The question is self-explanatory: How much do the astromech droids R2-D2 and BB-8 weigh, in-universe?
Wookieepedia says (citing "Star Wars Rebels: Head to Head") 32 kg.
Apparently the same number was in EU/Legends, in "Star Wars: Head-to-Head Tag Teams, pg 50" as per the same page.
couldn't find in-universe evidence in the film, novelization, or Visual Dictionary page.
Sphero's out of universe popular toy is ~200 g (at the height of 11cm, compared to real one's Databank height of 0.67m). If we proportionally size the weight up by 6^3=216 times, we get ~42kg.
Which probably isn't accurate, since I assume TGFFA has much better/lighter materials than Sphero (since R2-D2 could fly and BB-8 has liquid metal cables that hold its weight and pull him up).
Doing very back of the envelope guessing it looks like his body is ~20-30% of R2-D2 by volume. (they fit in same size X-Wing astromech socket.
- R2-D2's body seems to be exactly 1.5 times BB-8 body, since his body+head fits exactly 2 BB-8 bodies and his head is an exact hemisphere of the same radius. That gives us 9.42+4.19/2=11.5 for R2-D2 volume given radius of 1 (for simplicity of comparison), excluding appendages. BB-8 is 4.19 for the body and roughly 0.26 for head (half a sphere size of half the body), totalling 4.45.
Assuming he's not made of some advanced ultralight composites that severely affected his density by comparison the above gives us ~12.4kg. It may be less because of more advanced materials, or because I didn't count the volume of R2-D2's legs; or it can be less because I assume a spherical droid's internal space has less empty lacunae due to better space packing. Without better canon info, I'll call those factors a wash for hand-waved calculation purposes and settle on 12kg.