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Business and Financial Tools

Debt Prepayment and Early Payoff Calculator

Use remaining principal, rate, and term to estimate how an extra monthly payment can reduce interest and the number of installments.

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Private scenario calculator

Debt Prepayment Calculator

Change any assumption to recalculate instantly. Values stay in this browser and are not saved.

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Remaining loan

Prepayment comparison

Estimated interest saved—
Scheduled payment—
Payment with extra—
New payoff time—
Time saved—

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Quick answer

How much interest can an extra monthly debt payment save?

For reducing-balance debt, an extra amount applied to principal lowers the next period's interest base. This tool simulates the original schedule and the extra-payment schedule, then reports the difference.

How to use this tool

  1. Enter outstanding principal, rate, and remaining months from the latest statement.
  2. Enter the additional amount you plan to pay each month.
  3. Compare payoff time and interest, then confirm principal-application rules with the lender.

How the calculation works

The scheduled payment uses the annuity formula with monthly rate = annual rate ÷ 12. Each monthly simulation calculates interest on opening principal and applies the payment. The alternative adds a fixed amount each month. Interest and month differences are the estimated savings.

Examples

Example Input Expected result
Zero-interest debt 
THB 12,000, 0%, 12 months, THB 500 extra
 Scheduled THB 1,000, new payment THB 1,500, payoff in 8 months, 4 months saved, and THB 0 interest saved
No extra payment 
Any supported balance and rate, THB 0 extra
 The calculated term and interest do not change

Limitations

  • Supports equal-payment reducing-balance loans and a constant monthly extra amount.
  • Variable rates, actual dates, default interest, insurance, fees, and payment holidays are excluded.
  • The result is not a lender's settlement quote or account confirmation.

Frequently asked questions

Is every extra payment immediately applied to principal?

Do not assume so. Check the contract, posting date, and lender instructions for principal prepayment.

Can I use this for a floating rate?

The result holds the entered rate constant. Recalculate after any rate change.

Are early-settlement or refinancing costs included?

No. Consider appraisal, mortgage, stamp, contract, and other charges separately.

References

  1. Bank of Thailand — Reducing-balance loan interest Verified 2026-08-03
  2. Bank of Thailand — Managing debt Verified 2026-08-03

Content and calculation method last reviewed:

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