Can somebody tell me how Thai banks calculate the rent to pay for the loan?

mmm I believe that there is no 'rent' required for loans in Thailand

On the other hand if you are talking about INTEREST, then it would depend on what kind of loan. Is it for short-term finance or for a home loan ?

Term mortgages are calculated on a reducing table basis, same manner as western mortgages; however 2 main differences in setting repayments. 1; they typically set the amount to repay as a fixed amount that doesn't vary rather than variable - e.g. US loan repayment amounts can change each month depending on interest rates - whereas here they tend to set a higher repayment amount, with the extra each month going to repay principal - this results in fewer changes to the repayment amount (supposed to help people be less confused and more able to manage repayments) and (the second major difference) it results in the actual term being reduced - so instead of having say 20 years, if you repay a little more each month then by the end you will have repaid in say 18 years instead of 20. Most banks also offer introductory low rates which may or may not result in lower repayments. prepayment (refinance) within 3 years has penalty charges.

The pmt function in excel will generate with a loan repayment calculation.