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Background and Aims Intense post sternotomy pain delays mobilisation and increases peri operative opioid exposure. Ultrasound guided bilateral parasternal block provides targeted anterior chest wall analgesia; however, the dose-volume relationship of local anaesthetic for this block remains undefined. After establishing that 0.125% bupivacaine is clinically effective, we conducted a randomised controlled trial to evaluate whether varying injectate volumes at this fixed concentration modulate analgesic efficacy and opioid consumption.Methods This prospective, randomised, single-blind trial enrolled 44 adults for elective median sternotomy. After ethics approval (Pauls Stradiņš Clinical University Hospital, 281123–11L), patients received a bilateral parasternal block with 0.125% bupivacaine: 20 ml (n = 12), 40 ml (n = 20) or 60 ml (n = 12). Pain intensity (Numerical Rating Scale, NRS 0–10) was measured at 0, 4, 8, 12, 20 and 24 h. Secondary endpoints were 24-h opioid use and time-to-rescue analgesia.Results Median NRS at 12 hours postoperatively was 1 (IQR 0–3) after 20 ml, 1 (IQR 0–3) after 40 ml and 1 (IQR 0–2) after 60 ml (p = 0.801); pain scores did not differ significantly at any measured time point (p >0.05). Time to rescue was markedly shorter with 20 ml – median 120 min (IQR 120–300)—versus 40 ml – 995 min (IQR 440–1275) and 60 ml – 420 min (IQR 218–450); Repeated rescue opioid was needed in 25% of patients who received 20 ml, whereas none required it after 40 ml or 60 ml (p = 0.012).Conclusions Injectate volume was decisive ≥ 40 ml abolished rescue-opioid use; 40 ml produced the longest opioid-free period with no safety issues, whereas 60 ml offered no added benefit. Thus, a 40 ml bilateral parasternal block with 0.125% bupivacaine is the volume-efficacy sweet spot for post-sternotomy analgesia. Larger multicentre trials should validate these findings and refine practice.