Left-align an align environment in LaTeX.
By default, LaTeX centers every equation block in the align and align* environments. To push them to the left margin, load the amsmath package with the fleqn option, use flalign, or set a custom math indentation.
The short answer: add \usepackage[fleqn]{amsmath} to your preamble to make every equation block left-aligned, or use the flalign environment when you only need a specific block pushed left.
Make every equation left-aligned globally
If every equation in your document should sit at the left margin instead of centered, pass the fleqn option directly to the amsmath package in your preamble. You can also use mathtools since it loads amsmath internally and passes unknown arguments straight through.
By default, fleqn indents equations by a small amount. To remove this indent completely and push everything flush to the left margin, set the math indentation to zero right after loading the package.
Code for global left alignment
Using amsmath directly
Add this block to your preamble to pull all math blocks to the left across the entire document.
\usepackage[fleqn]{amsmath} \setlength{\mathindent}{0pt}Using mathtools
If your document loads mathtools for extra math tools, pass the same option there.
\usepackage[fleqn]{mathtools} \setlength{\mathindent}{0pt}
Left-align a single block with flalign
When most of your document stays centered but a single equation needs to sit on the left, switch from align* to flalign*. The flalign environment creates full-width column alignment, and adding an extra ampersand at the end of each line pushes the entire block against the left margin.
Note the trailing ampersand before the line break. Without it, the block will not pin correctly to the left edge of the page.
Code for left-aligning one block
Using flalign* with trailing ampersands
This code keeps the rest of your document centered while forcing this specific block to the left.
\begin{flalign*} x &= abcd &\\ &= abcd + efgh & \end{flalign*}
Aligning text descriptions inside equations
When you write equations with descriptive text on the right using \text{}, the align environment alternates between right-aligned and left-aligned columns. If your text descriptions line up on the right and you want them flush left, you need an extra ampersand before the text command to push it into the correct column.
Code for text column alignment
Double ampersand before text
Use two ampersands before \text{} to jump past the intervening column and left-align your descriptions.
\begin{align*} c(s) &= [x_c(s), y_c(s)]^T && \text{Centerline} \\ i(s) &= [x_{in}, y_{in}]^T && \text{Inner Border} \end{align*}
Questions
How do I left-align all equations in my document?
Add \usepackage[fleqn]{amsmath} to your preamble and set \setlength{\mathindent}{0pt} right below it. This moves every displayed equation block from the center to the left margin.
Can I left-align just one specific align block?
Yes. Use the flalign* environment instead of align*, and place an ampersand at the end of each line before the double backslash to push the entire block to the left.
Why is my text description aligned to the right inside an align environment?
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