This is an interesting tool I was playing with last week: Hamina Wireless. 

You must upload a floor plan, which you can create with any drawing application. I used RoomSketcher (free plan) to make it from my house plans (approximate because the original plans are from 1955, but the area is close enough). You can't save the plan as an image with a free account but you can just take a screenshot and use that.

Once you upload it to Hamina, you start by determining the scale by drawing a line over the plan and entering its length in real life. From there, you can start adding walls, doors and windows, basically just drawing those over your plan.

You then enter the routers, switches and access points with information about their Wi-Fi characteristics (Wi-Fi version, band, channel, channel width, etc.) and can add client devices. In my case I have the router plus a mesh node, using Ethernet for backhaul.

The result is a coverage heatmap that you can filter between 2.4 GHZ, 5 GHz, 6 GHz. 

This is what I got, which is pretty close for my 5 GHz coverage: