Every brake rotor has a minimum thickness below which it is unsafe to use - and that number is stamped right on the rotor. Knowing where to find it lets you decide whether a rotor can be reused or resurfaced. Here is where to look.
Where the number is stamped
The minimum thickness (often labeled "MIN TH," "min. thickness," or "discard at") is cast or stamped into the rotor - usually on the outer edge of the friction ring, on the rotor hat, or sometimes on the inside face near the hat. It is a specific number in millimeters, distinct from the rotor's nominal (new) thickness.
Minimum vs machine-to thickness
Do not confuse two numbers: the minimum (discard) thickness is the absolute limit below which the rotor must be scrapped; some rotors also list a "machine-to" thickness, the thinnest you can resurface it and still leave a safe margin. Always compare your measured thickness to the discard number - see rotor minimum thickness.
How to use it
Measure the rotor's actual thickness at several points with a micrometer - see how to measure rotor thickness. If it is at or below the stamped minimum, replace the rotor - it cannot safely be reused or resurfaced. This matters because thin rotors overheat, warp, and can crack - see when to replace rotors.
Why it is there
The stamp exists so anyone servicing the brakes can make a safe reuse-or-replace decision. Rotors thin out with wear and each resurfacing - see rotor resurfacing cost. Replace rotors as a pair when at the limit, with quality pads and fresh hardware.