Table of Contents

1 Introduction

This guidance explains how data types and units work together in XBRL, outlines the importance of proper modelling choices, and provides practical recommendations to taxonomy authors and preparers. It also highlights the role of the standardised units and the importance of unit validation in ensuring clear meaning, interoperability, and usability of XBRL data.

Accurate and consistent use of units is a foundational requirement for high-quality XBRL data. In an XBRL report, every numeric fact is associated with a concept that specifies its data type and a corresponding unit of measure. This relationship between data types and units plays a critical role in ensuring that the meaning of reported facts is clear, consistent, and machine-interpretable.

However, when units are not applied correctly, such as by using incompatible units, omitting required ones, or introducing non-standard custom units, data quality can suffer significantly. These issues can make reported data harder to analyse, reduce comparability across reports, and increase the burden on both preparers and data consumers.

2 The relationship between data types and units

In XBRL, concepts are defined using several properties, one of which is the data type. The data type determines the kind of values that can be reported using that concept in an XBRL report. For example, a concept with the data type "monetaryItemType" is used to report monetary values.

XBRL provides a mechanism for ensuring that units reported are appropriate for the data type. For example, currency units such as "USD", "GBP" and "EUR" are appropriate for a monetary type, but kilometres and kilograms are not. This compatibility is enforced as part of the XBRL validation process.

XBRL provides a standardised set of units that can be used with numeric data types. For example, units such as mile, inch, meter, and kilometre are available for use with the "lengthItemType" data type.

Not all data types have corresponding units, as units apply only to numeric data types.

If custom data types are defined in the taxonomy, it is recommended to derive these from an existing type. In this way, the new data type will inherit the relationship between the existing data type and its linked units.

3 Unit scaling

Values in XBRL reports will often be scaled from their base units, for example:

• Monetary values in millions
• Energy in gigajoules or megajoules
• Distances in centimetres, metres or kilometres

This can be for several reasons:

• Making the report easier to read;
• Fitting the natural unit of a particular measure;
• Avoiding very large or very small numbers; or
• Matching the underlying data standard.

To report scaled values in XBRL reports, the data preparer can use the appropriately scaled unit from the XBRL specification. For example, here are the scaled units for Joules recognised in XBRL:

Some units, particularly monetary units, do not have scaled units and are reported using their actual, unscaled figures in an XBRL report. In iXBRL reports, however, it is common practice to present monetary figures as scaled values (for example, in thousands or millions). A "scale factor" is applied in iXBRL allowing the presented (human-readable) value to be converted into an unscaled figure in the XBRL data.

For example, in an iXBRL report, a value of GBP 10 million would appear as shown below (with the tagged value highlighted).

Figure 1: Extract from iXBRL report

When tagging this value in iXBRL, an additional property scale="6" is specified, indicating that the reported value is expressed in millions. Upon converting iXBRL to XBRL for computer processing, the scale is applied automatically, and the figure is represented in its unscaled form as "10000000".

If the data collector or taxonomy author does not add rules to enforce the desired use of scaled units (or scale factors in iXBRL) then report preparers can use these features to report scaled numbers in their iXBRL report. For additional guidance, please refer to the following documents:

• iXBRL Tagging Features
• How to model and tag ESG Intensity Metrics

4 Recommendations for accurate unit declarations

Clear and consistent use of data types and units is essential for producing high-quality, comparable XBRL data. The following recommendations highlight key decisions that support effective taxonomy design and reliable reporting.

4.1 Decision 1: Selecting the appropriate data type

Taxonomies are often developed based on an underlying data standard that defines the data to be collected. It is essential that taxonomy authors accurately model concepts in a way that ensures the reported information is correctly understood and consumed by data users. For instance:

• If the reporting requirement is to disclose a monetary amount, the corresponding concept should use the monetaryItemType data type.
• If the requirement is to report a value such as cash as a percentage of total assets, the concept should use the percentItemType.
• If the requirement involves reporting greenhouse gas emissions, a concept with the ghgEmissionsItemType data type is appropriate.

In the typical case where taxonomy authors do not define the reporting requirements themselves, it is important to carefully select the correct data types to support the corresponding units of measure in the underlying standards. In the ideal case where the digital standard is part of the underlying standard, XBRL provides a means to specify the exact units that should be reported.

Proper modelling, including the use of suitable data types and units, not only ensures semantic clarity but also supports reliable data analysis and enhances the overall usability and comparability of XBRL reports.

4.2 Decision 2: Defining concepts for units of measure

XBRL provides a standardised set of units of measure for use in XBRL reports. However, it is not an exhaustive list of all possible units that are used in reporting. For example, there is no defined unit such as "employee", even though companies frequently report the number of employees.

The recommended approach is to define specific concepts for each type of reported fact, such as "Number of Employees", "Number of Divisions" or "Number of Plants" that are modelled with a data type of integer or positive integer. Each concept carries an unambiguous meaning, making it easier to incorporate into analytical models and improving consistency across filers. Under this approach, filers are still required to assign a unit to each fact, and it is generally recommended to use the unit "pure".

Conversely, for units of measure that are defined in XBRL, the unit should be provided at the XBRL report level, not embedded within the concept name or definition. For example, a taxonomy should not define multiple concepts such as "Cash measured in US Dollars", "Cash measured in Pesos" or "Cash measured in Euros". A single concept, such as "Cash" is sufficient, and the reported unit, such as USD, MXN or EUR, will specify the currency in the XBRL report.

This principle helps maintain semantic clarity and promotes reusability and consistency across different extension taxonomies and XBRL reports.

4.3 Decision 3: Further restrictions on units and scaling

In general, all a taxonomy author needs to do to handle units properly is to select the appropriate data type Decision 1 and use an appropriate concept definition Decision 2. This enforces standardised units and explains to taxonomy users how to interpret the data. However, further restrictions on what how figures are reported may be considered.

A common example of where this might be required is if monetary values should be reported in a specified currency. In these cases, it is recommended for the taxonomy author to:

• Enforce acceptable units using XBRL Formula.
• If additional unit restrictions are localised to specific items, a description of the restriction should be included in a concept guidance label.
• If additional unit restrictions are widespread or cover the entire report, the restrictions should be documented in filing rules.

It is recommended to use only the minimal restrictions on units required to reflect the underlying standard or legislation. Overly prescriptive unit scaling requirements can impose additional work on preparers, especially when internal systems do not match the units or scaling required. On the consumer side, XBRL defines machine-readable unit conversion factors that can be used to normalise the data if desired.

Making use of XBRL’s flexibility around units, supports a more open and adaptable reporting environment, reduces the burden on preparers, and allows data consumers to apply their preferred units without compromising data integrity or comparability.

In the case of iXBRL scaling (see Unit Scaling), the standard does not provide a means to enforce that any particular scaling is used. It is more valuable to allow values to be reported using scaling that is natural to the preparer and makes the rendered report easier to read, than to try to enforce iXBRL scaling through filing rules or some other means.

In summary, rather than mandating specific units and scales, taxonomies and regulators should focus on clear data type modelling and leave unit normalisation to downstream tools and users.

4.4 Decision 4: Validating units of measure

XBRL contains a list of data types, and which units¹ can be used for each. XBRL processors will validate the units applied to reported values to ensure they align with the data types defined in the taxonomy. This validation helps prevent errors such as applying an incompatible unit to a concept (for example, using a length unit with a monetary data type), improves data quality, and promotes comparability across filings.

4.5 Decision 5: Using units in XBRL reports

When preparing an XBRL report, preparers will need to select correct units with correct magnitudes and, in the case of iXBRL, scaling that matches the figures on the face of the report. They also need to take into account any unit-based filing rules.

If the value to be reported matches a unit and magnitude already defined in XBRL, that unit should be used. However, if the natural unit or magnitude is not defined in XBRL, preparers will either need to transform the data or create a custom unit. The use of custom units can introduce inconsistency and reduce data comparability across different filings. Therefore, it is recommended that preparers convert their data to one of the units included in XBRL and apply appropriate scaling to report the value. This process is usually assisted by tagging software, which supports valid units and applies the correct scale.

For example, if a preparer has a value measured in micrometres (a unit that was not included in the XBRL unit registry at the time this guidance was published), instead of creating a new custom unit, this should be reported in millimetres with appropriate scaling applied.

Note: If a preparer, data collector or taxonomy author believes a non-XBRL unit might have a wider application, then the unit can be submitted for inclusion in the XBRL Unit Registry. This provides a common, standardised definition of the unit that can be used by all preparers and consumers.