Move the text out of the <span>
with the glyphicon and apply vertical-align: middle;
to the <span>
<button class="btn btn-default">
<span class="glyphicon glyphicon-th" style="vertical-align: middle;"></span> Centered
</button>
Demo
This works in Bootstrap 4 with Font Awesome icons like so
<button class="btn btn-default" style="vertical-align: middle;">
<i class="fa fa-times"></i> Centered
</button>
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