Saying a number digit by digit

Is it okay in an informal conversation to say a number digit by digit?

For example, is it okay to say "two five six kilobytes" instead of "two hundred and fifty-six kilobytes"?

It depends. Normally, it's easier to understand numbers when presented in a familiar way.

• Dates are usually presented in groups of two: nineteen eightyfour
• Commonly-used numbers are sometimes abbreviated: one-twenty-eight, two-fifty-six
• numbers after decimals are usually said singly: three point one four
• regular numbers are almost never abbreviated: two hundred fifty six dollars

There are some exceptions when you'd want to spell the numbers out, mainly when you are trying to ensure that the number is received exactly correctly:

• phone numbers: dial eight six seven five three oh nine
• room numbers: room twenty-three fourteen
• any other time you're dictating a number and want to ensure correct transcription, such as reading your gas meter or if you need to be sure that the person on the other end gets the value correctly

When discussing regular numbers, it's difficult to understand a number if you just say the digits one after another. That's because you don't know how big the number is until the person stops saying it. 604893480264 is a big number, but if read aloud digit by digit the person has to wait until the last 4 before they can go back and try to remember what number they are receiving. Contrast that with six-hundred-four billion, eight hundred ninety three million, four-hundred eighty thousand, two hundred sixty four. Even if the person loses track of the number part way along, they already know the magnitude because you said "billion" right up front.

It's not too common to say a number digit-by-digit in informal conversation. It's usually only used to emphasize each individual number, like in an address for example. However, it is fairly common to say just the hundreds digit of a three digit number. So, saying "two five six kilobytes" would be unusual, but saying "two fifty-six" would be OK in some contexts.

It is possible that in a loud environment you would say digit by digit in this context, especially over the phone where being succinct makes a huge difference, but you would probably abbreviate to "kay" for the bytes (or "gigs," etc.).

Postal codes, phone numbers, and similar numeric identifiers are usually spoken as digits. (One indicator that you should read them as digits is that zeroes, especially leading zeros, are significant.) For all other numbers, probably not.

Sometimes, it is helpful to read numbers digit by digit for clarity. For example, "fifteen" and "fifty" sound alike, so in aviation, such numbers are spoken as "one-five" and "five-zero", respectively; 1500 is "one-five-hundred".

But doing that in informal conversation? You would sound weird, and you probably wouldn't be understood. You could say it redundantly "fifteen (that's one-five)".