Is a number easier to read with the thousand separator?

This may sound like a stupid question but I'm being taken to task for this at the moment. Personally I believe that it's easier to read 1,234.56 than 1234.56. However there are others that are saying that it is not.

Has the ease or difficulty of reading numbers with and without thousand separators ever been empirically tested and if so what were the results?

• At 4 digits, it makes very little difference. Beyond that, the separator should exist. Unless there is a mix of 4 and 5 digit numbers, in which case, for consistency all should have the thousands seperator Apr 20, 2012 at 20:23
• You could/should have flagged your Skeptics question for Migration to UX but it's fine here. I'm going with "duh" but I'd love to see experimental evidence Apr 20, 2012 at 20:23
• As a side note I seriously hate that the game Disgaea doesn't have thousands separator, I find it hard to even determine the order of magnitude for numbers like 52352554 Apr 20, 2012 at 20:24
• context is everything
– DA01
Apr 20, 2012 at 20:25
• To illustrate the importance of separators, consider that both phone and credit card numbers are conventionally displayed with separators that aid both reading and memorisation. Apr 21, 2012 at 8:26

Back to the question where the answer is yes, it improves readability, at least if you listen to Jakob Nielsen who (yet again) wrote an article on 113 Design Guidelines for Homepage Usability where #112 says:

112) Use a thousands separator appropriate to your locale for numbers that have five or more digits. For example, in the United States, fifty-three thousand should be written "53,000."

Now the locale thing is important, because what you write in the questions doesn't make sense to most of my fellow contrymen (Swedes). The 1,234.56 would mean just one and barely a quarter as in 1.25.

Still keep the thousand separator whether its 1,000,000 or just 1 000 000 (Swedish for a million).

You need to be very careful over this kind of issue if working internationally. What is used as a thousands separator in one country - may not be used in another.

I once saw a system which had been developed in Germany go horribly wrong when the English started inputting data...

http://userguide.icu-project.org/formatparse

For example, the number 12345.67 is

• "12,345.67" in the US,

• "12 345,67" in France and

• "12.345,67" in Germany.