I bought my first impact driver--a Worx 1/4 & 3/8 impact driver. I am trying to buy sockets and realised that these sockets come with drive sizes mostly 1/2".
I read more and I found that the head of this bit is the size, but the bottom size seems the same for all and seem to fit my driver or any driver as it is uni-size (unless the bottom side comes in different sizes and I haven't found that yet).
where is the difference between 1/2 and 3/8 if that bit fits all?
There are impact wrenches, drivers, and combos.
*Also, drills themselves are categorized as 3/8, 1/2, or 3/4, that means that the end of the drill (or "chuck") can twist open that wide for a bit that is that big around.
I will show a picture which may help:
Larger bolts need a beefier impact. The one on the right, is a standard impact for construction: screws, square heads, etc. In the middle they have adapters which will convert to 1/4, 3/8 and 1/2 square, for sockets. This works fine for light tasks, unscrewing large bolts on a table for example.
You can see the back of the sockets differ in size. More heavy-duty tasks like removing car wheels, will require beefier sockets and impact gun (left). The big impact on left has no issues removing wheels, but the impact on the right using the 1/2 adapter would struggle to do it.
So generally speaking, if you need to more heavy-duty work, then you need a large impact (1/2 inch). If you don't need to do this - then the adapters in the middle will work well, allowing you to undo/fasten any size bolt which are not high-torque
the head of this bit is the size, but the bottom size seems the same for all and seem to fit my driver
Yes, the "size" is the size of the square end, the hex end has to fit into the impact driver so it's always the same.
It's possible to buy impact-rated sockets with a hex shaft to mate with your impact driver.
Advantages: simpler, lighter if you have one or two, and the impact driver bit holder will grab them properly so unlike the adapter+socket solution it avoids the embarrassing situation where the socket stays stuck on the bolt then falls off, which is inconvenient when you're on a ladder, or if it falls into the machine you're working on.
Drawbacks: they're a bit more expensive, and if you have several, the lot will be heavier and take more space in the toolbox. And they're not compatible with a socket wrench. So you end up with two sets of sockets.
As mentioned by all the other answers, the head is what's being referenced in the size (1/2", 3/8", ...) and the bottom is intended to be inserted into the chuck of a drill/driver or even a manual handle.
The reason why it's a common size is for maximum flexibility of usage but in many cases not at maximum torque. You see this often in YouTube videos where a long wrench is needed to loosen a bolt, but then a small driver/drill is used to completely remove the bolt.
The pictured attachment is a good example of something to help insert/remove something that is "finger tight" a larger fastener but would probably not be reasonable to tighten to higher torque specifications required by most larger sizes. I will also work fine for smaller sockets and fasteners where the driver will stall before the fastener or socket breaks.
The size of impact driver you've purchased will be limited to lower torque values (e.g. less than 100 ft/lbs, 100 N.m) so this adapter will work for most of the sockets you will be able to purchase for that size of driver. Most impact sockets are 1/2" or larger because those fasteners usually require significant torque (i.e. a bar the length of your forearm or entire arm) and will break if too small.
Almost forgot to mention: Don't use a larger driver on something too small unless you enjoy breaking fasteners/nuts and having to remove them. Unless you know to use higher torque, it's cheaper to use less and let the tool/fastener tell you that it isn't enough, than to mash it and snap things off. Fixing a snapped bolt/screw or stripped head is much more annoying than one that's half in and the driver stopped because you have the setting too low.
