This thread says an OE front brake rotor weighs 18.5 lbs.

I just weighed one of my used OE rotors with 57k miles, and got 17.0 lbs. That was on a bathroom scale for what that's worth. The old rotor also have significant lips on their outer edges, so they're definitely significantly worn.

Does it make sense that a brake rotor could lose 1.5 lbs' worth of material in 57k miles, or is it more likely that something's amiss?

 

FWIW, I did some back-of-the-envelope calculations using some rough numbers, e.g. 60% of the rotor's mass in the friction ring, friction material accounting for two-thirds of the rotor thickness, and ~2 mm friction ring thickness lost. 1.5 lbs of mass lost seems vaguely plausible. Just wanted to see if anyone else had seen something similar.

 

To be honest, I haven't ever seen someone weigh their used rotors. Rotor weight interest usually ends at knowing what the brand new set weighs, if they are trying to reduce the overall weight (like with 2-piece rotors).

But yes, 1.5lbs is a hair over 8%, seems entirely plausible to me.