Calculators that do the sum and show how they did it
No sign-up, no email and no hidden paid version. They work in your browser, so what you type never leaves your computer. And every figure from the rules carries at the foot the link to the BOE it comes from.
The tools
Each one solves a specific sum and explains where the numbers come from, which is the part almost nobody writes.
VAT calculator
From the base to the total and from the total to the base, at 21%, 10% and 4%. Also with the eight Canary Islands IGIC rates and with IPSI.
IRPF withholding
What you will actually be paid for your professional invoice, with the 15% or with the 7% for a newly registered autónomo.
The one nobody else hasRecargo de equivalencia
The 5.2%, 1.4%, 0.5% or 1.75% surcharge and, above all, the real cost of the goods in your warehouse.
Self-employed social security contribution
Your band by net income, with the minimum and maximum base and the contribution for each. With the 2026 bands.
Form 303
The quarter's result with your output and input VAT, the credits to carry forward and the prorrata if it applies to you.
NewLate payment interest
What you can claim on an unpaid invoice, with the statutory rate applied half-year by half-year and the €40 of article 8.
Invoice template
Which fields the regulations require on a full invoice and on a simplified one, with an example that fills itself in.
Why a free calculator carries links to the BOE
Because 90% of the tax calculators out there do not say where their numbers come from, and quite a few carry rates that expired years ago. The Canary Islands' IGIC is the perfect example: almost none of them include the 5% rate, which has existed since 2024, or the 1% one, which has existed since 2026; and almost all of them still cite an article 51 of Ley 4/2012 that stopped being where the rates live in October 2025.
Here every page ends with the rule linked — article included — so that you can check the figure without taking our word for it. And where there is no verifiable figure, we say so instead of inventing one: the IPSI of Ceuta and Melilla has no national table of rates, so in that calculator you type the rate in yourself.
What a calculator cannot do for you
It can give you the VAT on an invoice. It cannot keep your record book, or remember that that client carries a 7% withholding, or add up the whole quarter without your dictating the totals to it, or comply with VeriFactu when the date comes.
That is a program, and that is what Cairos is for: invoicing, expenses, accounting and tax for autónomos, small businesses and also for associations, cooperatives and clubs, with a free plan that neither expires nor asks for a card.
When the sum stops fitting in a box with two numbers
These tools each solve one case. A business has hundreds a month, and there what you need is for them to come out on their own.
Invoices with VeriFactu
Sequential series, a chained record and a QR code, included in the free plan too.
Expenses with their VAT
A photo of the receipt, the supplier, the base, the VAT and the surcharge if it applies to you. Input VAT stops getting lost in a shoebox.
Forms worked out
The 303 and the rest come out of what you have recorded, and every box can be opened to see what makes it up.
Questions about the tools
Try it with your own invoices
Free plan up to 30 documents a year, no card needed. And if one day it stops working for you, your data leaves with you.
The sums, free. The work too, to start with.
Cairos's free plan does not expire and asks for no card. If you outgrow it, you can move up then.
No minimum term · VeriFactu included · Native support in Spanish, Catalan, Galician, Basque and English