Buy Now Pay Later Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

BNPL: Buy now, pay later is a plan that lets a shopper buy an item now and pay for it in scheduled parts.

Item price: The purchase amount before BNPL fees or interest, unless the plan also finances tax, shipping, or other charges.

First payment timing: Whether the first scheduled BNPL payment is charged on the purchase date or starts later.

Extra paid today besides scheduled payments: Money paid at checkout that lowers the amount put on the BNPL plan, such as a down payment or store credit.

Fee added to each payment: A fixed dollar charge added to every scheduled BNPL payment.

APR: Annual percentage rate. In this calculator, APR is treated as the yearly rate used to estimate interest between scheduled payments; APR can be different from a simple interest rate in real loan disclosures [2].

Estimated due today: The extra paid today plus the first scheduled BNPL payment if the plan starts today.

Cost above the item price: Total paid through the BNPL plan minus the item price. A positive number means fees or interest make the plan cost more than the item price.

Mistake: Putting the full cart total into Item price when sales tax or shipping is not financed by the BNPL plan.
Fix: Use the financed Item price only, or include tax and shipping only if the BNPL agreement includes them.

Mistake: Choosing First payment later when the first installment is charged at checkout.
Fix: Set First payment timing to First payment today so Estimated due today includes that scheduled payment.

Mistake: Counting only future payments in Number of scheduled payments.
Fix: Count every scheduled payment in the plan, including one due today if First payment timing is First payment today.

Mistake: Using 30 for Time between payments when the plan says every two weeks.
Fix: Enter 14 in Time between payments for a biweekly plan.

Mistake: Entering a down payment as Fee added to each payment.
Fix: Put a down payment in Extra paid today besides scheduled payments because it lowers the financed amount.

Mistake: Leaving APR as 0 when the offer charges interest.
Fix: Enter the plan APR so Interest charged by the plan and Cost above the item price include the interest estimate.

• The calculator estimates a level scheduled payment. A BNPL provider may use different rounding, exact calendar dates, late fees, promotional rules, or state-specific rules.
• The APR input is treated as a yearly rate converted to a simple payment-period rate using 365 days. It does not model daily compounding or lender-specific APR rules.
• Sales tax, shipping, discounts, returns, missed payments, and late fees are included only if you put them into Item price or Fee added to each payment.
• Extra paid today besides scheduled payments must reduce the BNPL amount. Do not use it for a fee that does not lower the financed amount.
• Dates are used only to count days until the last scheduled payment, not to adjust for weekends, holidays, or month lengths.

How the estimate is built

The calculator first subtracts Extra paid today besides scheduled payments from Item price to find the financed amount.

financed_amount = purchase_price - extra_due_today

APR is converted into a rate for one payment gap using the Time between payments value and a 365-day planning year.

period_rate = (apr_percent / 100) * (days_between_payments / 365)

If APR is 0, the base scheduled payment is the financed amount split evenly across the Number of scheduled payments.

base_payment = financed_amount / number_payments

If APR is above 0 and First payment timing is First payment later, the calculator uses the standard level-payment loan formula.

base_payment = financed_amount * period_rate / (1 - (1 + period_rate)^(-number_payments))

If APR is above 0 and First payment timing is First payment today, the first base payment is made immediately, so there is one fewer interest period after purchase. If there is only 1 scheduled payment, the base payment equals the financed amount.

base_payment = financed_amount / (1 + (1 - (1 + period_rate)^(-(number_payments - 1))) / period_rate)

The fixed fee is then added to each scheduled payment.

each_scheduled_bnpl_payment = base_payment + fee_per_payment

Total paid, fees, interest, and the extra cost are calculated from unrounded values before money is displayed to the nearest cent.

total_paid_for_plan = extra_due_today + each_scheduled_bnpl_payment * number_payments

plan_fees_charged = fee_per_payment * number_payments

loan_interest_charged = base_payment * number_payments - financed_amount

cost_above_item_price = total_paid_for_plan - purchase_price

Estimated due today adds the extra amount paid today and, only when First payment timing is First payment today, the first scheduled BNPL payment.

estimated_due_today = extra_due_today + first_payment_due_today

payments_left_after_today = number_payments - payments_made_today

days_until_last_payment = days_between_payments * payment_periods_after_purchase

Mini-example

For a $400 item, 4 scheduled payments, First payment today, 14 days between payments, no extra paid today, no fee, and 0% APR, the financed amount is $400. The base payment is $400 / 4 = $100. Estimated due today is $100, Total paid through the BNPL plan is $400, Scheduled payments left after today is 3, and Days until the last scheduled payment is 14 x (4 - 1) = 42 days.

Calculation choices

The calculator keeps internal amounts unrounded until the final display. APR is used only to estimate interest for the payment spacing entered by the user. Fixed fees are treated as cash charges added to payments, not as principal that reduces the financed amount.