🟢 Why “3% Interest” Can Mean Very Different Things
When you take out a loan — especially a car loan — the interest rate you are quoted may not be what you think. A “3% interest rate” can mean very different things depending on whether it is a flat rate or an effective rate. The difference can cost you tens of thousands of baht.
This article explains how each rate works, why flat rates look cheaper than they are, and how to compare them before signing anything.
What Is a Flat Rate?
A flat rate calculates interest on the original loan amount for the entire duration of the loan — regardless of how much you have already paid back.
Here is how it works on an ฿800,000 car loan at 3% flat rate over 5 years:
Interest = ฿800,000 × 3% × 5 years = ฿120,000
Total repayment = ฿800,000 + ฿120,000 = ฿920,000
Monthly payment = ฿920,000 ÷ 60 months = ฿15,333The calculation is simple — and that is exactly why flat rates are popular with lenders. They are easy to explain and the monthly payment is straightforward to calculate.
But there is a problem.
Why Flat Rates Are Misleading
When you make monthly payments, your outstanding balance decreases every month. After one year of payments, you no longer owe ฿800,000 — you owe significantly less.
But a flat rate ignores this. It keeps charging you interest on the full ฿800,000 for all 5 years, even though your actual debt is shrinking with every payment.
This means you are paying interest on money you have already paid back.
What Is an Effective Rate?
An effective rate calculates interest on the remaining balance each month. As you pay down the loan, the amount of interest you owe decreases too.
This is the standard method used by banks and financial tools for calculating the true cost of borrowing.
At 3% effective rate on the same ฿800,000 loan over 5 years:
Monthly payment: ฿14,361
Total interest: ฿61,634
Total repayment: ฿861,634Compare that to 3% flat rate:
Monthly payment: ฿15,333
Total interest: ฿120,000
Total repayment: ฿920,000Same “3%” — but the flat rate costs you ฿58,366 more in interest. That is nearly double.
A Note on How “Effective Rate” Is Calculated
Not all “effective rates” are calculated the same way, and this matters if you’re checking numbers against your own bank quote. There are two common conventions:
- Nominal rate (APR-style): the monthly rate is simply the quoted annual rate divided by 12 (e.g. 3% ÷ 12 = 0.25% per month). This is the convention most Thai lenders use in practice when they quote a monthly amortization schedule.
- True effective annual rate (EAR): the quoted annual rate already reflects compounding, so the monthly rate is derived as (1 + annual rate)^(1/12) − 1, which for 3% works out to roughly 0.247% per month.
The figures used in this article (฿14,361 monthly payment, ฿61,634 total interest) are calculated using the EAR convention. The two methods produce slightly different results — for this loan, the nominal/APR convention would give a monthly payment closer to ฿14,373 and total interest closer to ฿62,356. The gap is small, but if you’re comparing our numbers to a bank’s amortization table, ask which convention they use before assuming a discrepancy is an error.
The Real Comparison: What Does a Flat Rate Actually Equal?
A 3% flat rate on a 5-year loan is equivalent to approximately 5.79% effective rate. That is the true annual cost of the loan.
Here is a quick reference for common flat rates on 5-year loans:
Flat Rate Effective Equivalent
1.0% ~1.9%
2.0% ~3.8%
3.0% ~5.8%
4.0% ~7.7%
5.0% ~9.6%The rough rule: for a typical 5-year loan, the effective equivalent is roughly 1.9× the quoted flat rate. So when a dealer says “only 3%,” the real cost is closer to 6%.
Where You Encounter Each Rate
Flat rates are commonly used for:
- Car loans (hire purchase) — this is the most common place you will see flat rates
- Some personal loans from non-bank lenders
- Promotional financing offers
Effective rates are used by:
- Banks for mortgage loans
- Credit cards (interest on outstanding balance)
- Investment calculations and financial tools
- Regulatory disclosures (where required)
When a lender quotes you a rate, always ask: is this a flat rate or an effective rate? If they cannot answer clearly, that itself is a warning sign.
How to Protect Yourself
Always compare total cost, not just the rate. Two loans with the same headline “interest rate” can have very different total costs. The number that matters is: how much will I pay in total, and how much of that is interest?
Ask for the effective rate. If a lender only quotes a flat rate, ask them to provide the effective equivalent. If they refuse or cannot, you can calculate it yourself using our Loan Calculator.
Compare across lenders using the same rate type. Comparing a 3% flat rate from one lender with a 5% effective rate from another is meaningless — they might actually cost the same. Convert both to effective rates before comparing.
Read the full contract. Additional fees (processing fees, insurance requirements, early repayment penalties) can significantly increase the true cost beyond what any interest rate — flat or effective — suggests.
Key Takeaways
- A flat rate charges interest on the original loan amount for the entire period — even as you pay it down
- An effective rate charges interest on the remaining balance — decreasing as you repay
- A 3% flat rate on a 5-year loan equals approximately 5.79% effective — nearly double what it sounds like
- The flat rate makes loans look cheaper than they are — always ask for the effective rate or total cost
- Use the same rate type when comparing loans from different lenders
- Our Loan Calculator shows both rates side by side for any loan you are considering
Frequently Asked Questions
Why do lenders use flat rates if they are misleading?
Flat rates produce a lower headline number, which makes the loan more attractive. A “3% flat rate” sounds much better than “5.8% effective rate” — even though they cost exactly the same. Flat rates are also simpler to calculate, which made them practical before digital tools existed.
Is the flat rate illegal?
No. Flat rates are a legitimate and widely used way to calculate interest. However, some regulators require lenders to also disclose the effective rate so borrowers can make informed comparisons. Always check whether the effective rate is disclosed in your loan agreement.
Does the loan duration affect the conversion?
Yes. The equivalent effective rate depends on the loan term and repayment schedule, so there is no single conversion factor that works for every loan. The 1.9× rule is a useful approximation for 5-year loans, but always use a calculator for exact numbers.
What about credit card interest — is that flat or effective?
Credit card interest is always based on your outstanding balance — not a flat-rate calculation. In Thailand, credit card issuers may generally charge interest, penalties, service charges and related fees of up to 16% per year in aggregate under Bank of Thailand rules. This is why paying off credit card debt is usually the highest financial priority.
→ Try it yourself: Loan Calculator — Compare flat rate and effective rate side by side.