Disc rotor wear

[Gmeyer]

How do i know when my disc break rotor is worn out?

 

[9000ft]

How do i know when my disc break rotor is worn out?

Should be manufacturer specs that you can check with dial calipers or a micrometer. If you don't have the tools a bike shop should.

 

[dsut4392]

How do i know when my disc break rotor is worn out?

Does it look significantly worn? I.e. is the section where the pads bite significantly thinner than the rest? How many sets of brake pads have you worn out on the rotor?
If it's a standard 160mm steel rotor, it will last you through many, many changes of sintered metallic pads, and more still of organic pads. I'm on about my 5th set of sintered pads on one of my brake rotors and it's only barely worn. A larger rotor e.g. 185mm/203mm will wear more slowly still.

They aren't quite as over-engineered as car brake rotors, but those last for hundreds of thousands of km...