Enter the networks visible nearby, with their channels and signal strengths, and get a recommendation for the least congested channel.
2.4 GHz channels are 20 MHz wide but spaced 5 MHz apart, so neighbouring channels overlap. Only channels 1, 6 and 11 do not overlap each other.
This is a planner, not a scanner: a web page cannot scan Wi-Fi or see nearby networks, so the networks to consider are entered manually from a phone's Wi-Fi list or a router's status page. On 2.4 GHz each channel overlaps the four channels either side of it, which is why channels 1, 6 and 11 are the standard non-overlapping set; the planner scores those three against the entered networks, weighting strong nearby signals more heavily, and flags co-channel and overlapping entries. On 5 GHz channels are wider and mostly non-overlapping at 20 MHz; wider 40 or 80 MHz settings bond neighbouring channels together, so the guidance lists common channels and recommends the least used one among the entered networks.
Why can this page not scan for networks itself? Browsers give web pages no access to Wi-Fi scanning — a page cannot see nearby networks, their channels or their signal strengths. The networks have to be entered manually, copied from a phone or router list. Any site claiming to scan from the browser alone is not doing so.
Why are channels 1, 6 and 11 recommended on 2.4 GHz? 2.4 GHz channels overlap their neighbours, and 1, 6 and 11 are the only three channels in the band that do not overlap each other at all. Using one of them avoids the partial-overlap interference that mid channels cause.
What channel width should be used on 5 GHz? 20 MHz fits the most separate networks, 40 MHz doubles the width by bonding two channels, and 80 MHz bonds four for higher speed where the band is quiet. In a busy area a narrower width on a clear channel is often more reliable than a wide one overlapping neighbours.