Questions?
Is your Android phone drilling a hole in your pocket with the way it uses up data?
Is your Android device using up too much data?
If you forget to turn off mobile data on your device overnight, does your heart pound or do you get angry at your "silly mistake" when you suddenly realize in the morning?
If your answer is "YES" to any of the questions above, then I think I have a solution for you.
Lots of people have proffered this solution, however, to my amazement, few people
know about it and most of those posts do not explicitly explain, step by step, in clear terms how a new or intermediate user can go about these steps. So I'd try my best to be as detailed as
possible.

There are two solutions to reducing data usage on an andriod device…

First Solution
You need to install a little widget like a "Data switch " on your device. By simply tapping it, you can turn on or turn off you data. This actually simplifies the process of traditionally turning off your
data, which usually is a long process. Saves you the hassle. Right?
Unfortunately, if you inadvertently leave the data on for too long, you still find your Android device voraciously guzzling your data. That's not fair, is it? Android isn't data friendly in that aspect. Sorry.
There is however a hassle free method which leaves you bothering less about whether your data connection is left on overnight or even the whole day and here it is.

Second Solution
You need to install a firewall. Some firewalls require root access (e.g AFWall+ , DroidWall , etc), while some others don't require root access (e.g NoRoot Data Firewall , Mobiwol , NoRoot
Firewall , etc).
Now a quick note, if your phone is rooted, I suggest you download a firewall that requires root access, but if your phone is not rooted and you don't want it rooted, download a firewall that does not require root access.
If you are unsure about the root status of your device or you don't just know what rooting means, simply download any of the firewalls that don't require root access.

Follow the instructions below once you have the firewall installed. Note that the firewall typically allows you to Deny/Allow access to individual apps on your device when you're connected to the internet via Mobile Data or Wi-Fi (Note the icons).

Note: IF THE INSTALLED FIREWALL REQUIRES ROOT ACCESS : Grant your firewall root
access if you get a prompt.
1. Open the firewall app and select only the apps you want to grant access to the internet (thereby reducing the amount of overall data usage)
2. Ensure the firewall mode is set to "whitelist" which means, only apps you select will have access to the internet, all unchecked apps will not have access to the internet.
3. If you prefer to use "blacklist" however, it means apps you select will not be able to access the internet, while unchecked apps will be able to access the internet. You need to pay attention to this
part.
4. Finally, select "apply rules" and make sure the firewall is enabled. Usually, people do every other step, but forget to enable the firewall. Why on earth will you go through all these steps and forget that?
Note: If you get a notification saying your firewall can't get root access, it's either your phone isn't rooted (get it rooted/install a firewall that doesn't require root access) or the firewall is being blocked by the app handling root requests (Usually #SU binary installed while rooting your device). Check the app handling super user requests and ensure you set it to grant your firewall root access.]

Note: IF THE INSTALLED FIREWALL DOES NOT REQUIRE ROOT ACCESS :
1. Open the firewall app, click the part that says you understand what you're doing and you trust the app, then enable the app to filter all traffic by clicking "enable" or "start" depending on
the app.
2. Next, try to use any of your apps that normally access the internet. The firewall will keep a list of apps that try to access the internet in a "Pending access" tab for you to review. Allow those you want to grant internet access and deny others. Your choice will affect the apps' subsequent attempt to connect to the internet. If you change your mind about an app later, you can always return to grant or deny it access.
3. Now you are done, you can go to bed leaving your mobile data on. However, you must remember that leaving it on drains your battery faster
Read more: http://www.wizloaded.com/2015/11/steps-to-reduce-data-usage-on-andriod.html

fineyemi:
Questions?
Is your Android phone drilling a hole in your pocket with the way it uses up data?
Is your Android device using up too much data?
If you forget to turn off mobile data on your device overnight, does your heart pound or do you get angry at your "silly mistake" when you suddenly realize in the morning?
If your answer is "YES" to any of the questions above, then I think I have a solution for you.
Lots of people have proffered this solution, however, to my amazement, few people
know about it and most of those posts do not explicitly explain, step by step, in clear terms how a new or intermediate user can go about these steps. So I'd try my best to be as detailed as
possible.

There are two solutions to reducing data usage on an andriod device…

First Solution
You need to install a little widget like a "Data switch " on your device. By simply tapping it, you can turn on or turn off you data. This actually simplifies the process of traditionally turning off your
data, which usually is a long process. Saves you the hassle. Right?
Unfortunately, if you inadvertently leave the data on for too long, you still find your Android device voraciously guzzling your data. That's not fair, is it? Android isn't data friendly in that aspect. Sorry.
There is however a hassle free method which leaves you bothering less about whether your data connection is left on overnight or even the whole day and here it is.

Second Solution
You need to install a firewall. Some firewalls require root access (e.g AFWall+ , DroidWall , etc), while some others don't require root access (e.g NoRoot Data Firewall , Mobiwol , NoRoot
Firewall , etc).
Now a quick note, if your phone is rooted, I suggest you download a firewall that requires root access, but if your phone is not rooted and you don't want it rooted, download a firewall that does not require root access.
If you are unsure about the root status of your device or you don't just know what rooting means, simply download any of the firewalls that don't require root access.

Follow the instructions below once you have the firewall installed. Note that the firewall typically allows you to Deny/Allow access to individual apps on your device when you're connected to the internet via Mobile Data or Wi-Fi (Note the icons).

Note: IF THE INSTALLED FIREWALL REQUIRES ROOT ACCESS : Grant your firewall root
access if you get a prompt.
1. Open the firewall app and select only the apps you want to grant access to the internet (thereby reducing the amount of overall data usage)
2. Ensure the firewall mode is set to "whitelist" which means, only apps you select will have access to the internet, all unchecked apps will not have access to the internet.
3. If you prefer to use "blacklist" however, it means apps you select will not be able to access the internet, while unchecked apps will be able to access the internet. You need to pay attention to this
part.
4. Finally, select "apply rules" and make sure the firewall is enabled. Usually, people do every other step, but forget to enable the firewall. Why on earth will you go through all these steps and forget that?
Note: If you get a notification saying your firewall can't get root access, it's either your phone isn't rooted (get it rooted/install a firewall that doesn't require root access) or the firewall is being blocked by the app handling root requests (Usually #SU binary installed while rooting your device). Check the app handling super user requests and ensure you set it to grant your firewall root access.]

Note: IF THE INSTALLED FIREWALL DOES NOT REQUIRE ROOT ACCESS :
1. Open the firewall app, click the part that says you understand what you're doing and you trust the app, then enable the app to filter all traffic by clicking "enable" or "start" depending on
the app.
2. Next, try to use any of your apps that normally access the internet. The firewall will keep a list of apps that try to access the internet in a "Pending access" tab for you to review. Allow those you want to grant internet access and deny others. Your choice will affect the apps' subsequent attempt to connect to the internet. If you change your mind about an app later, you can always return to grant or deny it access.
3. Now you are done, you can go to bed leaving your mobile data on. However, you must remember that leaving it on drains your battery faster
Read more: http://www.wizloaded.com/2015/11/steps-to-reduce-data-usage-on-andriod.html

u can also resrict background data(only the apps u open get access to the internet)