How to Bold Math Symbols in LaTeX

LaTeX uses different commands for bold text and bold mathematical symbols. The right choice depends on what you want to bold: a Latin letter, a Greek letter, a relation sign, or an entire formula.

For most documents, use \mathbf for upright Latin letters and digits, and use \bm from the bm package when you need to bold Greek letters or other math symbols. The \boldsymbol command from amsmath is a useful alternative for many symbols.

Choose a command for the symbol

Command	Best for	Example
\mathbf{...}	Upright Latin letters and digits	\mathbf{A}
\boldsymbol{...}	Many bold math symbols, including Greek letters	\boldsymbol{\alpha}
\bm{...}	Broad support for bold math symbols and expressions	\bm{\nabla}
\boldmath	Switching a whole formula to the bold math version	\boldmath $x+y$

\mathbf changes the math alphabet to bold upright. It does not make Greek letters bold: \mathbf{\alpha} generally will not give you bold alpha. For Greek symbols, use \boldsymbol or \bm instead.

The \boldsymbol command is provided by amsmath. The bm package is often the most convenient option when you need to bold a variety of math symbols, including Greek letters, operators, and delimiters. Load it in the preamble with \usepackage{bm}.

A complete example

This minimal document shows the common choices and compiles with standard LaTeX:

\documentclass{article}
\usepackage{amsmath}
\usepackage{bm}

\begin{document}

Upright bold Latin letter: $\mathbf{A}$.

Bold Greek letter: $\boldsymbol{\alpha}$ or $\bm{\alpha}$.

Bold vector and ordinary index: $\bm{v}_i$.

Bold operator: $\bm{\nabla} f$.

A whole formula in the bold math version: {\boldmath $x + y = z$}.

\end{document}

The amsmath package supplies \boldsymbol, while bm supplies \bm. In $\bm{v}_i$, only the vector symbol is inside the command, so the index stays unbolded. In general, place the command around exactly the part you want to change.

How to bold a whole formula

Use \boldmath when you want to select the document’s bold math version for an entire formula. It is a declaration, so put it outside math mode. Braces keep the change local:

{\boldmath $x + y = z$}

The declaration must take effect before LaTeX enters math mode. Writing \boldmath inside $...$ is too late and produces an error. Also, the exact appearance depends on the bold math fonts available in the document; not every font family provides a bold version of every symbol.

For selected symbols in a formula, \bm or \boldsymbol is usually more precise than changing the whole formula’s math version.

Common mistakes

Using \textbf in math mode

\textbf is for text, not mathematical symbols. It does not reliably produce bold math letters or Greek symbols. Use a math command such as \mathbf, \boldsymbol, or \bm instead.

Expecting \mathbf to bold Greek letters

\mathbf selects bold upright Latin letters and digits. For example, use \bm{\alpha} for a bold Greek alpha. If you want a bold italic vector letter, \bm{v} is generally more suitable than \mathbf{v}, which is upright.

Putting \boldmath inside a formula

Because \boldmath changes the math version, it must be issued in text mode, before the math expression begins. To limit its scope, enclose the declaration and formula in braces.

Bolding more than intended

Commands apply to their arguments. Compare \bm{A}_i, which leaves the index unbolded, with \bm{A_i}, which applies the bold command to the entire grouped expression. Keep subscripts, superscripts, and punctuation outside the argument unless they should also be bold.

Which one should you use?

For a single upright matrix name such as $\mathbf{A}$, use \mathbf. For bold Greek letters or symbols such as a gradient, try \bm if you have loaded the package; use \boldsymbol when you want the amsmath command. For a formula that should be bold throughout, use \boldmath before entering math mode and check that your chosen fonts provide the desired bold symbols.