I am a DIY n00b ... help with putting up a shelf please :D

I have an IKEA bracket set with shelf which has been sitting around the house for an eternity not put up. The reason I have put it off for so long is because it didn't come with screws. Now I have decided to put it up I am very unsure what length screws to use.

This is the bracket. The wood is about a centimetre and a half thick. So what length screw do I need to securely fasten it to the wall. I am not going to put anything seriously heavy on it other than maybe a few books.

P.s I do have a spirit level so don't worry about that :D

What's behind the wood this is going to screw into?

How thick is the wood of the bracket? looks to be about 15-20mm. Gut feeling is 30mm screws should be absolutely fine. Probably 3-4mm thick.

I would say a red rawl plug is about 40mm long, with a drilling size of 6mm

I always try and have the screw in the whole length.

So for your shelf I would probably use a screw that is between 40mm and 50mm long. Ensures that the rawl plug is fully spread out and secure.

A number 6 guage screw around 5mm wide should do the trick.

Hope that all makes sense and is useful to you.

The main force holding up the shelf is friction, between the edge of the shelf and the wall. The purpose of the screw is to provide the force to give that friction. If the screw is tight, there should be no downward force (shear) on the screw at all.

on the bracket you show, there will be a turning moment about the bottom of the bracket that will tend to pull the top screw ot of the wall, so that is the most important on to get right.

A 4mm screw, about 30 to 40 mm long would probably be enough, with an appropriate plug into plaster. If it is a drywall, then you would need to use an appropriate drywall fitting (if you are in the US, Home Depot or Lowes is your friend. In the UK, any of the big name DiY stores will have a range of products.

The most important bit is to ensure the screws can be done up tightly, whatever size or method you use.