How to type subscript numbers on iPhone
H₂O and CO₂ are typeable on iPhone with no formula editor and no app, but subscript is a much smaller Unicode set than most people expect walking in — solid for digits, thin for letters, and missing several everyday ones that superscript actually has.
A chemistry formula, a variable with an index, a quick H₂O in a text — and the number needs to sit low and small, not on the line with everything else. The iPhone keyboard has no button for that, in Messages, Notes or any app it is installed in. What actually lands on screen when you see H₂O is not a shrunken, dropped “2” — it is its own character, sitting at a separate address in Unicode from the ordinary digit next to it.
Why there is no subscript key
A toggle needs a place to store the instruction “everything from here down is lowered” — a formatting layer sitting underneath the text itself. A text field in Messages, in Notes, or in almost any app on a phone holds nothing but plain characters, with no such layer to write that instruction into. So the only way a subscript effect gets onto the screen is by typing a character that is already drawn small and low to begin with, the same workaround behind bold, small caps and every other fancy alphabet a phone can produce.
Digits and a few signs: the part that holds up completely
This is the one corner of subscript Unicode with no gaps in it. Every digit from 0 to 9 has its own subscript character, and so do the plus sign, the minus sign, the equals sign, and both parentheses. That is already the whole job for most of what people actually type it for: H₂O, CO₂, O₂, a variable written x₁ and x₂, a rate constant kₐ. If a subscript number is all you are after, nothing here runs short.
Letters: a noticeably smaller set than superscript
Superscript letters cover most of the lowercase alphabet, because they were built for ordinal endings like 1ˢᵗ and 2ⁿᵈ, which need a wide spread of everyday letters. Subscript letters were built for a narrower job — phonetic transcription and mathematical notation, things like xₙ or aᵢ — and the set that survives is noticeably thinner: a, e, h, i, j, k, l, m, n, o, p, r, s, t, u, v and x, seventeen letters in total. Ordinary letters like b, c, d, f, g, q, w, y and z were simply never added, because nobody assembling this set letter by letter for a specific notation needed them enough to bother. As with superscript, no capital letter exists in subscript at all — a word or an abbreviation that needs one has to leave that letter at normal size and normal height.
In practice this rarely gets in the way, since the letters that survive happen to be the ones chemistry and maths actually reach for, but it is worth knowing before assuming a subscript version of any letter you like is out there waiting to be typed.
Typing one on iPhone
For a single character, the usual route works: find the subscript number or symbol you need in a reference or a web generator, copy it, switch back to the app you were writing in, and paste. For something typed often — a chemical formula in particular — that gets old fast.
The faster route: a Text Replacement shortcut
Settings → General → Keyboard → Text Replacement saves any text, subscript characters included, as a short shortcut that expands automatically as you type, on any keyboard, in any app. Paste H₂O or CO₂ into the Phrase field, set the Shortcut to something like “h2o” or “co2”, and typing that shortcut followed by a space drops in the real subscript character from then on — no copying, no pasting, no hunting for it again. The same trick covers a footnote-style variable like x₁ or a rate constant like kₐ, and the full setup, including the mistake that makes a shortcut fire when it should not, is covered in the guide on Text Replacement shortcuts.
Where Kibo fits
Kibo carries a range of fancy text styles directly on the keyboard — Bold, Italic, Script, Small Caps, Circled and the rest — but subscript is not one of them, for the same reason superscript is not either: there is no complete alphabet behind it to draw the whole style from, letters especially. What Kibo does carry is a snippets panel built for exactly this kind of text — save a chemical formula or a footnote-style variable once and drop it into any app straight from the keyboard afterward, the same panel used for an address or an email signature. It works with Full Access left off, since inserting saved text is drawing text, not reading the clipboard. Kibo is free to download.
Frequently asked questions
Is there a subscript button on the iPhone keyboard?
No. Plain-text fields on iOS have no formatting layer for a toggle to write an instruction into, so what looks like subscript is a separate Unicode character, typed or pasted in, rather than a style applied to the ordinary digit.
Can I type H₂O or CO₂ directly on iPhone without copying and pasting?
Not with a single keypress, but you can get close: save the character once in Settings → General → Keyboard → Text Replacement as a short shortcut like “h2o”, and typing that shortcut anywhere expands it automatically from then on.
Why does subscript have fewer letters than superscript?
Superscript was built mainly for ordinal endings like 1ˢᵗ, which need a wide spread of everyday letters. Subscript was built for narrower notation — phonetics and maths — so only about seventeen lowercase letters exist, and common ones like b, c, d, f, g, q, w, y and z were never added. Neither one has any capital letters at all.
Try it on your own keyboard
Kibo is a free custom keyboard for iPhone with real mechanical key sounds, a wardrobe of themes and RGB backlight effects. No account, no ads, and it works with Full Access left off.