Why people lowercase text
- To compare text fairly. Apple, APPLE and apple are different strings to a computer. Lowercasing both sides first makes them match.
- To clean lists. Email lists, tag lists and keyword lists are full of the same entry in different capitalization. Lowercase them, then remove the repeats with the duplicate line remover.
- To normalize usernames, handles and file names, where capitalization should not create a second copy.
- To undo shouting. Text typed with caps lock on becomes readable again. For proper sentences afterwards, use the sentence case converter.
Hashtags are a special case. Platforms treat them as case insensitive, so lower case works, but a hashtag written as #LowerCaseConverter is easier to read, and screen readers can read it as separate words. #lowercaseconverter cannot.
Email addresses and URLs
The domain part of an email address is not case sensitive. The part before the @ technically can be, but practically every mail provider treats it as case insensitive. Lowercasing an address is safe in practice, even though the standard does not promise it.
Be more careful with URLs. The domain is case insensitive, but the path often is not: on many servers /Report.pdf and /report.pdf are different pages. Lowercase the domain, and leave the path alone unless you know the server ignores case.
Lowercase versus casefold
For comparing text in code, programmers sometimes prefer a casefold function, such as Python's str.casefold(), to ordinary lower casing. Casefolding is more thorough: it turns the German ß into ss, so that 'Straße' and 'STRASSE' match. This page applies ordinary lower casing, in which ß stays ß. That is what you want when you are rewriting text for people rather than matching it.
In Excel and Google Sheets the formula is =LOWER(A1). In JavaScript it is text.toLowerCase(), in Python text.lower() and in SQL LOWER(column).