What is iXBRL?
How does iXBRL work?
An iXBRL file is a normal web page that a person can read, with machine-readable tags wrapped around the numbers and key facts. Each tag says what the number is (a concept from a taxonomy), which period and entity it belongs to (a context), the unit (usually GBP) and how to scale it.
A tagged figure for net assets looks like this inside the HTML:
<ix:nonFraction name="core:NetAssetsLiabilities" contextRef="CY_end"
unitRef="GBP" decimals="0" format="ixt:num-dot-decimal">25,000</ix:nonFraction>The contextRef points to a context defined elsewhere in the file, which holds the date (here, the balance sheet date) and any dimensions, such as a share class or "creditors due within one year".
Where do you find iXBRL in real filings?
Most accounts filed electronically at Companies House are iXBRL documents. When you open "View PDF" on the register you see a rendered copy; the underlying file is published in Companies House's free accounts bulk data as daily and monthly ZIP archives. Tag names come from the FRC taxonomy suite, for example:
| Balance sheet line | Typical tag |
|---|---|
| Net assets | NetAssetsLiabilities |
| Total assets less current liabilities | TotalAssetsLessCurrentLiabilities |
| Creditors due within / after one year | Creditors with a within or after one year dimension |
| Cash at bank and in hand | CashBankOnHand |
| Turnover | TurnoverRevenue |
Filings made before the FRS 102 taxonomy used older UK GAAP names, such as ShareholderFunds, CashBankInHand and CreditorsDueWithinOneYear.
Worked example: why naive reading fails
Each filing carries the current year and the prior year, and often dimensional values too. Example from our own pipeline: for TESCONS LTD (10623966), accounts to 31 May 2025, a parser that took the first tag it found read the period end as 31 May 2024 and equity as 100, which was one share class only. Resolving the contexts properly gave equity of 183 at 31 May 2025. After that fix, net assets equalled equity in 99.4% of filings, up from 66.6%.
The rule: take facts whose context has no dimensions and whose date equals the balance sheet date, then apply the scale and sign attributes. Our iXBRL parsing guide walks through the code.
Why it matters when checking a company
iXBRL is what makes multi-year financial history searchable. We have parsed about 37 million machine-readable accounts filed from 2008 to 2026, covering about 7 million companies, and about 70% of active companies have at least one iXBRL filing. The rest include companies that have not filed accounts yet and large groups, which often file PDF only. Our accounts type breakdown and methodology explain what that covers.
If a company's accounts are only a scanned PDF, the figures cannot be read automatically and must be checked by eye.
Common confusions
- iXBRL is not XBRL. Plain XBRL is a data file with no human-readable layout; iXBRL embeds the same tags in a readable document.
- The tag is not the label. A line printed as "Shareholders' funds" may be tagged
Equity. - Not every number is tagged. Untagged text and notes are invisible to software.
Developers can get these figures as JSON with a free API key instead of parsing files themselves.
Sources
- HMRC: Taxonomies accepted by HMRC
- HMRC Company Taxation Manual COM130010: accounts
- GOV.UK: Companies House changes to accounts filing from April 2028
FAQ
Do UK companies have to file accounts in iXBRL?
For corporation tax, HMRC requires accounts filed with the Company Tax Return to be in iXBRL, with limited exceptions such as some non-resident companies and unincorporated clubs. Companies House currently also accepts accounts on paper, but it has announced that from April 2028 all companies must file accounts in iXBRL using commercial software.
How can I read an iXBRL file without code?
Open it in a web browser: it displays like ordinary accounts. To see the tags, use an iXBRL viewer such as the open source Arelle tool, which lists every tagged fact with its concept, period and dimensions. For many companies at once, a parser or a hosted API is more practical than viewing files one by one.
What is the difference between iXBRL and a PDF of the accounts?
A PDF is a picture of the accounts: a person can read it but software cannot reliably extract the figures. An iXBRL file shows the same accounts to a reader while also carrying tags for each number. That is why figures from iXBRL filings can be compared across years and companies automatically.
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