How to Define a Subscripted Unit in siunitx

To make a unit symbol with a subscript in siunitx, declare a custom unit and include the subscript in its definition. The example below defines the symbol “m” with the upright subscript “eff” and uses it after a number.

Minimal working example

\documentclass{article}
\usepackage{amsmath}
\usepackage{siunitx}

\DeclareSIUnit{\meff}{\text{m}_{\text{eff}}}

\begin{document}

The measured value is \qty{3.2}{\meff}.

\end{document}

The output is a number followed by the unit symbol $\text{m}_{\text{eff}}$, with siunitx handling the spacing. The example uses the current siunitx command \qty. To print the unit without a number, use \unit{\meff}.

What the definition does

\DeclareSIUnit{\meff}{...} creates a unit command named \meff. The second argument supplies the printed symbol:

• \text{m} prints the letter upright.
• _{...} places the following material as a subscript.
• \text{eff} keeps the letters in the subscript upright, as is usual for a descriptive label.

The braces around eff make it a single subscript group. If the subscript is a mathematical variable that should be italic, write it as math instead—for example, \text{m}_{i}.

The example loads amsmath to provide \text. The siunitx package provides \DeclareSIUnit, \qty, and \unit.

Using the unit

Once declared, use \meff anywhere siunitx expects a unit:

\qty{3.2}{\meff} % a value and its unit
\unit{\meff}     % the unit alone

For a document using the older siunitx interface, the corresponding value command is \SI{3.2}{\meff}. The custom declaration itself is the same. New documents should generally use \qty and \unit.

If the subscript contains more than one word or a longer expression, group it explicitly. For example:

\DeclareSIUnit{\mref}{\text{m}_{\text{ref}}}

Then use \qty{12}{\mref}. You can choose a command name that is clear in your source; it does not determine how the symbol is printed.

Common mistakes

• Putting the subscript outside the declaration: Writing \unit{m}_{\text{eff}} leaves the subscript outside the unit argument. Define the full symbol as one unit instead.
• Leaving the subscript ungrouped: In math syntax, m_eff makes only the next character a subscript. Use braces for a multi-character label: m_{\text{eff}}.
• Using a unit prefix as a subscript: Prefixes such as kilo- are handled by siunitx as part of unit parsing. A custom subscripted symbol is a distinct unit symbol and should be declared explicitly.
• Assuming the new symbol is an SI unit: \DeclareSIUnit teaches siunitx how to typeset a command; it does not establish that the symbol is an officially defined SI unit. Use this for specialized or field-specific notation, and follow the conventions of your discipline.

If you need a compound unit with ordinary multiplication, division, or prefixes, keep those parts as separate siunitx unit tokens where possible. A custom symbol defined using text is treated as a single item, so siunitx cannot inspect or modify its internal subscript.