Paying an invoice that contains a mistake is easy to do and annoying to undo. A wrong quantity, a tax line that does not add up or a total that is slightly off can slip through when you are approving a pile of invoices. This checklist shows how to check an invoice for errors in a few minutes, and how to let a tool do the arithmetic for you.
Why invoice errors are so common
Most invoices are produced by hand, by spreadsheet or by accounting software that someone configured once and never looked at again. Errors creep in when:
- a quantity or unit price is edited but the line amount is not updated
- a discount is applied in one place and forgotten in another
- tax is calculated on the wrong amount, or at the wrong rate
- a total is typed in instead of calculated
None of these are fraud. They are ordinary mistakes, and they cost real money if nobody notices. For the other kind of problem, see our guide to spotting a fake invoice.
The 10-step checklist
1. Confirm the basics
Check that the invoice number, issue date, due date and supplier name are all present. A missing invoice number makes it hard to reference later, and a due date earlier than the issue date is a clear sign something is wrong.
2. Match it to what you ordered
Compare the invoice with the purchase order, quote or contract. Are the items, quantities and agreed prices the same? A correct calculation of the wrong price is still an overcharge.
3. Recalculate every line
For each line, multiply quantity × unit price and compare the result with the line amount. This is where most errors live, so do not skip lines because they look fine.
4. Add up the lines
Add all the line amounts and compare the result with the subtotal. If the invoice lists discounts or shipping, treat them as separate lines.
5. Check discounts
If a discount applies, work out whether it should come off before or after tax and whether it applies to the whole invoice or only some lines. Then check that the discount shown matches the agreed percentage.
6. Check the tax
Multiply the taxable amount by the tax rate and compare the result with the tax shown. Our guide on how to calculate VAT on an invoice has the formulas and a note on rounding.
7. Check the grand total
The total due should equal the subtotal, minus any discount, plus shipping, plus tax. Recalculate it yourself rather than trusting the printed number.
8. Check the currency
Make sure the currency is the one you agreed, and that every amount on the page uses the same one.
9. Check the payment details
Compare the bank details with the ones you have on file for this supplier. If they changed, confirm the change by calling a number you already trust, not one printed on the invoice.
10. Keep a note of what you checked
If you find a problem, write down which figure is wrong and what it should be. That makes the conversation with the supplier short.
A worked example
Take this small invoice:
| Item | Qty | Unit price | Amount shown |
|---|---|---|---|
| Design hours | 3 | £45.00 | £135.00 |
| Stock photos | 12 | £8.50 | £112.00 |
- Line 1: 3 × £45.00 = £135.00. Correct.
- Line 2: 12 × £8.50 = £102.00, but the invoice says £112.00. A £10.00 error.
- The subtotal printed is £247.00, which matches the wrong line amounts. The correct subtotal is £237.00.
- VAT at 20% should be £47.40, not £49.40, and the correct total is £284.40.
One mistyped digit in a single line changes three other figures. That is why checking only the final total is not enough.
Let a tool do the arithmetic
Steps 3 to 7 are pure arithmetic, which is exactly what software is good at. InvoiceInspect reads a PDF invoice, recalculates every line total, the subtotal, the tax and the grand total, and shows you each mismatch with the expected and actual values and the page it was found on. It is free, needs no signup, and for guests nothing is saved.
It does not replace steps 2 and 9. Only you know what you ordered and who you normally pay.
When you find an error
Do not pay the difference and move on. Ask the supplier for a corrected invoice or a credit note. Our post on what to do when an invoice total is wrong includes an email you can copy.