Most late payments are not disputes. They are invoices that could not be processed and sat in someone's inbox until chased.
What has to be on it
| Field | Why |
|---|---|
| Unique invoice number | Required in most jurisdictions; how it is tracked |
| The word "Invoice" | Distinguishes it from a quote or a statement |
| Your name, address, contact | Legal requirement and dispute route |
| Client name and address | Must match their records to be matched |
| Invoice date and supply date | Tax point; these can differ |
| Description of goods or services | Vague descriptions get queried |
| Net, tax rate, tax amount, gross | Shown separately, not just a total |
| Payment terms and a due date | A date, not "30 days" |
| Bank details or payment link | The single most-omitted field |
Requirements vary: a VAT-registered business in the UK or EU must show its VAT number and a breakdown, and thresholds apply for simplified invoices. In the US, sales tax appears only where nexus applies. Check what your jurisdiction requires — this list is the common core, not a compliance checklist.
Why invoices are actually paid late
The instinct is to blame the client. Usually it is a processing failure that nobody reported back.
- No purchase order number, where the client requires one. Most payables systems cannot match it and it goes to an exceptions queue.
- Sent to the wrong address. Your day-to-day contact is rarely accounts payable, and forwarding depends on them remembering.
- Terms stated as "Net 30" rather than a date. It is one more calculation for someone with two hundred invoices.
- No bank details, which means a reply, a wait, and a restart.
- A description too vague to approve. "Consulting services" gives the approver nothing to check against.
- Amounts that do not reconcile — a rounding error between line items and the total stops it dead.
Almost all of it is preventable at the point of writing. Ask for the PO number and the payables address before the work starts, and state a due date rather than a term.
PDF, and not an image
This matters more every year, because payables is increasingly automated.
Modern accounts payable systems read invoices, extract the fields and match them against purchase orders. That works on a PDF with a real text layer. It works poorly on a scan and not at all on a screenshot pasted into an email.
An invoice a machine cannot read becomes an invoice a person has to key in, which means a queue, a delay and a chance of a typo. Export from your accounting software or a document tool — do not photograph, do not screenshot, and do not send a Word file that renders differently on their machine and may be edited.
The design decisions that matter
An invoice is a form to be processed, not a piece of design work. Three things affect whether it processes cleanly.
Font. A clean sans or serif at 10–11pt. Avoid anything with ambiguous digits — a 1 that looks like a 7, or a 5 like an S — because extraction misreads them and so do people. Embed the font, or a substitution can shift a column and break alignment; see PDF font problems.
Alignment. Amounts right-aligned with a consistent decimal position. This is the fastest way to make a total scannable by eye.
The total. One unambiguous "Amount due" figure. If there are three bold numbers on the page, someone will pay the wrong one.
Colour, logos and layout flourishes are fine and change nothing about payment speed. Legibility does.
Tax arithmetic
The most common error on an invoice is calculating tax backwards from a gross figure by subtracting the rate. It is wrong every time.
To remove 20% VAT from a gross amount you divide by 1.2, not subtract 20%. On £120 that is £100 net and £20 VAT — subtracting gives £96 and a £4 discrepancy that will bounce the invoice.
Rounding also matters: rounding each line and summing gives a different total from summing and rounding once, and payables systems check. Pick one method and apply it consistently. The full arithmetic is in how to calculate VAT backwards, and the VAT Calculator works in both directions.
Purchase orders and numbering
A purchase order is the mirror image — the buyer issues it before the work, committing to pay for specified goods at a specified price. It carries the same fields plus a delivery date and a PO number.
That PO number is what makes an invoice match automatically. Quote it prominently on the invoice, in its own labelled field rather than buried in a description line.
On your own numbering: make it sequential and gapless. Gaps invite questions in an audit, and non-sequential numbering makes it harder to prove nothing is missing. A simple 2026-001 pattern is sufficient, and restarting each year keeps the numbers short.
Frequently asked questions
What must legally be on an invoice?
A unique number, both parties' details, the date, a description, and the amount with tax shown separately. Specific requirements vary — a VAT-registered business in the UK or EU must show its VAT number and a breakdown. Check your own jurisdiction; the common core is not a compliance checklist.
Should I send an invoice as PDF or Word?
PDF, always. A Word file renders differently on their machine and can be edited. Export a real PDF with a text layer rather than photographing or screenshotting — modern payables systems read invoices automatically, and one they cannot read goes to a manual queue.
Why do clients pay my invoices late?
Usually a processing failure rather than a dispute: no PO number so it cannot be matched, sent to a contact rather than accounts payable, terms given as "Net 30" instead of a date, or missing bank details. Nearly all of it is preventable when writing the invoice.
How do I calculate VAT on an invoice?
To add it, multiply the net by 1.2 at 20%. To find it from a gross total, divide by 1.2 — do not subtract 20%, which is wrong by £4 on £120. Round consistently, either per line or on the total, since payables systems check the arithmetic.
What is the difference between an invoice and a purchase order?
A purchase order is issued by the buyer before the work, committing to pay for specified goods at a specified price. An invoice is issued by the seller afterwards requesting payment. Quoting the PO number on the invoice is what lets it match automatically.
Should invoice numbers be sequential?
Yes, and gapless. Gaps invite questions in an audit and make it harder to show nothing is missing. A simple 2026-001 pattern restarting each year is enough.