How to know the wifi channel already used

When you will install wi-fi (access point) device, especially if the wifi device more than one, two or three, then you should pay attention to the wifi channel, because if the channel setting of each device is not correct, the  interference may be happen that may cause the connection to be slow or even dropped.

In general, default settings of wifi device  for the channel is set to automatically, when there are existing wifi signal, the new wifi device  will seek and occupy unused channel.

But just to be safe if you are confident in your area has wifi signal and you will install the new one, the channel should be set up manually. to avoid interference between the devices.
Channel menu on wifi device generally will be found in Wireless section, refer to your device  manual instruction.

The easy way to know wi-fi channel already used in the area, I use Wifi Analyzer, the App for Android. Easy to use and has good measuring result.
Take your Android device, go to Play Store,  search using keyword:  Wifi Analizer , choose the Wifi Analizer from farproc with the logo:

After download and installed, run it, and the app will inform you the wifi detected in your area.

Sample captured in my area :

The sample above show the wifi use the channel 1 and channel 6, the next recommended channel is channel 11, because 1,6,11 have no interference.

For details about the relation between the Wi-Fi channel at 2.4 GHz can be seen in the following graph:

Graph 2.4 GHz Wi-Fi Chanel

The width of each channel frequency is 22 MHz,
Choose the channel with no interference
2, 7,12
3,8,12 and so on.