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Load the bm package and use \bm{...}: it finds a bold version of Greek letters, operators, and symbols while keeping each glyph's shape - italic letters come out bold italic, upright operators come out bold upright. Do not rely on \mathbf - it only affects letters and digits, uprights them, and silently leaves Greek letters and symbols unbolded. For the rare symbol \bm cannot handle, \pmb (poor man's bold) fakes the weight by overprinting.
Why \mathbf is not the answer
\mathbf switches to the bold upright text font inside math mode. It works for Latin letters and digits - \mathbf{x} gives a bold upright x - but it has no bold upright Greek or symbol glyphs to switch to, so $\mathbf{\alpha}$ and $\mathbf{\nabla}$ come out unchanged. It also changes shape, not just weight: math variables are conventionally italic, and \mathbf makes them upright, which can clash with journal notation that expects vectors and matrices bold italic.
The bm package: the standard fix
The bm package (load it after amsmath with \usepackage{bm}) provides \bm{...}, which finds the bold version of nearly every math glyph while preserving its shape and spacing class: $\bm{\alpha}$ comes out bold italic, and upright symbols like $\bm{\nabla}$ and $\bm{\sum_{i=1}^{n} x_i}$ come out bold upright. The package documentation recommends \bm over the older \boldsymbol command from amsmath wherever both work, because \bm handles more symbols and spacing classes. In practice \boldsymbol{\alpha} produces the same glyph for common cases; \bm is the safer default.
When \bm is not enough: \pmb
Occasionally a symbol has no bold variant in the fonts you are using. For those, amsmath's \pmb (poor man's bold) prints the glyph several times with tiny offsets to simulate weight, as in $\pmb{\wp}$. It is a visual compromise, so use it only when \bm genuinely fails, not as a first choice.
A complete example you can compile
\documentclass{article}
\usepackage{amsmath}
\usepackage{bm}
\title{Bold Math Symbols in LaTeX}
\date{October 2026}
\begin{document}
\maketitle
The command \verb|\mathbf| makes letters upright and bold, as in
$\mathbf{x}$, but it does not affect Greek letters or symbols:
$\mathbf{\alpha}$ and $\mathbf{\nabla}$ stay unbolded.
For bold math that covers Greek letters and symbols, load the
\emph{bm} package and use \verb|\bm|: $\bm{\alpha}$ comes out bold
italic, while $\bm{\nabla}$ and $\bm{\sum_{i=1}^{n} x_i}$ come out
bold in their original upright shape - \verb|\bm| bolds each glyph
without changing its shape or spacing class.
The older \verb|\boldsymbol| from \emph{amsmath} also works, as in
$\boldsymbol{\alpha}$, but the bm package documentation recommends
\verb|\bm| for most cases. When \verb|\bm| has no bold glyph for a
rare symbol, \verb|\pmb| (poor man's bold) fakes one by overprinting:
$\pmb{\wp}$.
\end{document}We verified this file compiles as-is with a local Tectonic 0.17.0 compile on October 30, 2026, and inspected the rendered page: the \mathbf Greek and nabla visibly stay thin while the \bm and \boldsymbol versions come out bold.
FAQ
How do I make a Greek letter bold in LaTeX?
Use \bm{\alpha} with the bm package loaded. \mathbf{\alpha} does nothing to Greek letters, which is the most common reason a bold Greek command "does not work".
What is the difference between \bm and \boldsymbol?
Both produce bold math glyphs that keep each symbol's shape: bold italic for italic letters, bold upright for upright operators. \boldsymbol ships with amsmath; \bm comes from the bm package, which its documentation recommends as the more capable option - it covers more symbols and preserves spacing classes better. For everyday Greek letters they look the same.
Why is my bold math command not working?
The usual cause is \mathbf applied to a Greek letter or symbol, which it cannot bold. Switch to \bm. If \bm errors, check that the bm package is loaded after amsmath and that the symbol exists in your fonts.
How do I bold a whole equation?
Wrap the math in \bm{...}, for example $\bm{E = mc^2}$. For entire blocks of bold math there is also the \boldmath switch, which makes all following math bold until \unboldmath, but per-expression \bm is easier to control.
Can I use \bm with XeLaTeX or LuaLaTeX?
With the unicode-math setup those engines use, bold math is normally handled by \symbf{...} instead; the bm package is the pdfLaTeX-era solution. This example was compiled with Tectonic, which uses a XeTeX-derived engine, and bm worked as shown - but if your project loads unicode-math, use its \symbf family instead.
Further reading
Companion guides in this series cover typing backslashes and tildes, and citing websites with BibTeX. Both are coming to this series. More how-tos are on the Bibby AI blog. Source question: How can I get bold math symbols? on TeX Stack Exchange.
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Last updated: October 30, 2026. By the Bibby AI editorial team.