drillscrewsshelvingwalls
I have a shelf upright that has 7mm sunken holes, like this one here: https://www.twinslotshelving.co.uk/shelving-uprights/1m-white-upright
I'm just unsure about the size of the things I need for fixings.
What size drill bit should I use for the wall hole? and then what size wall plug and screw do I need?
You need to make the hole the "right" size - neither too big nor too small.
You should choose the size (diameter and length) of screw based on the job you are doing. You should then choose the size of wall plug based on this.
The internal diameter of the wall plug is determined by the size(s) of screw it is designed to take. The external diameter of the wall plus is determined, in part, by the load it's designed to take. A "heavy duty" plug will be bigger than a "light duty" one.
You should then choose the drill bit required to fit the plug snugly. This should be printed somewhere on the packaging. You might find you need to go one drill size larger if you are using the maximum size (diameter) screw recommended for the plug.
A standard #10 screw has an outer thread diameter of 3/16". The #10 countersink you are referring to is a combined tool designed for applications where the screw is to pass completely through the material without digging the threads into the wood and have the head sit flush. If you wish to use a screw that is actually going to grip the material, such as a wood screw or self-tapping screw, do not buy the item. Instead you will need an appropriately sized #10 pilot drill (3/32" for soft wood, 7/64" for hardwood) and a separate countersink bit designed for your chosen screw size.
So, the answer depends on the application. If you intend for the screw to pass completely through the first work piece with nothing but the head securing it (no threads biting into the wood) then the combined #10 drill/countersink is great. If you intend for the screw to bite the material, you need a separate pilot drill and countersink bit.
Best Answer
I'd try my best to screw those into studs with the largest screw that will sit flush in the countersink.
If you can't line it up with studs, get the largest anchor that will fit the countersink.
Something like: https://www.renodepot.com/en/hollow-wall-anchors-0484236 and NOT the plastic ones. This anchor will bend out in 4 directions behind the wall board and will distribute the weight on a larger surface area.
Of course, the one I linked does not have a countersunk screw so you'll want to pick some up as well unless you don't mind the head sticking out a little.
Again, the best would be to screw these directly to a stud.