For all you tank building engineering types....

[ScubaSteve]

The aquatools calculator assumes rimless though they don't state it, so a safety factor of 5 should be fine. 7 to 9 is overkill. I use 10 mm glass on my rimless 24x24x20 tank. My friend has a rimless that is about 31.5x14x18 and it uses 6 mm glass (though I could not say whether it is tempered or not.