I am building a table and I have something like the following:
I want to know if I have a 3/8 inch bolt, how large should the hole on the diagonal piece be? I already secured it on the table leg.
It's important to remember the difference between a bolt and a screw (this is handy if you don't know the size of the bolt or screw)
This is a rare case where you want the hole to be somewhat larger than the bolt diameter, to allow some shifting and settling. For a 3/8" bolt the hole should be 1/2" or 5/8" diameter. You must use a fender washer under the nut.
The squared and smoothed (deburred) ends of the side and end frame rails are what hold the leg square and prevent wobbling. The bolt is only to keep the leg pressed firmly against the rails. Do not rely on the side loading of the bolt for anything.
Wood is notorious for not holding its dimensions while in use. Changes in temperature and humidity, or just drying and aging, will cause joints to shift and loosen over time. (In fact when a leg gets a little shaky I recommend first loosening then tightening the fastening nut, to allow the leg to resettle into the corner.) If the bolt is tight in the hole, it may prevent the leg from fitting snugly against the rails. Then the washer will cut into the angle brace and the leg will wobble under any serious stress.
Another cause of poor fit is the difference among the legs. It's very difficult to make all four corner braces, or all four legs, exactly the same. Some day someone is going to remove the legs without marking their positions, and if the bolt holes are a tight fit he will have only 1 chance in 24 of ever having a solid strong table again.
It is called 10mm here, but the rule still applies - you drill for the advertised size of the bolt. Neither the hole, nor the bolt, nor the drill bit are exactly 10mm, but everything will fit.
The only notable exception is when you drill in stone/concrete/masonry with a hammer drill. The hole gets +10% - +100% larger and you never know how much.
You drill holes to the advertised size of the bolt. So a 3/8" bolt gets a 3/8" hole.
In your case, alignment may prove tricky. You may need to oversize a bit for that reason.
I like the hole to be snug on the threads, with these screws you don’t have a lot of shoulder so if the screw is a wood screw with the shank close to the screw head it is better to have the screw tight in the hole so the head won’t pull thru the hole of over sized. I was talking about the 4 wood screws that attach the brace not the single wood/machine screw/bolt a washer can easily be fitted to the single screw/bolt but not the smaller wood screws.
I drilled holes for a living.
For very small holes (numbered drills), a clearance hole should be 1 or 2 drill sizes up. For holes under 1/2 inch, drill a clearance hole that is 1/64 larger. For holes larger then 1/2, drill clearance hole 1/32 larger.
You should never be drilling the same dia clearance hole as the screw size. No to 3/8 hole for 3/8 screw. Screws and holes are not always straight and you dont want the two mating pieces to bind on the screw.
You can use a drill & tap chart to see what size your holes should be. This will show you not only what size to drill for a tap, but also what size for the clearance. The one I found happens to include information for different materials, including specific numbered or lettered drills. The companion one is for metric.
Randomly found online charts:
In my experience, these are just guides. They are often really good guides, but due to the quality (or lack thereof) of the tools I'm using, I've needed to go up or down a size to make things work right. Sometimes it's a personal preference, such as liking a snug fit or a looser fit. Sometimes it depends on the materials or how much precision the project needs.
There are a wide variety of these types of charts. I'd recommend trying one out and see if it does what you need. If not, try another one. Or if it's reliably off, stick with it and make sure you remember (or write down) what the adjustment is. Since the one I linked above makes allowances for soft vs hard materials, it seems like it could be a pretty good chart. I haven't used it, so that remains to be seen.