Although barium swallow imaging is established in the investigation of Zenker’s diverticulum (ZD), no agreed measurement protocol exists. We developed a protocol for measuring ZD dimensions and aimed to correlate measurements with symptoms and post-operative outcomes. This prospective study included patients with confirmed ZD who underwent flexible endoscopic septal division (FESD) between 2014 and 2018. ZD was confirmed on barium radiology with measurements reviewed by two consultant radiologists. Symptom severity pre- and post-FESD was measured using the Dysphagia, Regurgitation, Complications (DRC) scale. Regression analyses were conducted to identify dimensions associated with therapeutic success, defined as remission (DRC score ≤ 1) 6 months after index FESD. In total, 67 patients (mean age 74.3) were included. Interobserver reliability (intraclass correlation coefficients—ICCs) was greatest for pouch width (0.981) and pouch depth (0.934), but not oesophageal depth (0.018). Male gender (60.9%) was associated with larger pouch height (P = 0.008) and width (P = 0.004). A positive correlation was identified between baseline DRC score and pouch depth (ρ 0.326, P = 0.011), particularly the regurgitation subset score (ρ 0.330, P = 0.020). The index pouch depth was associated with FESD procedure time (rho 0.358, P = 0.041). Therapeutic success was achieved in 64.2% and was associated with shorter pouch height (median 14.5 mm vs. 19.0 mm, P = 0.030), pouch width (median 19.9 mm vs. 28.8 mm, P = 0.34) and cricopharyngeal length (median 20.2 mm vs. 26.3 mm, P = 0.036). ZD dimensions may be feasible and were evaluated using Barium radiology. Specific parameters appear to correlate with severity and post-FESD outcomes, which aid with pre-procedural planning.