Invoice
Learn what an invoice is, what to include, and how tax invoice and InvoiceNow rules work in Singapore.
Published Wednesday 30 September 2026
Table of contents
Key takeaways
- An invoice is a document that asks your customer to pay for goods or services and records the details of the sale
- A unique invoice number, a clear due date and set payment terms help you get paid on time and keep cash flow steady
- GST-registered businesses in Singapore must issue tax invoices that meet IRAS rules, generally within 30 days of the supply
- InvoiceNow e-invoicing is being phased in, with all remaining GST-registered businesses joining between April 2028 and April 2031
What is an invoice?
An invoice is a document that asks your customer to pay for goods or services you’ve provided. It’s also called a bill, and it sets out what you sold and when your customer needs to pay.
Say you run a catering business and cook for a client’s office launch. After the event, you send an invoice listing the menu, guest numbers, price per head and total, plus your bank details and due date. For a closer look at the format, read this invoice basics guide.
What an invoice is used for
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Disclaimer
This glossary is for small business owners. The definitions are written with their requirements in mind. More detailed definitions can be found in accounting textbooks or from an accounting professional. Xero does not provide accounting, tax, business or legal advice.
An invoice’s main job is to get you paid, and it also gives you a record for your books and your tax. Every unpaid invoice adds to your accounts receivable, which is the money customers owe you.
An invoice also reminds your customer they haven’t paid yet and tells them how long they have. That window is the payment term, such as 14 or 30 days, and it’s often written as ‘Net 14’ or ‘Net 30’.
Because of this, accurate invoices sent on time keep cash coming in at a steady pace. When your customer has every detail they need, there’s less back-and-forth before they pay.
What to include on an invoice
A complete invoice gives your customer everything they need to approve and pay it without a follow-up email. Most invoices include:
- the word ‘Invoice’ and a unique invoice number
- the issue date and the payment due date
- your business name, address and contact details
- your customer’s name and address
- a description of each item, with the quantity and rate
- the total amount owed
- your payment terms and the ways your customer can pay
GST-registered businesses add a few more details to create a tax invoice, which the GST section below covers.
Invoice vs quote, bill and receipt
Invoices sit alongside a few similar documents, and each one plays its own part in a sale. Here’s how they compare:
- a quote gives an estimated cost before the work starts, while an invoice shows the actual time taken and the actual cost
- a bill is usually the same document as an invoice, seen from the customer’s side
- a receipt confirms you’ve received payment, so it comes after the invoice is settled
- a sales invoice and a purchase invoice are one document: it’s a sales invoice for you as the supplier and a purchase invoice for your customer
Types of invoices
The type of invoice you send depends on the job and whether you’re registered for GST. Common types include:
- a tax invoice, which GST-registered businesses issue to GST-registered customers
- a simplified tax invoice, which you can use when the total including GST is $1,000 or less
- an interim invoice, which bills part of a long project, followed by a final invoice when the work is done
- a recurring invoice, which charges the same amount on a regular schedule, such as a monthly retainer
- a pro forma invoice, which lists expected charges before a sale is confirmed
- a commercial invoice, which travels with exported goods so customs can assess them
- a credit note, which reduces or reverses a charge on an invoice you’ve already issued
Some customers need to see costs before they commit, for example, to get internal approval. A pro forma invoice gives them those figures, and you send the real invoice once the sale goes ahead.
Tax invoices and GST in Singapore
If you’re registered for Goods and Services Tax (GST), your invoices must meet rules set by the Inland Revenue Authority of Singapore (IRAS). You charge GST at the current GST rate of 9%.
Under the IRAS tax invoice rules, you must issue a tax invoice to GST-registered customers, generally within 30 days of the supply. Zero-rated supplies, exempt supplies and sales to customers who aren’t GST-registered sit outside this requirement.
Along with the standard invoice details, a tax invoice must show:
- the words ‘Tax Invoice’
- your GST registration number
- the GST rate applied
- the total excluding GST, the total GST and the total including GST
For sales where the total including GST is $1,000 or less, you can issue a simplified tax invoice instead. It asks for fewer details, which suits quick, low-value sales at a counter or market stall.
Singapore is also moving to e-invoicing through InvoiceNow, a nationwide Peppol-based network the Infocomm Media Development Authority (IMDA) introduced in 2019. Since 1 April 2026, new voluntary GST registrants must send invoice data to IRAS through InvoiceNow, according to this 2026 IRAS announcement.
All remaining GST-registered businesses will be phased in between April 2028 and April 2031. Small and medium-sized enterprises (SMEs) can use InvoiceNow-Ready Solutions for free until March 2031, so check whether your invoicing software is one of them.
How to create and send an invoice
Once you have a routine, an invoice takes only a few minutes to put together. A consistent invoicing process helps you bill every job the same way, so follow these steps each time:
- Add your business details, your customer’s details and a clear description of the goods or services, with quantities and rates.
- Give the invoice a unique number that follows on from your last one, so you and your customer can track it.
- Set the issue date, the due date and your payment terms, and list the ways your customer can pay.
- Send the invoice as soon as you’ve delivered the work, while the details are fresh for both of you.
- Follow up on the due date if payment hasn’t arrived, with a polite reminder and a copy of the invoice.
Create and send invoices with Xero
Accurate invoices sent on time keep your cash flow steady and your GST records in order. With Xero’s online invoicing, you can create customised invoices in minutes and set up automated reminders for overdue ones.
Offering online payments helps too: Xero customers who use online invoice payments get paid up to twice as fast. Try Xero and get one month free to see how it fits your business.
FAQs on invoices
Here are quick answers to common questions about invoices.
Is an invoice a bill or a receipt?
An invoice and a bill are usually the same document, and a receipt is a separate one you send after payment. Keep each receipt with its matching invoice so your records show the full sale, from charge to payment.
Does an invoice mean you’ve paid?
An invoice is a request for payment, so the amount stays outstanding until your customer settles it. Marking the invoice as paid in your records, or sending a receipt, confirms the payment.
How should you number invoices?
Use a sequential system where each invoice gets a unique number, such as INV-0001 followed by INV-0002. Adding a prefix for the year or the client can help you sort records, as long as every number stays unique.
How long should you keep invoices in Singapore?
IRAS requires GST-registered businesses to keep the tax invoices they issue and receive for at least five years, as its GST record-keeping guidance explains. Storing them digitally in accounting software makes them easy to find if IRAS asks to review your records.