Inverse Case Converter

Flip the case of every letter instantly

Inverse case - also called toggle case, swap case, or flip case - is a transformation that examines each letter individually and replaces it with its opposite case: uppercase becomes lowercase, lowercase becomes uppercase, and anything without a case is left alone. "Hello World" becomes "hELLO wORLD". The defining property is that the output depends entirely on the input's existing casing, which is what separates it from every other case converter.

The single most common reason people look for it is the Caps Lock accident. You type a paragraph without noticing the key is engaged, and because Shift now produces lowercase, the result comes out as "hELLO, mY nAME iS jOHN." Every letter is precisely wrong, which means one pass through an inverse case converter restores the intended text exactly. No other operation does this: forcing lowercase loses your capitals, and title case would capitalise words that should have stayed lowercase.

How it differs from its neighbours:

• vs. alternating case - alternating case ignores the input and flips on position (aLtErNaTiNg), so it produces the same pattern no matter what you feed it. Inverse case is input-dependent.
• vs. UPPERCASE / lowercase - those are lossy and idempotent: run them twice and nothing more changes. Inverse case is lossless and self-inverse.
• vs. Sentence case or Title Case - those apply grammatical rules about word boundaries and stop words. Inverse case applies no rules at all.

Is inverse case really its own inverse?

For English text, yes - it is an involution, meaning f(f(x)) = x. Run it twice and you are back where you started, which is why it works as a safe toggle you can undo without an undo button. Unicode adds a few genuine exceptions worth knowing about. The German ß has no single-character uppercase in traditional orthography and expands to SS, so the round trip yields ss. Turkish distinguishes dotted and dotless i, so a locale-aware conversion turns i into İ rather than I. And Greek final sigma ς and medial σ both uppercase to Σ, collapsing a distinction that cannot be recovered on the way back down.

Beyond fixing typing mistakes, the effect gets used deliberately: sarcasm markers on social platforms, deliberately jarring typography in memes and band names, and generating case-variant wordlists for testing that a login form or search index really is case-insensitive. Conversion is performed character by character in your browser - nothing is uploaded, whatever you paste.