You validate an invoice by checking, before sending it, four things: that the client's tax ID is valid (its check digit adds up), that VAT and amounts sum correctly, that the numbering follows the previous one with no gaps or duplicates, and that all mandatory fields for the invoice type are present. All four are mechanical checks: they can happen on their own, the moment you save, without sending your data anywhere.
The idea isn't to distrust you. It's to let the machine review the mechanical part —pure calculation and cross-check— so you spend your attention on what really needs judgement. That prevents most corrective invoices, rejections and awkward calls, which almost always come from a small error nobody looked at.
Why validate before, not after?
Because invoice errors don't show the day they're made, but weeks later and far away: in the VAT quarter, in the client's accounts, in a request. The later they surface, the more expensive they are to fix and the worse they look. A mistyped tax ID costs five seconds to check when issuing and a wasted afternoon if found in September.
What do you check, step by step?
In this order, from quickest to finest:
- Validate the tax ID. The Spanish tax identifier carries a letter or check digit computed from the rest. If it doesn't add up, it's mistyped. It doesn't confirm the ID exists, but it rules out the typo on the spot.
- Reconcile VAT with amounts. Base times VAT rate gives the tax due; base plus tax gives the total. With several lines, an equivalence surcharge or a withholding, it stops being trivial: recompute and compare with what the invoice says.
- Check the numbering. That the number follows the previous one in the series, with no gaps or duplicates, and that the date doesn't go backwards within the series. A long-standing requirement, and critical with VeriFactu.
- Check the mandatory fields. Issuer and recipient identified, date, clear description, tax breakdown and total. Miss one and the invoice is incomplete even if the amount is right.
- Mark the invoice type correctly. Full, simplified or corrective. A corrective invoice has to say which invoice it corrects; if not, it's guaranteed trouble.
- Leave judgement for last. That the description reflects what you sold and that the VAT rate is right for that product, you check yourself. It's the only thing that can't be automated.
What can be automated and what can't?
Almost everything mechanical can; judgement can't:
| Check | Automatable? |
|---|---|
| Tax ID check digit | Yes, instantly, with no external lookup. |
| VAT and amounts reconciliation | Yes, it's pure arithmetic. |
| Sequential numbering and dates | Yes, comparing with the last invoice in the series. |
| Mandatory fields present | Yes, checking no field is missing. |
| That the description reflects the sale | No: needs human judgement. |
| That the VAT rate suits the product | Not fully: the machine warns, you decide. |
There are free tools that run these checks in the browser: you paste an invoice's data and they tell you what fails and why, recording nothing. It's a good way to see the kind of validation I mean before building it into your own flow.
When is it NOT worth setting up?
If you make four invoices a month and already use a program that validates as you issue, you don't need to set up anything: you've got it. Automatic validation pays off when you invoice in volume, or when you issue through several paths (a program, a backup Excel, invoices made by more than one person) and want a single door that checks everything that goes out. Below a certain volume, a careful eye is enough.
What mistakes do people usually make?
- Copying the tax ID without checking it. One changed digit and the invoice goes to an identifier that doesn't exist.
- Trusting the total without looking at the breakdown. The total can be right and the per-line VAT wrong, and that throws off the quarter.
- Reusing a number when redoing an invoice. The previous one is badly voided and a duplicate appears.
- Validating only when something bounces. By then you've already looked bad to the client; the check has to come before sending.
Frequently asked questions
How do I know a tax ID is right without calling the Tax Agency?
By its check digit: the Spanish tax identifier carries a letter or number computed from the rest, so it can be checked on its own. If it doesn't add up, it's mistyped. That doesn't confirm the ID exists or is active —that needs a query to the Tax Agency— but it rules out most copy errors.
What fails most on an invoice?
VAT reconciliation when there are several lines with different rates, an equivalence surcharge or a withholding. The total can look right and the per-rate breakdown be wrong, and that's what throws off the quarter's VAT. Recomputing each segment catches it.
Can I validate invoices without installing anything?
Yes. There are free tools that check tax IDs, VAT reconciliation and numbering in the browser, recording nothing. They're good for one-off checks; if you invoice in volume, it's worth integrating that validation into your own issuing flow.
Does this relate to VeriFactu?
Yes, partly. VeriFactu requires numbering to be sequential and each invoice to be recorded and sealed. Validating data and numbering well before issuing is exactly what gets you to VeriFactu without surprises.
Validate before sending, not after it bounces
Four checks prevent almost every invoice error: the tax ID check digit, VAT-and-amounts reconciliation, sequential numbering without duplicates, and mandatory fields by type. All four are mechanical and happen on their own when you save.
Automating the mechanical part doesn't take away your control: it gives it back. The machine rules out the impossible and you spend your attention on what needs judgement. And on the way you reach VeriFactu with your homework done.
How many corrective invoices do you issue a year for errors a check would have caught?
If you review invoices by eye —or not at all— there's room to automate the mechanical part without changing how you work. I'll show you how to build it into your flow.
See the automation service Let's talk about your invoicing