Decorative bark (or nuggets) is great for those areas of your garden you want to show off and look smart. 

If you put weedmat under it, 75 mm (3 Inches) thick will be enough. If no weedmat is used 100 mm (4 inches) thick is recommended.
 

As a rule: You need 1 cubic metre for every 10 squared metres at 100mm thickness!