An invoice is a short document with a long list of jobs: it asks to be paid, it records a sale for both sides' accounts and, in many countries, it is the paper trail the tax office follows. This article covers the parts every invoice needs, how to calculate tax and discounts so the numbers survive an accountant's check, the mistakes that cause invoices to be rejected, and what is different in Vietnam, where VAT invoices must be electronic. The examples were calculated with the logic of the invoice generator, which works in whole dong and whole cents.
The legal part is a summary as of 13 October 2026, checked against secondary sources (law libraries and accounting firms' newsletters), not the official gazette. Rules change: check the current rules, or ask your accountant, before relying on it.
What every invoice should show
Whatever the country, a buyer's accountant will look for the same things:
- The word "Invoice" (or "Quotation", "Receipt": say what the document is).
- A unique number from one unbroken sequence, such as INV-2026-001, 002, 003. Gaps and duplicates are the first thing an auditor asks about.
- The date of issue and, if you give credit, the due date or payment terms.
- The seller: legal name, address and tax identification number if you have one.
- The buyer: name and address, plus their tax number when they are a business that will claim the tax back.
- One line per product or service: a description someone else can understand, the quantity, the unit, the unit price and the line amount.
- The tax rate and tax amount, separately from the price, when tax applies.
- The total to pay, in one currency, and how to pay (bank account, reference to quote).
A worked example in dong
A small design studio bills a client for three things: a logo (1 package at 5,000,000 dong, 8% VAT), a year of website hosting (12 months at 150,000 dong, 10% VAT) and 2.5 hours of consulting at 600,000 dong an hour that the studio treats as untaxed. The client gets a 10% discount on the whole invoice.
| Line | Calculation | Amount |
|---|---|---|
| Logo (8%) | 1 × 5,000,000 | 5,000,000 |
| Hosting (10%) | 12 × 150,000 | 1,800,000 |
| Consulting (no tax) | 2.5 × 600,000 | 1,500,000 |
| Subtotal | 8,300,000 |
The 10% discount is 830,000. The question is how much of it belongs to each tax rate. The fair way, and the way the invoice generator does it, is to spread the discount over the lines in proportion to their amounts: 500,000 off the logo, 180,000 off the hosting and 150,000 off the consulting. Tax is then charged on what is left in each rate:
- 8% on 4,500,000 = 360,000
- 10% on 1,620,000 = 162,000
The total is 8,300,000 − 830,000 + 360,000 + 162,000 = 7,992,000 dong.
If you charge tax first and discount afterwards, you get 400,000 + 180,000 = 580,000 of tax and a total of 8,050,000: the client pays 58,000 dong of tax on money they never paid you. If your contract says the discount applies to the total including tax, write that discount as its own line so everyone can see what was done.
Rounding: once per rate, not once per line
Small amounts make rounding visible. Take three lines of 0.35 dollars each, all at 7% tax. Per line, the tax is 0.0245, which rounds to 0.02, and three of them make 0.06. Calculated once on the 1.05 base, the tax is 0.0735, which rounds to 0.07. The second figure is the one most accounting software and tax offices expect, and it is what the tool prints: one tax line per rate, each rounded once. The line amounts themselves (quantity × price) are rounded half up to the cent, or to the dong, which has no smaller unit.
What Vietnam requires: electronic invoices
In Vietnam, VAT invoices are electronic invoices governed by Decree 123/2020/ND-CP, which was amended by Decree 70/2025/ND-CP, in force from 1 June 2025. According to the summaries we checked:
- Article 10 lists the content of an invoice: the invoice name, its symbol and template symbol, the invoice number; the seller's name, address and tax code; the buyer's name, address and tax code (after the 2025 amendment, a buyer can also be identified by a budget-relationship code or a personal identification number); and for the goods or services the name, unit, quantity, unit price, amount before VAT, the VAT rate and the total VAT.
- Article 9 sets the time an invoice must be issued: for goods, when ownership or the right to use passes to the buyer, whether or not the money has been received.
- Decree 70/2025 replaced the cancellation of an erroneous e-invoice with issuing a replacement or adjustment invoice.
- Household and individual businesses with annual revenue of 1 billion dong or more, and businesses selling directly to consumers, fall under the rules for e-invoices generated from cash registers, which send data to the tax authority.
The consequence for anyone using a PDF invoice maker: a PDF you make yourself is not a Vietnamese VAT invoice, however complete it looks. The official invoice is issued through e-invoice software connected to the tax authority, and for invoices that need the authority's code, the code is issued by that system. In Vietnam, use a PDF invoice as a quotation, a payment request, a pro-forma, or a commercial invoice to a foreign client, and issue the e-invoice through your provider.
Common mistakes
- Reusing or skipping numbers. Keep one sequence per year or per series and never delete a number; void it instead.
- Tax before the discount. As in the example above, it overcharges tax. Spread the discount, then tax each rate.
- Rounding every line's tax. It can drift by a cent or a few dong per rate. Round once per rate.
- Two currencies on one invoice. Invoice in one currency. If the client wants a conversion, give it in the e-mail and name the rate and date; the currency converter helps.
- Missing payment details. An invoice without a bank account and a reference gets paid late. Put both on the document.
- Treating a PDF as a Vietnamese VAT invoice. It is not one; see above.
Sources and how to check
The Vietnamese rules above come from summaries of Decree 123/2020/ND-CP and Decree 70/2025/ND-CP published by Thư viện Pháp luật and by Grant Thornton Vietnam's June 2025 tax newsletter, cross-checked against each other. We did not verify later amendments. Before relying on any of it, read the current text on the government's legal document portal or ask your accountant or e-invoice provider.
To make the invoice itself, open the invoice generator; for a quick price with or without VAT, the VAT calculator does it in one step.