Method, sources, and what is missing

A calculator that will not say where its numbers came from is asking to be trusted. This page is the alternative: the sources, the checks, the two decisions taken where the sources were silent, and a plain list of what this site does not answer.

Where the figures come from

Every number this site ships is read from a primary source — an Act, a statutory instrument, or Revenue’s own published pages — and carries that source with it in the site’s data, not just in the prose. The sources page lists all of them with the passage each figure was read from.

What each part of the site rests on.
Figures	Primary source
The five VAT rates	Value-Added Tax Consolidation Act 2010, s. 46(1) and Revenue’s historical-rates table — two independent sources, because Revenue’s five rate pages state no percentage at all
What sits at each rate	Revenue’s five rate pages, with the effective dates each states
The 1 July 2026 hospitality move	Finance Act 2025, s. 71, as enacted
Registration thresholds	Revenue’s VAT thresholds page
The nine older public holidays	Organisation of Working Time Act 1997, Second Schedule
The February public holiday	S.I. No. 50/2022, regulations 4 and 5
The public-holiday entitlement	Organisation of Working Time Act 1997, s. 21
Motor tax rates	Finance (Excise Duties) (Vehicles) Act 1952, Schedule Part I, paragraph 6

What each part of the site rests on. Every one of these is fetched and stored as text in the build repository, with a checksum, so a reviewer can refetch and diff.

How the arithmetic is checked

Three separate mechanisms, and they check different things.

A hand-derived test suite. Ninety-three cases whose expected answers were worked out by hand from the statute before the code was run — including both sides of every band boundary in all three motor-tax tables, and every public holiday date for a decade counted from that year’s 1 January weekday. A test whose expectation was generated by the thing it is testing proves nothing.
A frozen golden master. 229 outputs — every rate in both directions across a spread of amounts, thirteen years of calendars, both sides of every duty band — captured and committed. Any future change that alters any of them has to be justified individually before the file is regenerated.
A build-time data validator. It fails the build on a number with no source, a missing effective period, a band table whose bounds do not ascend, or a date-bearing value that has expired. It also scans the site’s own source code for numeric literals that look like legal constants — which is why no percentage or threshold is typed into a page on this site.

The one decision we had to make ourselves

Rounding. No Revenue or statutory statement of the convention a VAT amount is computed to was located. The Act fixes the percentages; Revenue’s pages fix the categories; neither says what happens at the third decimal place.

No Revenue or statutory statement of the rounding convention a VAT amount is computed to was located. VATCA 2010 s.46 fixes the percentages and Revenue’s rate pages fix the categories; neither says what happens at the third decimal place, and Revenue’s "What happens when the VAT rate changes?" page addresses WHICH rate applies, not how the result is rounded. This calculator rounds half up to the cent and says so on every result and on the methodology page. It is a disclosed modelling decision, not a rule this estate found.

So this site rounds half up to 2 decimal places, and says so on every result the calculator produces. It is a disclosed modelling decision, not a rule we found, and it can differ from an invoice by at most one cent per line.

The related decision is which figure gets rounded. Three numbers — net, VAT, gross — and only two can be independent if they are to reconcile. This site keeps the amount you typed exactly as typed, rounds the VAT, and takes the third as the sum or the difference. So net plus VAT always equals gross, in both directions, with nothing unaccounted for. The alternative convention gives a VAT figure one cent different on a small proportion of inputs; neither is more correct.

The visible edge of that is the one-cent case. Taking VAT out of a gross price of €0.01 at the standard rate gives VAT of €0.00 and a net of €0.01 — the VAT rounds away entirely. That is correct under the stated convention and the site does not pretend otherwise.

What this site will not tell you

Three things, each of them a deliberate refusal rather than an omission.

Not answered here

What does a half-year or quarterly motor tax disc cost?

The proportions of the annual rate charged for a half-year, quarterly or monthly licence were NOT located in the Finance (Excise Duties) (Vehicles) Act 1952 Schedule, which is where every rate in this payload comes from. The Department of Transport publishes them — "the half-year payment is 55.5% of the annual rate, the quarterly payment is 28.25% of the annual rate, the monthly cost is 10% of the annual rate" — on a page last updated 5 October 2022. A published administrative figure is not promoted to a statutory one here by wrapping it in sourced(): the site states the published proportions as the Department’s, with that date, and does not compute a part-year price.

Where the answer lives: the instrument that sets the fractional-licence proportions.

What “updated” means in the footer

It is the month this site’s rate data last changed — not the month a page was edited, and not the month the build ran. It tracks one file, and it is never bumped to look fresh.

You will not find the word “verified” anywhere on this site, and that is deliberate. Verification means a named person checked a figure against its source and signed for it. That has not happened here: the figures are read from primary sources and cited, which is a weaker and more honest claim. The about page says who does what.

Corrections

If a figure here disagrees with the source it cites, the source is right and this site is wrong. Say so and point at the source — contact details are here. A correction with a citation attached is the most useful thing anyone can send.