Hi, I want to buy the following plug-

http://www.screwfix.com/p/fischer-plasterboard-plugs-hdf-4mm-8ga-pack-of-100/58219

Now I know they say use screw size 8 (4mm) which is the diameter- however how are you meant to know what length screw to use as 8G screws come in different lengths and there is now length size written for the plug. Thanks for your help

you need plug length plus fitting length give or take 5mm

so your shelve is 18mm thick and your plug is say 39mm long the total will be 57mm so your ideal size will be 52-62mm or 2" -2 1/2" 2 or 3mm more or less wont matter but better longer than shorter

Thanks for the reply, is there a danger that if the screw is too long it will burst through the bottom of the plug, if this happened would this ruin the plug or would it not matter? Thanks

It won't matter.

thanks folks