It also handles the detail people forget: rounding. When a total does not divide evenly, the calculator keeps every installment identical and adjusts only the last one by a few satang, so the schedule adds up to the exact amount owed.
How it is calculated
Installment = (T + T × f) ÷ n, with the final installment adjusted so the total is exact
T is the total amount owed before interest.
f is the optional flat interest rate for the whole plan, as a decimal — 4% is 0.04. Flat interest is charged once on the full amount, not per period.
n is the number of installments.
Each installment is rounded to the nearest satang (or smallest currency unit); the final installment absorbs the rounding difference.
For zero interest, the formula is simply T ÷ n.
Example
A customer owes your shop 25,000 THB and you agree on 8 monthly installments with 4% flat interest. The total becomes 25,000 + 25,000 × 0.04 = 26,000 THB, which divides evenly into 8 payments of 3,250 THB each.
Rounding case: splitting 10,000 THB into 3 equal installments gives 3,333.33 THB per installment, but 3 × 3,333.33 = 9,999.99 THB. The schedule therefore runs 3,333.33, 3,333.33, and a final installment of 3,333.34 THB so the borrower repays exactly 10,000 THB.
Understanding your result
The per-installment amount is what goes into the installment payment agreement, alongside the number of payments and the due date of each. Copy the figures exactly — including the adjusted final installment — so the written plan matches the math.
If you added flat interest, record the total with interest in the agreement too. Both sides should see the same final number, not just a rate.
Pair the amounts with dates using a payment schedule, and have both parties sign it. A dated, signed schedule is the single most useful document when a payment is later missed or disputed.
Frequently asked questions
What is flat interest and how is it different from a monthly rate?
Flat interest is a one-time percentage added to the whole amount, regardless of how long repayment takes — 4% flat on 25,000 THB is always 1,000 THB. A monthly rate keeps charging as time passes, so it grows with the term. Flat interest is simpler and common in informal installment plans.
Why is my last installment slightly different from the others?
Because most totals do not divide evenly. Rather than using awkward repeating decimals, the calculator rounds each installment to the nearest satang and corrects the difference — usually a few satang — in the final payment, so the plan totals exactly what is owed.
Can I use this for an existing debt someone already owes me?
Yes, that is one of its main uses. When a borrower cannot repay a lump sum, converting the debt into written installments recovers the money gradually instead of not at all. Combine the schedule with a debt acknowledgment so the amount owed is confirmed in writing.
How many installments should I choose?
Pick the shortest plan the payer can realistically afford — fewer installments mean less can go wrong. Check the resulting amount against their monthly budget: an installment they can only just cover will eventually be missed. There is no legal standard; it is whatever both sides agree in writing.
Should installments be weekly or monthly?
Match the payer's income. People paid monthly should pay monthly, ideally a day or two after their salary arrives; people with weekly or daily income often find smaller weekly installments easier. The calculator works the same either way — only the number of installments changes.
What should the written agreement say about missed installments?
Decide in advance and write it down: how many days of grace, whether late interest applies, and whether the whole remaining balance becomes due after repeated misses. Agreeing on consequences while everyone is still friendly is far easier than negotiating them mid-dispute.
Results are estimates for general information only and may differ from a lender's exact figures. They are not financial or legal advice.