Spaces are the usual disagreement
Most forms state a limit "including spaces", but plenty state it without, and some do not say at all. The gap between the two is large — roughly a sixth of a typical paragraph is spaces.
Write to the smaller number when a form is unclear. Being under a limit costs you nothing, while being over usually means the form truncates your text silently at submission.
Line breaks are counted, and they are invisible
A line break is a character. Some systems store it as one character, others as two, and none of them show it to you. If you are near a limit and cannot account for the difference, your paragraph breaks are a likely explanation.
This is also why text pasted from a word processor can suddenly run long. Soft wrapping in the editor becomes hard line breaks when it lands in a plain text field.
When the limit is in bytes
Some forms — public sector applications in particular — limit the field by storage size rather than by character count. This matters enormously for languages outside the basic Latin alphabet.
In UTF-8, a plain Latin letter takes one byte while a Korean syllable takes three. A 300-byte field holds 300 English letters but only 100 Korean characters. If a limit looks strangely small for the text you are writing, it is probably a byte limit.
Older systems may use a different encoding where a Korean syllable takes two bytes instead of three, so a byte limit is only meaningful once you know which encoding the form uses. When the form does not say, treat the UTF-8 figure as the cautious one — it is the larger of the two.
Check against the form itself before you submit
No external counter can know a particular form's rules. Use one to draft and to get close, then paste into the real field and read whatever number the form shows you. That number is the one that decides whether your text is accepted.
Leave a margin of a few percent. Forms that trim trailing whitespace, normalize quotation marks, or convert line endings can shift the count between your draft and the submission.
Why the counter here does not upload your draft
An application essay is personal writing, often naming your employer, your history, and your circumstances. It should not need to travel to a server to be counted.
The counter on this site runs in your browser. The text stays in the page and is gone when you close it.