The formula
For a loan of amount P, repaid over n months at a monthly interest rate r (the annual rate divided by 12), the equal monthly payment is:
payment = P × r ÷ (1 − (1 + r)^−n)Worked example
A loan of 100,000,000 Toman over 24 months at 18% a year (an example rate):
- Monthly rate: 18% ÷ 12 = 1.5% = 0.015
- Monthly payment: 4,992,410 Toman
- Total repaid: 119,817,845 Toman
- Total interest: 19,817,845 Toman
Where each payment goes
In month 1, interest is charged on the full balance: 100,000,000 × 0.015 = 1,500,000 Toman. The rest of the payment, 3,492,410 Toman, reduces the balance. Each month the balance is smaller, so the interest part shrinks and the repayment part grows.
What changes the cost
- A longer term lowers the monthly payment but raises the total interest.
- A higher rate raises both.
- Fees and required deposits — some loans require a blocked deposit or charge fees, which raise the real cost. Ask the bank for the full terms.
Tools for this
Examples use illustrative rates. Your bank's contract decides the actual payment — check the rate, fees and any required deposit before you sign.
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Asaiejadoo — everyday calculation and AI guides, part of the ZIBADIS network founded by Masoud Moghaddam in Tehran.
