8mm is Good For Small Windows
Tempered or toughened glass is heat treated so that it is stronger and more resilient than untreated glass. Glass that is 8mm thick weighs 20kg per square meter, and is perfectly adequate for use in double glazing, shower screens and general household use. If it is broken, it will shatter into small pieces rather than dangerous shards. Its lighter weigh means it is easier to install and handle. Glass manufacturers Pilkington recommend that 8mm glass should not be used in panes that exceed 1100mm square.
Bigger Panes Need Thicker Glass
Thicker, 10mm tempered glass is larger and still as strong. This thickness of glass weighs around 25kg per square meter. Tempered glass that's 10mm thick can be used in panes up to 2250mm square. Larger glass panes allow more natural light into the area.
With toughened glass around five times stronger than untreated glass, tempered or heat-treated safety glass is the only sensible option. It's only necessary to consider the merits of 8mm versus 10mm tempered glass only when a pane of glass comes close to exceeding the advised size.