The HTML <sub> tag
The HTML <sub> element renders text as subscript — lowered and smaller — for meanings that require it, such as chemical formulas (H2O) and mathematical indices. Use it semantically, not for decoration.
Overview
The <sub> element renders text as subscript — lowered and reduced in size — where that position carries meaning. The genuine use cases are specific: chemical formulas such as H2O, mathematical and variable notation, and certain footnote markers.
It is the counterpart of <sup> (superscript). Reserve it for cases where subscript changes the meaning — in chemistry, "CO2" and "CO2" are not interchangeable.
Do not use it merely to make text small and low for visual effect; that is a presentation concern best handled with the CSS vertical-align and font-size properties.
Syntax
<p>Water is H<sub>2</sub>O.</p>Example
<p>The formula for water is H<sub>2</sub>O and carbon dioxide is CO<sub>2</sub>.</p>Best practices
- Use
<sub>where subscript is meaningful — chemical formulas, math notation, some footnotes. - Pair it conceptually with <sup> for superscripts.
- Do not use it for purely visual lowering — use CSS vertical-align for that.
- For complex equations, consider the <math> element (MathML).
Frequently asked questions
What is the sub element for?
How do I write a chemical formula like H2O in HTML?
<sub>: H<sub>2</sub>O renders as H2O.What is the difference between sub and sup?
<sub> lowers text (subscript); <sup> raises it (superscript).