The most I've ever used is around 700GB, otherwise it's 500-600 GB. I cancelled Directv and have been streaming movies from Prime all day everyday, plus my PC pretty much stays online all day, not to mention my Ring cams are always active. Are you streaming 4k resolution or something?
I tried out their data calculator and you can stream almost 14 hours of HD content per day without breaking past the data limit, but of course this might depend on how many days in the monthly billing cycle they use. They might use 31 days one month (causing you to go over the limit, and 30 days for another billing cycle, leaving you under the limit. I suggest bookmarking the link to your data usage and check it everyday.
Maybe someone is stealing your bandwidth. It's not unheard of. Google it for how to find out.
Meanwhile, change your passwords on everything connected such as your wifi router and your computer. If someone nearby hacked your wifi router, they could be using up your data plan. Then start monitoring your data usage often, to see if changing your passwords made a difference.
St8kout: Thanks for the reply. This is exactly why I know my data isn't being calculated correctly. I don't stream in 4K, only 1080p. Per your example of using 14 hours per day of HD streaming without going over and Cox's own example of using 10 hours of HD video streaming plus 7 hours of streaming music per day to being equivalent of 1280 GB per month is exactly why I know my data isn't being calculated correctly. Most people I've talked to with Cox internet have numerous devices & use much more data, like yourself & don't have the problem like I do of going over the data limit. Like I mentioned, I can only use one device-streaming Hulu on tv & it's no where near 14 hours per day, most days not half that much (yet I still will go over their 1280 data limit unless I shut down my modem & router for at least 4 days out of the cycle which I'm about to do right now for this cycle as I'm right at the limit with 4 days left to go through Sept 2). I can't use WiFi on my iPhone to backup, update iOS & I only get 2 bars at my house from Verizon's cellular data, my iPad has become absolute because it only works with WiFi. Cox always uses 31 day cycles. I already have their data usage link bookmarked & check it daily. I already changed my WiFi password per Cox's suggestion and it hasn't made a difference. All I know for sure is that one person living in one residence, using one device shouldn't have this issue. And Football season hasn't even officially started yet.
