Which Home Loan Prepayment Technique Saves the Most Interest and Pays Off Debt Fastest?

Owning a "home" is a dream, but a "home loan" spanning 20–30 years can turn into a nightmare without proper planning. Financial experts often recommend "prepaying the loan," meaning paying more than the bank’s scheduled installment so that the extra amount directly "reduces the principal" quickly.

Because home loan interest is calculated on a "reducing balance" basis (Effective Rate), the faster we reduce the principal, the less interest we pay in the following installments.

This article from Thairath Money compares clearly which method—"monthly extra payments" or "annual lump-sum payments"—helps save more money.

Example case: Regular payments vs extra payments.

Assuming you borrow 3,500,000 baht to buy a home, with a fixed interest rate of 5.00% per year throughout the loan term, and a normal monthly installment of 21,000 baht (total 286 installments).

Why does "monthly prepayment" win?

From the table, even though the total annual amount used is the same—36,000 baht—making an extra 3,000 baht payment each month (Option B) saves 25,546 baht more in interest than a single annual prepayment and shortens the loan term by about 2 installments.

This is because when we prepay monthly, the principal reduces gradually with each installment, which continuously lowers the interest calculated for the next month.

Formula: (Remaining principal × Annual interest rate × Number of days in the month) / 365.

This differs from annual prepayment where the principal drops sharply only at year-end, losing the opportunity to reduce monthly interest earlier.

Which method suits you?

1. Monthly prepayment (paying extra every installment).

• Suitable for: Those with stable regular income, strong saving discipline, and extra funds each month.
• Advantages: Saves the most interest, allows stable expense planning, and builds good financial habits.

2. Annual lump-sum prepayment.

• Suitable for: Those with irregular income or receiving lump sums periodically, such as bonuses or commissions.
• Advantages: Highly flexible; if an emergency expense occurs in some months, you don’t have to force extra payments and can save up to pay at the right time.

3 checklist items before starting to prepay your home loan.

However, before using extra money to prepay, to maximize efficiency, you should assess the following.

1. Check liquidity: Ensure you have no high-interest debt (e.g., credit card debt) and have enough "emergency savings."
2. Calculate proportions: The sum of your installment plus extra payment should not exceed 30–40% of your income to avoid impacting daily living.
3. Choose the right day to prepay: Prepay on the "same day the bank closes your statement cycle" so all funds go directly to reduce principal rather than daily accumulating interest.

If you have discipline and steady income, monthly prepayment is the most cost-effective. Regardless of method, "starting prepayment" today is better than just paying the minimum over time.

Source: Krungsri Bank.

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