As an only English speaker, I don't think I've ever seen someone put a space before a percent sign. Or at least, not that I can recall. It definitely looks weird to me.
The space between a number and the percent symbol is completely foreign to me. I have never ever seen anyone do that, but then again I hail from the USA, and it is convention here to not put a space between the number and percent symbol. Thus, we here in America write 5%, 10%, 100%, etc.
I've ranted about percentages in Lemmings for years. Indeed, 97 % is less useful than the positive save requirement 39/40 or the negative count, lose 1. But 100 % is still so common colloquially and needs no denominator (40) to compute; I feel like 100 % is special. I'll have to sleep over it. Probably just say "all".
That makes sense that 100% is special, considering that percent means "per hundred." It also does have connotations in the sense of the accomplishment of getting everything correct on a test or completing everything for examples, which can be very satisfying depending on what it is.
And I wouldn't say that physical units always have a space in English, writing 220V or 60Hz is definitely normal, though adding the space there is probably more common than doing it for %. This might also depend on the unit though? I'm trying to think if there's any I would expect a space.
In fact, it looks like Google insists you don't put the space
Yea, Simon's post got me to pause and think about how I write quantities with physical units. I honestly have always written them together with no space, e.g, 5ft for 5 feet rather than 5 ft. The latter is therefore strange and foreign to me as well. However, I read up on this, and the convention is to always put a space. In that case I've been writing them "wrong" for years!
The exception is when you are dealing with notation symbols, like with the percent sign. In this situation, it's convention to not put a space between the number and it. Thus, that's why you see 10%. Then again, can we really say whether one way is truly incorrect and the other way is correct in either situation?
I think it's just a matter of personal preference/style. You can't really say either one is incorrect in writing them. The most common example I always see is the date format. Here in America, it's month/date/year, but in other countries, like in Europe, it's instead date/month/year. There's still even other variations, such as in my parents' home country of Vietnam, where the format is
year/date/month edit: Wow, I apparently don't know what I'm talking about
I'm wrong, it's the same as Europe, date/month/year, and also there's no place, it appears, that uses year/date/month, but there are places that use year/month/date. So, can we really say that any of these is incorrect in writing out the date shorthand? As I am accustomed to the American way of writing it due to where I grew up, there's absolutely no chance of me adopting the European way or any other country's way of writing the date other than month/date/year. However, since differing formats can lead to confusion, the safest way to avoid any is to simply write it out completely in words, e.g, November 2, 2022.