See True Cost Upfront
Reveals the exact total amount paid across all installment periods, preventing surprises from flat-rate interest accumulation.
Installment & Instalment Calculator
Calculate monthly instalment payments, total flat-rate interest, and total purchase cost from a monthly rate, down payment, and term.
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Retail Flat-Rate Installment Formula
Last reviewed: September 2026Calculation Formula
Financed Principal = Purchase Price × (1 - Down Payment %)
Monthly Payment = (Financed Principal ÷ n) + (Financed Principal × Monthly Rate)
Total Repayment = Down Payment + (Monthly Payment × n)
Variables & Definitions
- Purchase Price:
- Total sticker price of the retail merchandise or service
- Financed Principal:
- Remaining balance after deducting your upfront cash down payment
- n:
- Number of monthly installment payment periods (3 to 36 months)
- Monthly Rate:
- Flat interest percentage assessed per month on the original financed balance
- Total Interest:
- Financed Principal × Monthly Flat Rate × n
Core Assumptions
- Flat-rate interest convention common in consumer electronics, furniture, and retail installment plans.
- Interest is calculated each month on the initial financed balance rather than a declining balance.
- Payments are made in equal monthly installments over the chosen duration.
- Down payment is settled immediately at point of sale.
Disclaimer: Retail installment contracts that charge flat monthly interest produce an effective APR roughly 1.7× to 1.9× higher than the stated flat rate because you pay interest on the full principal even after repaying part of it. Always compare flat installment quotes against standard amortized loans. Results are informational estimates.
Worked Example: $1,200 Electronics Purchase (20% Down, 12 Months @ 1.5% Flat)
A consumer buys a $1,200 laptop on a 12-month installment plan, paying 20% down at purchase with a 1.5% flat monthly finance rate:
| Down Payment (20% upfront) | $240.00 cash at checkout |
| Financed Principal Balance | $960.00 ($1,200 - $240) |
| Monthly Principal Component ($960 ÷ 12) | $80.00 per month |
| Monthly Flat Interest ($960 × 1.5%) | $14.40 per month |
| Total Monthly Installment | $94.40 per month |
| Total Lifetime Interest Paid ($14.40 × 12) | $172.80 in finance charges |
| Total Purchase Outflow | $1,372.80 ($240 down + $1,132.80 installments) |
💡 Although advertised as "just 1.5% per month," paying flat interest on the full starting balance results in an effective APR of approximately 27.7%, highlighting the importance of calculating total finance cost before choosing an installment plan.
How to use Installment & Instalment Calculator
- 1
Enter the total retail purchase price of the product or equipment.
- 2
Set your upfront down payment percentage (from 0% up to 90%).
- 3
Enter the retailer flat monthly interest rate and select an installment term (from 3 to 36 months).
- 4
Review your monthly installment payment, flat interest charges, and total overall purchase cost.
Why use this tool?
Flexible Term & Down Payment Modeling
Test 3, 6, 12, 24, or 36-month terms with custom down payments to find an installment plan that fits your monthly budget.
Flat Rate vs Declining Balance Insight
Clearly breaks down the principal repayment and monthly interest so you can evaluate consumer installment financing against credit cards or personal loans.
Continue your workflow
Want to see if an amortized personal bank loan is cheaper than a retail installment plan? Use the Compare against a declining-balance personal loan.
Evaluating competing quotes from multiple merchants or BNPL providers? Try the Compare 3 competing financing offers.
Weighing an upfront cash discount against an installment promotion? Use the Calculate cash discounts vs financing.
Frequently asked questions
Is it spelled installment calculator or instalment calculator?
"Installment" (with double l) is standard American English, while "instalment" (with single l) is standard in British, Australian, Canadian, and Commonwealth English. Both refer to the identical monthly installment repayment calculation for retail purchases and Buy Now Pay Later (BNPL) plans.
What is the difference between flat-rate interest and declining-balance interest?
In a flat-rate installment plan, interest is calculated on the original starting principal for every single month, regardless of how much you have already repaid. In a declining-balance (amortized) loan, interest is recalculated each month only on the remaining unpaid balance, making declining-balance loans significantly cheaper for identical nominal rates.
How do I calculate the effective APR from a flat monthly interest rate?
A rough rule of thumb for a 12-month flat installment plan is to multiply the annual flat rate (monthly flat rate × 12) by approximately 1.85. For instance, a 1.5% flat monthly rate equals 18% nominal annual interest, but translates to an effective APR of roughly 27.7% because the average debt balance over the year is only about half the starting principal.
Are Buy Now Pay Later (BNPL) 0% interest plans really free?
True 0% APR plans (like "Pay in 4") are free of interest if every scheduled installment is made on time. However, many 12-to-36-month merchant plans charge flat monthly interest or tack on mandatory processing and late fees if a payment is missed. Always review the merchant terms before agreeing to financing.
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