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How to Calculate a Loan Payment (Home & Car Loans)

5 min readUpdated 5 October 2026

To calculate a loan payment, you need three numbers: the amount you borrow, the interest rate and the term. Put them into Loan Calculator, choose Home / Personal for a reducing-balance loan or Car (flat rate) for Thai hire purchase, and you see the Monthly payment, Total interest and the month the loan is Paid off.

It is free, needs no account and works on any phone or computer, so you can try a bigger down payment or a shorter term in seconds. Your figures stay on your device. The result is an estimate: banks round, charge fees and change floating rates in their own way, so always check the final numbers in your lender's offer.

Do it now with Loan CalculatorFree, no sign-up, works on any device.

Step by step

  1. 1Choose the loan typeOpen Loan Calculator and pick Home / Personal (reducing balance — interest on what you still owe) or Car (flat rate) (interest fixed on the original amount). The currency starts on THB and can be changed.
  2. 2Enter how much you borrowUse Price & down to type the Price and Down payment in baht or %, and the tool shows the amount you are financing. Or switch to Loan amount and type it directly.
  3. 3Add the rate and termType the Interest rate (% per year), or the Flat rate for a car loan, and the Term in years or months. Car loans also get quick 3–7 year buttons. Set First payment to date the schedule.
  4. 4Add stepped rates or extra payments (optional)For home and personal loans, turn on Stepped rates for a promotional rate that later floats, or Extra payments (โปะ) to pay a fixed extra each month or one-off lump sums at a chosen payment number.
  5. 5Read the results and scheduleSee Monthly payment, Total interest, Total paid and Paid off, plus a chart of principal against interest by year. The Amortisation schedule opens year by year; download it as CSV or use Copy summary.

Reducing-balance loans: the formula and an example

Home loans and most personal loans charge interest each month on the balance you still owe. The payment that clears the loan exactly on time is payment = P × r ÷ (1 − (1 + r)^−n), where P is the loan amount, r the yearly rate ÷ 12 and n the number of monthly payments.

Example — the one the tool opens with: a ฿3,500,000 home with 10% down means borrowing ฿3,150,000. At 6% over 30 years the payment is about ฿18,886 a month, and the total interest comes to about ฿3.65 million. In the first month ฿15,750 of the payment is interest, which is why the balance falls slowly at the start and faster later on.

Tip: The schedule shows this clearly: open Year 1 and then the last year, and compare the Interest column.

Thai car loans: how the flat rate works

Thai hire-purchase loans (ดอกเบี้ยคงที่) work differently. Interest is loan amount × flat rate × years, fixed for the whole contract, and the instalment is (loan amount + interest) ÷ number of months.

Example: a ฿800,000 car with 25% down leaves ฿600,000 to finance. At 2.5% flat for 5 years, interest is ฿75,000 and the instalment is ฿11,250 a month. Because you keep paying interest on money you have already paid back, the tool also shows the Effective rate (APR) — about 4.73% a year on a reducing balance for this example.

Tip: Compare loans by effective rate, not by headline rate: a flat rate is roughly half the equivalent reducing-balance rate.

Stepped rates and extra payments

Many home loans start on a fixed rate for a few years and then float (for example MRR minus a margin). With Stepped rates on, you can set up to three tiers; the tool recalculates the payment for the remaining term each time the rate changes and shows Avg. rate, first 3 years, the figure banks often advertise.

With Extra payments (โปะ), your regular instalment stays the same and the loan simply ends sooner. In the example above, an extra ฿2,000 a month saves roughly ฿920,000 in interest and finishes about six and a half years early — the tool shows this as Interest saved and Time saved.

Extra payments and stepped rates apply to reducing-balance loans only. For a flat-rate car loan, ask the lender for an early payoff quote; the tool notes that Thai rules give a discount on interest not yet due.

Compare two or three loans

The Compare tab puts 2–3 loans side by side, each as reducing or flat, with the monthly payment, total interest, total paid and effective rate. The cheapest overall gets a Lowest total cost badge, with the difference spelled out.

For example, ฿1,000,000 at 6% costs about ฿5,996 a month over 30 years but ฿7,164 over 20 years. The shorter loan costs more each month, yet saves around ฿440,000 in interest. Seeing both numbers together makes the trade-off easier to judge.

  • Thai Tax Estimator — home loan interest can reduce your income tax, up to ฿100,000 a year; take the yearly interest from the schedule.
  • Percent & Discounts — work out a down payment percentage or a price discount.
  • Currency Converter — check a foreign-currency loan in baht.

Questions people ask

Why is my bank's figure slightly different?
Banks round, count days and charge fees in their own way, and floating rates change over time. Use the tool to plan and compare, then check the exact payment in your lender's offer or contract.
Is a 2.5% flat rate the same as a 2.5% interest rate?
No. A flat rate charges interest on the full original amount for the whole term, so it costs more than the same reducing rate. The tool's Effective rate (APR) shows the reducing-balance equivalent.
Does paying extra lower my monthly payment?
In this calculator, no: the instalment stays the same and the loan ends earlier, which is what saves the interest. Some banks offer to lower the instalment instead, so ask yours which option applies.
Can I export the payment schedule?
Yes. Press CSV above the Amortisation schedule to download every payment with its date, rate, principal, interest, extra and balance, ready for Excel or Google Sheets.
Do it now with Loan CalculatorFree, no sign-up, works on any device.

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