costs are interest: The total-interest result does not add separate charges or account for fees unless they are reflected in the APR or principal used.
• Rounding differences: Payment schedules may adjust the final payment by a small amount because actual lenders calculate and round according to their own procedures. This estimate uses a cent-rounded regular payment.
• Irregular repayment: Extra payments, skipped or late payments, variable rates, and payment dates other than monthly change the balance and total cost. This calculator assumes regular level payments.

The formula is useful for estimating standard amortizing payments, but it is not a payoff schedule or a substitute for the lender’s full disclosure. A lender’s calculations may also reflect fees, timing conventions, and contract terms not represented here.

Frequently Asked Questions

How do I calculate interest on a personal loan?
For a fixed-rate loan with equal monthly payments, calculate the monthly payment using the amortization formula, then multiply that payment by the number of months and subtract the principal. This calculator applies that method and rounds the payment to cents.

Is APR the same as the interest rate?
Not necessarily. APR is an annualized borrowing-cost measure, and its treatment of charges can differ from the stated interest rate. This calculator uses the APR entered as if it were the nominal annual rate for monthly interest calculations, so its result may not reproduce every lender disclosure.

Does a longer loan term reduce total interest?
With the same principal and rate, a longer term usually lowers the scheduled monthly payment but increases the number of periods over which interest accrues. The calculator illustrates this mathematical relationship; actual loan offers may have different rates or costs.

Does this calculator include fees?
No separate fees are added. If a fee is financed, include it in the principal input to estimate its effect on the payment, but that will not model every fee treatment or APR calculation.

Why does my lender’s payment differ?
The lender may use a different APR or principal, include charges, use different rounding or payment dates, or adjust the final payment. Check the loan disclosure and repayment schedule for the contract’s exact figures.

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