Business and Financial Tools
Installment Payment and Interest Calculator
Enter a purchase price, down payment, annual rate and number of months, then choose flat-rate or reducing-balance interest to see the installment, total interest, total repayment and estimated schedule.
Two methods · one explicit assumption set
Plan an installment schedule
Select the method stated by the lender; the same numeric rate does not mean the same borrowing cost.
Flat Rate
Estimated payment schedule
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| No. | Payment | Interest | Principal | Balance |
|---|
Quick answer
How is flat-rate interest different from reducing-balance interest?
Flat-rate interest is calculated from the original financed principal for the full term, while reducing-balance interest is calculated from the outstanding principal each period. The same numeric rate must not be compared directly across methods. This calculator separates the methods and shows its formula, schedule and satang rounding, but remains an estimate before fees, taxes, insurance and contract-specific terms.
How to use this tool
- Choose Flat rate or Reducing balance to match the lender document.
- Enter the price, down payment, annual rate and 1–360 installments, then check the financed principal and visible formula.
- Compare the estimate with the quotation after adding fees, taxes, insurance, advance payments and settlement rules omitted here.
How the calculation works
Financed principal equals purchase price minus down payment. Flat-rate total interest is principal times annual rate times months divided by 12; principal and interest are allocated across installments with deterministic one-satang remainder distribution. Reducing-balance mode derives a level payment from a monthly rate, calculates each period's interest from the outstanding balance, rounds half up to one satang, and adjusts the final installment to close the balance. The entered annual rate belongs to the selected method and excludes all other cash flows.
Examples
| Example | Input | Expected result |
|---|---|---|
| Legacy flat-rate example | |
THB 5,000 financed, THB 125 total interest, THB 5,125 financed repayment and THB 512.50 per installment |
| Five-year reducing balance | |
A regular payment of about THB 9,666.40; the final payment may differ slightly because each period is rounded |
| Zero interest | |
THB 12,000 total repayment and THB 1,000 per installment in either method |
Limitations
- This is an educational planning calculator, not a credit offer or financial, legal or accounting advice.
- Price is capped at THB 999,999,999,999.99; down payment cannot exceed price; rate is 0–100%; and term is 1–360 installments.
- Flat mode holds original principal constant. Reducing-balance mode assumes a fixed rate, on-time monthly payments and no extra payments.
- VAT, fees, insurance, deposits, advance installments, balloon payments, floating rates, actual dates, default interest and early settlement are excluded.
Frequently asked questions
Is a 3% flat rate the same as a 3% reducing-balance rate?
No. Flat rate uses the original principal for the entire term; reducing balance uses the outstanding principal. The Bank of Thailand recommends checking both the calculation method and effective rate before comparison.
Why can the final installment differ?
Interest and payments are rounded to one satang each period. The final installment is adjusted to the remaining principal and interest so the schedule closes at zero.
Can this replace a quotation or contract?
No. It excludes fees, taxes, insurance, deposits, advance installments, floating rates, actual payment dates, default, prepayment and early-settlement rules. Use the lender's documents for a decision.
Does the website store my price or debt values?
No. Calculation and file generation happen in the browser; values are neither sent to the server nor stored in browser storage.
References
- Bank of Thailand — Flat-rate loan interest Verified 2026-07-31
- Bank of Thailand — Reducing-balance loan interest Verified 2026-07-31
- Bank of Thailand — Factors to consider before taking debt Verified 2026-07-31
- Legacy THcount — Installment Calculator Verified 2026-07-31
Content and calculation method last reviewed: