What weight medicine ball should I start with?

Mainstream reviewers and r/crossfit threads generally suggest 8–12 lb for general fitness and rotational core work, 14–20 lb for wall balls (with 14 lb being the CrossFit women's RX and 20 lb the men's), and 20–30 lb for slamming if conditioning is the goal. Rehab and mobility users frequently go lighter — 4 to 8 lb — to protect joints during high-rep work.