Fixed Installments (Flat Rate) Calculator
The Fixed Installments (Flat Rate) Calculator estimates a loan and interest estimate for your scenario from the quantities, rates, period, units and other assumptions requested on the page. Replace the example values with your own fixed installments flat rate figures and recalculate after changing an assumption. Use it to compare scenarios, then verify any current rate or fee before acting.
Enter the loan, flat rate interest and number of months to see the installment, total interest and actual interest rate estimates before you decide to accept the offer.
Quick answer.
Calculation of fixed installment: 800,000 baht 3.5% 60 monthly installment 15,666.67 baht/month
Total interest 140,000 baht Total payment 940,000 baht and converted to EIR is about 6.54% per year. This figure makes it easier to compare with other effective rate offers.
Use this answer as a starting point and enter the numbers from the actual offer in the form below.
Flat Rate Offering
Look at the numbers you should ask before accepting a fixed-rate offer.
If the offer is flat rate, look at the installment, total interest, contract and EIR payments, and estimates. Don't compare the flat rate figures with the effective rate.
Checklist 4 before accepting the offer
1. Really.
What's the total amount after down payment and discounts?
Use this to calculate the installment, not the full price of the car if you have a down payment.
2. Total interest.
How many baht interest is paid during the contract?
Flat rate. The figures look low, but interest is calculated on the full principal for the duration of the contract.
3. EIR or real interest.
The contract or offer specifies the effective rate per year.
Compared to other proposals that are closer to the initial interest rate.
4. Non-interest expenses.
Any fees, insurance, registration, or early closure fees?
Actual cost may be higher than the instalment value shown in the first line.
Calculate immediately
Fixed monthly payments.
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Packages sent to contractors or consultants.
After calculating, copy this result and ask about EIR, other expenses, and early closure terms.
Ready after calculation.
The gate before the offer.
Use this calculation to check the fixed-rate offer before signing.
- You're a witch.
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EIR approx.
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Total Interest
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After calculating, the system will summarize the points to ask the contractor and send the numbers to calculate the car loan.
EIR in contract.
Have the contractor confirm the EIR or actual interest rate calculation in writing, not just using flat rate figures.
Other expenses
Ask about fees, pick-up fees, insurance, or anything that's not included in the monthly payment.
Early closure of account.
Ask about interest, discounts, or fines if you're forced to close your account before the contract expires.
Use the real offer from the finance or car dealer before you decide.
What is a flat rate?
Flat Rate is the calculation of interest from the full principal amount for the duration of the contract. The rate is equal every month. Formula: Principal = (principal + principal x flat rate x year) / number of instalments. This tool converts the result to an approximate EIR to help compare with the offer that specifies the effective rate.
The Bank of Thailand explains that flat rate is used for car or motorcycle leasing and comparison with effective rate must be converted to the same method.
Frequently asked questions
Flat Rate 3.5% per year 60 month equivalent EIR how much?
Why do you have to convert Flat Rate to EIR before comparing?
Can it be used for a car loan?
Official reference source
English calculator guide
How to use the Fixed Installments (Flat Rate) Calculator
Use this page to test a specific fixed installments flat rate scenario. Enter your own values in the calculator above, review the units beside each field, and recalculate after changing an assumption. The result updates the fixed installments (flat rate) estimate instead of relying on a generic example.
- Build a baseline fixed installments flat rate case with figures that match your situation.
- Run a second fixed installments flat rate case after changing the rate, period, quantity or other key assumption.
- Record the difference between the two fixed installments flat rate results and the input responsible for it.
Inputs for the fixed installments flat rate estimate
For this fixed installments flat rate calculation, check the quantities, rates, period, units and other assumptions requested on the page. The result is designed to show a loan and interest estimate for your scenario, not a guaranteed provider price or official decision.
How the fixed installments flat rate result is calculated
The calculator models principal, interest, repayment timing and term using the assumptions shown on the page. Reusing the same fixed installments flat rate inputs will produce the same estimate, which makes the page suitable for controlled scenario comparisons.
Checks for a fixed installments flat rate decision
Confirm any current rate, threshold or provider rule that could change the final amount. For fixed installments flat rate, Use the result to compare affordability, not as a lending offer. Check the lender’s effective rate, fees, insurance, payment schedule and approval conditions before committing. Check the important inputs and any current provider or authority rules before using the result for a decision.
- Language QA
- Terminology, numeric values and calculator controls checked
- Calculator scope
- Loan and interest
- Last English review
- 26 July 2026