Loan Repayment Calculator

Monthly repayment schedules for loans: declining balance, equal instalments or flat rate, promotional rates, early repayment fees and the real annual rate.

Result

Press “Run” to see the result.

The loan calculator builds the full repayment schedule of a loan month by month: opening balance, principal, interest, payment and closing balance. It supports the three methods used by Vietnamese banks and consumer-finance companies: declining (dư nợ giảm dần: equal principal, interest on the remaining balance, the usual home and car loan), annuity (trả góp đều: the same payment every month) and flat (lãi phẳng: interest on the original amount every month, common for instalment purchases).

Real offers often start with a promotional rate: promo_rate=7.5&promo_months=12 applies it for the first year and the regular rate afterwards, recalculating the instalment. prepay_month and prepay_fee simulate an early full repayment with its penalty, and start_date adds payment dates. Every response also compares the three methods for the same loan.

effective_annual_rate is the internal rate of return of the payments: a flat 12% loan really costs about 21% a year, which is the number to compare offers. Amounts are rounded to the đồng in each row.

Use this in your code

curl "https://apibox.io.vn/v1/loan-calc?amount=120000000&months=12&rate=12&schedule=false" \
  -H "Authorization: Bearer YOUR_API_KEY"

const res = await fetch('https://apibox.io.vn/v1/loan-calc?amount=120000000&months=12&rate=12&schedule=false', {
  method: 'GET',
  headers: {
    Authorization: 'Bearer YOUR_API_KEY',
  },
});
const data = await res.json();
console.log(data);

import requests

res = requests.get(
    "https://apibox.io.vn/v1/loan-calc?amount=120000000&months=12&rate=12&schedule=false",
    headers={"Authorization": "Bearer YOUR_API_KEY"},
    timeout=30,
)
print(res.json())

<?php
$ch = curl_init('https://apibox.io.vn/v1/loan-calc?amount=120000000&months=12&rate=12&schedule=false');
curl_setopt_array($ch, [
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => ['Authorization: Bearer YOUR_API_KEY'],
]);
$response = curl_exec($ch);
print_r(json_decode($response, true));

API documentation → Get a free API key

FAQ

Declining balance or equal instalments?

With declining balance (dư nợ giảm dần) the first payments are higher and the total interest lower. Equal instalments (trả góp đều) keep the same payment every month but cost slightly more interest.

Why is the flat rate so expensive?

Interest is computed on the original amount even though you repay part of it every month. 12% flat ≈ 21% effective; see effective_annual_rate.

How is the promotional period handled?

The promo rate applies to the first promo_months; afterwards the regular rate applies to the remaining balance and, for annuity loans, the instalment is recalculated.