Interest-Only Loan Calculator (VND)
An interest-only loan lets you pay only the interest for an initial period — common in Vietnam as a "grace period" (ân hạn gốc) on home loans — before switching to full principal-plus-interest repayment. During that window your monthly outgoing is small and flat, because you are not yet chipping away at what you borrowed. The catch every borrower must internalise: the principal does not shrink at all. When the interest-only window closes you still owe the entire original amount, and only then does real repayment begin.
That creates a payment jump exactly when the grace period ends. On an illustrative 11%/year loan of 800.000.000 VND with a 24-month interest-only window, you pay 7.333.333 VND/month while interest-only, then 12.566.741 VND/month once you amortise the full balance over the remaining 96 months — roughly 1.7× the earlier figure, a step up of 5.233.408 VND a month. For anyone living on a VND salary, that jump is the single biggest cash-flow risk of an interest-only structure, and the reason to model it before you sign rather than be surprised by it.
The calculator above takes the loan amount, the annual rate, the number of interest-only months, and the number of months you then repay over. It returns the interest-only payment, the higher post-grace payment, the total interest across the loan's life, and a comparison with repaying from day one. Note that the 11%/year used in every example is an illustrative assumption, not a quote — Vietnamese lending rates vary by bank, product, reset schedule and date, and many loans float after a teaser period. Enter the rate your bank actually offers for a realistic result.
How the calculator works
Monthly rate
The tool converts the annual rate to a monthly rate = annual rate ÷ 12 — the simple convention most Vietnamese loan contracts use to compute each period's interest. At 11%/year that is 11% ÷ 12 ≈ 0.9167% per month.
The interest-only payment
Because no principal is repaid, each month you pay interest on the entire outstanding balance:
Monthly interest = Principal × monthly rate
This figure is constant through the grace period, since the balance never moves. For 800.000.000 VND at 11%/year: 800.000.000 × 0.9167% = 7.333.333 VND/month.
The payment after the grace period
When the interest-only window ends, the full 800.000.000 VND principal is amortised into a level principal-plus-interest payment over the remaining months, using the standard annuity formula:
Payment = Principal × r / (1 − (1 + r)^(−n))
where r is the monthly rate and n the number of amortising months. Over 96 months this is ≈ 12.566.741 VND/month.
Total interest and the comparison
| Component | How it is computed |
|---|---|
| Interest during IO window | monthly interest × IO months |
| Interest during amortisation | level payment × months − principal |
| Total interest (with IO) | sum of the two rows above |
| Total interest (repaying from day one) | principal amortised across the whole term |
The "repaying from day one" benchmark assumes you spread 800.000.000 VND over all 120 months with no grace period. The gap between the two totals is the price you pay for the easy start.
What the model leaves out
It assumes a constant rate for the whole term, uses the annual ÷ 12 monthly convention, and ignores arrangement fees, early-repayment penalties, loan insurance, and rate resets. Many Vietnamese loans float after a promotional period, so treat the output as a scenario, not a guarantee.
Worked example: 800.000.000 VND at an illustrative 11%/year, 24-month grace period
Illustrative assumptions: a constant 11%/year rate, a 24-month interest-only window, then level principal-plus-interest repayment over 96 months (a 120-month life in total).
| Phase | Monthly payment | What is happening |
|---|---|---|
| Interest-only (months 1–24) | 7.333.333 | interest only; principal frozen at 800.000.000 |
| After grace (months 25–120) | 12.566.741 | principal + interest over 96 months |
| The jump | +5.233.408 | about 1.7× the earlier payment |
The balance when the grace period ends is still 800.000.000 VND — exactly what you borrowed, because you repaid no principal during the first 24 months. That is the part that catches people out: two years of steady payments and you owe the same as on day one.
Now compare total interest across the loan's life:
| Option | Total interest | Difference |
|---|---|---|
| With a 24-month grace period | 582.407.090 | +60.006.982 |
| Repaying from day one (120 months) | 522.400.108 | benchmark |
The grace period costs an extra 60.006.982 VND of interest versus repaying from the start — the price of keeping the full 800.000.000 VND balance outstanding for 24 extra months. In exchange, your first two years are far lighter (7.333.333 versus the full payment). A sensible play, if your contract permits it: during the grace period, channel the cash you save into voluntary principal prepayments, keeping the cash-flow flexibility while trimming this extra interest.
Frequently asked questions
What is an interest-only loan?
A loan where, during an initial grace period, you pay only the interest and repay no principal. Your monthly payment is lighter, but the balance does not move — when the window ends you still owe the full amount and only then start full principal-plus-interest repayment. In the example (800.000.000 VND at an illustrative 11%/year, 24-month window) the interest-only payment is 7.333.333 VND/month.
How much principal is left when the interest-only period ends?
The entire original principal — you repaid none of it during the interest-only window. In the worked example, after 24 months the balance is still exactly 800.000.000 VND. From that point the 800.000.000 VND is amortised into level payments over the remaining 96 months.
How big is the payment jump after the grace period?
In the 800.000.000 VND example at an illustrative 11%/year, the payment rises from 7.333.333 VND/month (interest-only) to 12.566.741 VND/month (principal + interest over 96 months) — a step up of 5.233.408 VND, about 1.7× the earlier figure. Budget for it before the grace period ends.
Does an interest-only loan cost more in total interest?
Yes. Because the full 800.000.000 VND balance stays outstanding through the grace period, you pay interest on a larger principal for longer. In the example, total interest is 582.407.090 VND — 60.006.982 VND more than repaying from day one (522.400.108 VND).
How is the monthly interest during the grace period calculated?
Monthly interest = Principal × (annual rate ÷ 12). For 800.000.000 VND at an illustrative 11%/year, the monthly rate is 11% ÷ 12 ≈ 0.9167%, so each month you pay 800.000.000 × 0.9167% = 7.333.333 VND. It stays constant through the interest-only window because the balance never changes.
When does an interest-only structure make sense?
It suits borrowers who need light cash flow early on — say you have just bought a home and are renovating, your income is about to rise, or a rental property is not yet earning. The trade-off is more total interest (60.006.982 VND in the example) and readiness for the jump to 12.566.741 VND/month. If your income is stable and you want the lowest total cost, repaying from day one is usually cheaper.
Is the 11% rate in the examples a real bank rate?
No. The 11%/year figure is an illustrative assumption chosen so the math is easy to verify. Real Vietnamese lending rates vary by bank, product, reset schedule and date, and many loans float after a promotional period. Enter the rate your bank actually quotes to get a realistic result.