Defence ?

[Ortega]

If u have 10bio on the open how much defence will u need to not get attacked ?

[urogard]

generally the rule of thumb works about 1 bil naq for 10 bil defence.
obviously other factors like planets for example can change this.
someone with 10 attack planets might attack a 10 bil defence even for 400 mil, since his losses will be minimal

[Setesh]

If u have 10bio on the open how much defence will u need to not get attacked ?

There is no general 'have this much defense to not get attacked'.
It all, really, depends on who you are, your alliance, your enemies, and then your defense.

Some people, Ethaniel and Su come to mind, that I wouldn't attack if they were defenseless and allianceless with 60b sitting out.

But 1b per 10b defense, as stated by Uro, is the general idea. Some idiot has been hitting me recently for 8b or so and has suffered more losses in UU, repairs, and mothership repairs than he's gotten naq for.