What constitutes a gigabyte

A gigabyte (GB)  = 10⁹ or 1 billion bytes or 1,000 megabytes (MB).  The term has also been used in information technology for 1,073,741,824 (1024³ or 2³⁰) bytes.   For the purpose of resolving this confusion, IEC 80000-13 was published as the international standard in November, 2009 to:

               Decimal                                                Binary

10³ = kilobyte (KB)     1,000 bytes      2¹⁰ = kibibyte (KiB)      1,024 bytes (used in usage calculation)

¹⁰⁶ = megabyte (MB) 1,000 KBs         2²⁰ = mebibyte (MiB)  1,024 KiBs

10⁹ = gigabyte (GB)   1,000 MBs        2³⁰ = gibibyte (GiB)     1,024 MiBs

10¹² = terabyte (TB)  1,000 GBs        2⁴⁰ = tebibyte (TiB)      1,024 GiBs

See Binary vs Decimal Data Measurements for more information.  

Cox uses a mix of 1,024 binary (KiB) and decimal to calculate data usage limit, but under-reports the actual established limit in TBs.  Their calculation simply sets the data limit at 1.25 billion KiBs rather than use the actual number of bytes in a TB.  Cox websites report 1.25 TB = 1,280 GB.  However 1,280 GB is 30 GB more than 1.25 TB.   Either could be the data limit, but not both.

Cox Internet packages include a monthly data plan of 1.25 TB (1,280 GB).

"All Cox Internet plans include 1.25 TB (1280 GB) per month of data usage".   <==  Cox website near the bottom of the page.==>  Learn About Cox Internet Data Usage | Cox

1,024 bytes * 1.25 billion  = 1,280 billion bytes = 1,280 GB = 1.28 TB = 1,192 GiB = 1.16 TiB

You might get an extra 30 GB before exceeding the 1,280 GB limit reported on Cox websites.  However, Cox's data usage algorithm doesn't read Cox websites.  If the monthly data usage limit is actually set to 1.25 TB in code, you'll exceed your data usage limit if you go over 1,250 GB.  

I have a feeling this may be happening.  Cox should investigate to determine if this is the case.  If yes, the best thing they could do, from a public relations standpoint, would be to officially raise their data usage limit to 1.28 TB and modify their algorithm code accordingly.  But, they may choose not to acknowlege errors in website reporting and just make the code change.