What must be on an invoice for it to be valid and get paid?
The short answer
Nine things. GOV.UK's list1: a unique number, your name, address and contact details, their name and address, a description, supply and invoice dates, amounts, any VAT and the total. A VAT invoice needs more2, and three extras get it paid: a due date, their purchase order number and your bank details. With no date agreed, the 30 days start on the later of the work and their notice of the amount5, so an invoice sent late moves your due date.
England and Wales only. WolfX is software, not a law firm.
What you can add
A £3,600 invoice sent two weeks late
Say you finish a £3,600 job for another business on Friday 3 July 2026, with no payment date agreed. You plan to invoice that day, but the invoice goes out, and arrives, on Friday 17 July. The 30 days begin5 on 17 July, the later day. The last day to pay without interest is 15 August, not 1 August, and interest starts on 16 August. The rate for a debt that goes late in the second half of 2026 is the 3.75% Bank Rate12 plus 8%11, which is 11.75%. They pay on 28 August, 13 days late.
Sent on 3 July, the same invoice would have owed £3,701.29 on 28 August: 27 days of interest, £31.29. The £16.22 gap is the interest on £3,600 for the 14 days it took to send, about £1.16 a day. Interest is the small part. A customer who pays on the last day pays on 15 August, not 1 August: two weeks longer to wait for £3,600. The £70 fixed sum is owed only once interest starts6, so it moves back two weeks too.
The road ahead
Six steps. All free.
Step 1: Pick the list that applies
Step 2: Put the nine on it, under the right name
A limited company shows its full registered name8 and, if it names directors, all of them1. A sole trader using a business name adds their own name and an address for legal papers9.
Put the supply date, the day you finished or delivered, on its own line. Describe the work in the words of their order.
Step 3: Add the VAT items if you charge VAT
A VAT invoice adds your VAT number, the time of supply (tax point), each item's quantity, unit price and VAT rate, the price before VAT, any cash discount rate and the total VAT in sterling2. Number invoices in running order, and normally issue one within 30 days of the tax point2.
Your customer needs it to reclaim the VAT. With an item missing, they must give HMRC other proof of the sale, and HMRC decides whether to allow the claim.
Step 4: Add the three that get it paid
The nine on GOV.UK's list don't include a due date, purchase order number or bank details; the Small Business Commissioner's list has all three3. Write the due date as a date, 1 August 2026, not '30 days'.
Ask early for the purchase order number, as a late one can delay both the invoice and the payment3. The terms to pick and the late payment lines have their own pages.
Step 5: Public customer? Match the law's four items
On a contract with a council or other authority, the law defines a valid invoice7: your name, a description, the sum and a unique number, plus anything else the contract asks for. An electronic invoice in the required form counts too.
If the authority thinks an invoice invalid or disputes it, it must say so without undue delay, and its 30-day payment term stops applying7.
Step 6: Send it to the person who pays, the day you finish
The person who ordered the work may not be the one who pays3. Ask who handles invoices and email it there, so the sent copy is dated.
The 30 days begin5 on the later of the day you finish and the day they have notice of the amount. An agreed rule5 such as '14 days after a valid invoice' runs from that invoice, so check what your contract calls valid.
A letter you can copy
An invoice layout, ready to copy
Copy it into your invoicing app or a document for every invoice to a business, and fill the gaps.
Tap a highlighted gap to see what goes in it.
Not VAT-registered? Delete the VAT number, the 'Amount before VAT' line and the 'VAT' line, and make 'Total to pay' the price. Email it as a PDF to the person who pays, with the subject line above, and keep the sent copy. It shows the day you sent it.
When not to bother
When it isn't worth it
- You're not VAT-registered. Leave off the VAT number and the VAT lines, because only a VAT-registered business can issue a VAT invoice2. The nine, with the name rules in step 2, are your whole list.
- Your customer is a private person. You needn't give a VAT invoice to a customer who isn't VAT-registered2. Selling to a private person follows other rules, which this page doesn't cover.
- It's construction work under the VAT reverse charge. Add a note that the customer pays the VAT10, such as 'Customer to pay the VAT to HMRC'. Show the VAT due, or its rate, outside your total. Ask the customer first if it applies.
- It's a small slip on an invoice already sent. Send a credit note or a new invoice naming the first one's number and date, as HMRC asks for VAT errors2. A slip does not cancel the debt4.
- The invoice is right and they still haven't paid. A correct invoice starts the clock; it does not make anyone pay. When the date has passed, see what to do first.
The Wolf's note
A missing detail does not cancel a debt, but it gives an accounts team a reason to wait, and the day you send the invoice decides when your clock starts. Put the list in a template once. Then send each invoice the day the work is done.
Mr. Wolf · the AI inside WolfX
What comes next
Your next move
What payment terms should I put on my invoice?
Fourteen or thirty days, agreed before the work starts.
5 min read · Letter included
When it first goes quiet
When is an invoice officially late?
An invoice is late the day after the agreed date.
5 min read · Letter included
What you can add
Do I add VAT to late payment interest and compensation?
No. HMRC says no tax is due on interest because of late payment.
4 min read · Letter included
Don't want to do this yourself?
The Wolf does every step for you. He works out the sums, writes the letters and keeps track of every date. Nothing goes without your yes.