Invoicing and documents
Invoice number
An invoice number is a unique, sequential code assigned to a sales document to identify a specific transaction, track payments and maintain accurate accounting records.
What is Invoice number?
Every time you issue a bill to a customer, you must assign it a unique invoice number. This identifier acts as the primary reference point for both your accounting records and your customer's accounts payable department. Tax authorities require these numbers to follow a strict, unbroken sequence. You can organise this sequence using prefix and series schemes to categorise different revenue streams. For example, you might use 'RET' for retail sales and 'WHL' for wholesale transactions, followed by a sequential number like 001. If you delete an invoice and create a gap in this sequence, tax inspectors may suspect you are hiding income. By maintaining a continuous numerical order, you prove that all sales are properly recorded and reported in your general ledger.
How Invoice number works
The process begins when you configure your invoicing system before making your first sale. You establish a primary numbering format, often combining a date or department prefix with a starting sequential value. When you finalise a sale, the system automatically generates the next available number in that specific series and applies it to the document. You then send this numbered document to your buyer. When the customer pays, they quote this exact number on their bank transfer or remittance advice. You use this reference to match the incoming payment against the open receivable balance in your accounts. Finally, during financial reporting or a tax inspection, an auditor reviews the complete log of these numbers to verify that no documents are missing or duplicated.
- Choose a logical prefix system to identify different branches or sales channels.
- Set a starting numerical value to begin your continuous billing sequence.
- Generate the final document to lock the number and prevent future edits.
- Share the document with your customer to secure a clear payment reference.
- Reconcile the customer payment against the specific code in your accounting software.
Why it matters for your business
A disorganised numbering system directly harms your cash flow and compliance. If your customers cannot clearly identify the bill they are paying, they will delay the transfer, increasing your days sales outstanding. More importantly, missing numbers are a red flag for tax authorities. An auditor who spots a gap in your sequence will assume you deleted a cash sale to evade taxes, which can trigger a full investigation and heavy penalties. Using a reliable system protects you from these risks. Paper & Pen creates invoices, quotations and receipts, and Sales and Invoicing is free forever.
Questions
Common questions
Can I use different invoice number sequences for different projects?
What should I do if I make a mistake on an invoice?
Related terms
- Tax invoice A tax invoice is a legal document issued by a registered business to a buyer, detailing the goods or services provided and the specific amount of tax collected on that sale.
- Credit note A credit note is a commercial document issued by a seller to a buyer, reducing or cancelling the amount owed on a previously issued invoice due to errors, returns, or damages.
- E-invoicing E-invoicing is the automated creation, exchange and processing of invoices in a structured digital format between a supplier and a buyer, often directly integrated with government tax authorities.
- Accounts receivable Accounts receivable represents the total amount of money owed to a business by its customers for goods or services that have been delivered but not yet paid for.
- Debit note A debit note is a commercial document issued by a buyer to request a credit, or by a seller to increase the amount owed on an existing invoice.
- Delivery note A delivery note is a commercial document accompanying a shipment of goods that lists the description and quantity of items enclosed without displaying their financial value.