Writing large percentages

So, I have a reason to display percentages in the thousands and greater

1000%, 20000%, etc

should they be written with or without thousand separators?

1,000%, 20,000%, etc

I have been Googling and can't find an answer; I keep ending up on sites talking about Excel number formatting.

Numbers up to 9999, whether they are percentages or not, should not use a separator, and from 10,000 on should (and a comma is actually not standard, but it is still common).

I see five references on the first page of Googling for Numbers up to 9999 comma.

My reference to using a space separator corresponds to the SI standard, and, as mentioned in the comment, this is based upon ISO 31-0.

EDIT: I should say, in tabular form, such as in Excel of course, including the separator for 1,000-9,999 makes sense when there is likely to be numbers greater that 10,000.

I think using percentages over 200% just confuses people.

After all it takes quite a while to work out that when you say "20000% of quoted cost" you really mean "200 times the original estimate", you also imply a degree of precision which is probably not true -- are your calculations really accurate to five significant digits?

Certainly if you have to use them then use comma separators or anything else that makes it easier to read.

Normally, you should put thousands separator to improve readability.

The only guide I found (on wikipedia) is on exceptions to digit grouping where it states:

The International Bureau of Weights and Measures states that "when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit".[11] Some manuals of style state that thousands separators should not be used in normal text for numbers from 1000 to 9999 inclusive where no decimal fractional part is shown (in other words, for four-digit whole numbers), whereas others use thousand separators, and others use both. For example, the Publication Manual of the American Psychological Association stipulates a thousands separator for "most figures of 1,000 or more" except for page numbers, binary digits, temperatures, etc.

There are always common-sense exceptions to digit grouping, such as postal codes, page numbers, and ID numbers of predefined nongrouped format, which style guides usually point out.

However, the way you will present large percentages is entirely at your discretion. Percentage stands for a factor of 1/100 and you can choose to express very big percentages using it if you wish so (for example to be consistent with presentation of other values).

Also, I would not call using big percentages confusing, just harder/slower to parse.

Alternatively you can of course, drop the percentage sign and use the regular number with no factor, but this might get unnoticed if you have a bigger table, where huge percentage would visually stand out. The question is if this data is anomaly or it is the focal point of the data presented.

For ratios greater than 1:1, I have seen for example 10x to mean 1000%, and 200x to mean 20000%.

I don't know if this is standard, but it would save you having to worry about comma separators... at least up to 100000% :)