Installment True APR Calculator

Use this installment plan true APR calculator to turn a flat handling fee or 0% installment offer into a real annualized borrowing cost. It helps you compare BNPL real interest rate offers with a simple APR threshold before you commit.

Inputs

Cash price

The pay-now price without financing.

Number of installments

Monthly payments in the plan.

Payment per installment

Used first when filled in.

Total amount paid

Used when payment per installment is blank.

Upfront fee

Any fee paid today outside the monthly payments.

Acceptable APR (%)

Your personal cutoff for this purchase.

Verdict

Enter a plan to see the true APR.

True APR0.0%

Total financing cost$0

Monthly payment used$0

Total paid$0

How it works

The calculator treats the cash price as money you receive today, then treats each installment and upfront fee as repayments. It solves the monthly internal rate of return and annualizes that rate to show the true APR.

This is why a flat fee is usually more expensive than it looks: the balance falls every month, but the fee is charged on the original cash price.

Methodology

The calculator treats the cash price as the amount financed, then solves the monthly internal rate of return from the installment payments and any upfront fee. The monthly rate is annualized into an estimated APR.

Assumptions

The default cash price of 10,000, 12 installments, and total paid of 10,300 are example values for testing the model.
The default acceptable APR of 10% is a personal warning threshold, not a recommendation or market benchmark.
Payments are treated as equal monthly payments unless you enter a separate payment per installment.

Example calculation

If a 10,000 cash price is paid back as 10,300 over 12 monthly installments, the flat fee is 300. Because the balance falls during the year, the estimated APR is higher than a simple 3% fee.

Limitations

Late fees, penalty interest, credit-card rewards, taxes, and missed cash discounts are not included.
Real lender APR disclosures may use jurisdiction-specific rules that differ from this simplified estimate.

Last updated: July 7, 2026

Estimate disclaimer: This tool provides estimates only. It is not financial advice. Results depend on the numbers you enter and the simplified assumptions listed here.

This tool runs entirely in your browser. Your input never leaves your device.

Frequently asked questions

Why is a 3% handling fee not a 3% APR?

APR is based on how much principal is outstanding over time. With monthly installments, you repay principal each month, so a flat 3% fee over 12 months can work out near 5.6% APR.

What cash price should I enter?

Use the price you would pay today without financing. The calculator treats that as the amount you are effectively borrowing through the installment plan.

Should I enter payment per installment or total with fee?

Either works. If payment per installment is filled in, the calculator uses it. If it is blank, it divides the total amount paid by the number of installments.

Does this BNPL real interest rate include missed payment fees?

No. Add only known upfront or handling fees. Late fees, penalty interest, and lost discounts can make the real cost higher.

What acceptable APR should I use?

Use the rate you would be comfortable paying for short-term borrowing. The default 10% is a simple warning threshold, not financial advice.