Loan Interest Rate Calculator (VND)
In Vietnam, lenders and salespeople almost always quote a loan as a monthly payment, not as an interest rate. A dealer tells you "just 6.500.000 VND a month"; a bank officer hands you a repayment schedule. The number that actually tells you whether the deal is good is the one nobody volunteers: what annual interest rate is hidden inside that payment? This calculator answers exactly that. You enter the amount borrowed, the fixed monthly payment and the number of months, and it back-solves the implied interest rate.
Take the default case: you borrow 200.000.000 VND and repay 6.500.000 VND every month for 36 months. Over the life of the loan you hand over 234.000.000 VND in total, so 34.000.000 VND of that is pure interest — roughly 17% of the principal. Expressed as a rate, the loan costs 10.49%/year (about 0.87%/month). Only once you have that figure can you compare one bank against another, or weigh a "0% instalment" plan against a normal loan.
Back-solving the rate is especially useful for Vietnam's ubiquitous "0% interest" instalment offers. They frequently bundle an arrangement fee, a processing fee, or a higher sticker price, so the true rate is rarely zero. Enter the cash you actually receive and the contractual monthly payment, and the tool exposes the real rate. One caveat: the result reflects the numbers you type. If the contract also carries hidden fees, loan insurance, or early-repayment penalties, your effective cost can be higher still.
How the calculator back-solves the rate
The amortizing-loan model
An amortizing loan is an annuity: the outstanding balance shrinks over time, and each period you pay a fixed amount that covers both interest and principal. The amount borrowed and the monthly payment are linked by:
Loan amount = Monthly payment × (1 − (1 + r)^−n) / r
| Symbol | Meaning |
|---|---|
| Loan amount | Principal borrowed (default 200.000.000 VND) |
| Monthly payment | Fixed instalment (6.500.000 VND) |
| r | The monthly rate we want to find (decimal) |
| n | Total number of payments = 36 months |
You cannot isolate r algebraically — it sits inside both an exponent and a denominator. So the calculator uses a binary search: it guesses an r, computes the loan amount that rate implies, and narrows the interval. Because the implied loan amount decreases monotonically as r rises (a higher rate means the same payment can only support a smaller loan), the search converges quickly — 200 iterations pin the rate down to many decimal places.
From monthly rate to annual rate
Once r is found per month, the nominal annual rate = r × 12. In the default case r ≈ 0.87%/month, so the annual rate ≈ 10.49%. This is the nominal convention Vietnamese banks typically use when they advertise a yearly rate and divide by 12 to get the monthly figure.
Total interest
Total interest = Monthly payment × number of months − Loan amount. This doesn't depend on solving for r at all: subtract the principal from everything you pay. Here, 6.500.000 × 36 − 200.000.000 = 34.000.000 VND.
What the model leaves out
The tool assumes a constant, level payment for the whole term, interest charged on the declining balance, and no arrangement fees, prepayment penalties or loan insurance. Many real Vietnamese loans charge interest on the original principal (a flat rate), in which case the true effective rate is meaningfully higher than the advertised number. Read the output as the effective declining-balance rate and reconcile it against the method written into your contract.
Worked example: borrow 200.000.000 VND, pay 6.500.000 VND/month for 36 months
You're offered 200.000.000 VND with a payment of "only 6.500.000 VND a month for three years". The calculator solves for 0.87%/month, which is 10.49%/year.
| Metric | Value |
|---|---|
| Loan amount | 200.000.000 VND |
| Monthly payment | 6.500.000 VND |
| Number of months | 36 |
| Total paid | 234.000.000 VND |
| Total interest | 34.000.000 VND |
| Implied annual rate | 10.49% |
Three things stand out.
- The interest is larger than it looks. 6.500.000 VND a month feels light, but across 36 months you pay 34.000.000 VND extra — about 17% of the principal.
- For the same loan, the rate drives the payment. At an 8%/year rate the monthly payment would be 6.267.273 VND; at 12% it's 6.642.862 VND; at 15% it's 6.933.066 VND. The sensitivity chart above marks where your 6.500.000 VND payment lands on the rate axis.
- Use it to unmask "0% instalments". If a rival lender offers the same 200.000.000 VND but at 6.267.273 VND a month, you're actually borrowing far more cheaply despite the identical "instalment" label.
Enter the exact cash you receive and the contractual payment to get the real rate. FiMo also has loan-payment and loan-comparison calculators if you want to work in the opposite direction.
Frequently asked questions
How do I find the real interest rate from just a monthly payment?
You need three inputs: the amount borrowed, the fixed monthly payment, and the number of months. The calculator back-solves the hidden rate by binary-searching for the monthly r where Loan = Payment × (1 − (1 + r)^−n)/r. Example: borrowing 200.000.000 VND and paying 6.500.000 VND/month for 36 months works out to 10.49%/year (about 0.87%/month).
What rate am I paying on 200,000,000 VND at 6,500,000 VND/month for 3 years?
The implied rate is about 10.49%/year, or 0.87%/month. You repay 234.000.000 VND in total on a 200.000.000 VND loan, so 34.000.000 VND is interest — roughly 17% of the principal. That's the effective declining-balance rate; if your contract charges interest on the original principal (a flat rate), the advertised number will look lower than the true cost.
What is the formula to solve for an interest rate from a payment?
It starts from the amortizing-loan identity: Loan = Monthly payment × (1 − (1 + r)^−n)/r, where r is the monthly rate and n the number of payments. Because r appears in both an exponent and a denominator, there is no closed-form solution, so the tool uses binary search to converge on r. The nominal annual rate is then r × 12, and total interest = payment × n − loan = 34.000.000 VND in the example.
Is a "0% interest" instalment plan really free?
Usually not. Many "0%" offers bundle an arrangement fee, a processing fee, or a marked-up sticker price, so the true rate isn't zero. To check, enter the cash you actually receive (or the real price of the item) and the contractual monthly payment. If payment times the number of months exceeds the amount borrowed, the gap is interest — and the calculator converts it into an annual rate so you can compare it against a conventional loan.
How are monthly and annual interest rates related?
The nominal annual rate equals the monthly rate × 12 (and conversely, monthly = annual ÷ 12). In the example, 0.87%/month corresponds to 10.49%/year. That's the nominal convention most Vietnamese banks quote. Note that if interest compounds monthly, the effective annual rate (APY) is slightly higher than simply multiplying by 12.
Why can my effective rate be higher than the advertised bank rate?
Two common reasons. First, many loans charge interest on the original principal (flat rate) rather than the declining balance, which makes the effective rate nearly double the headline number. Second, processing fees, loan insurance and prepayment penalties sit outside the quoted rate but still raise your real cost. This tool computes the effective declining-balance rate straight from your payment, so it usually tracks reality more closely than the advertised figure.