Purpose <p>Posterior spinal fusion (PSF) for scoliosis correction can be associated with substantial blood loss. Accurately characterizing risk factors and identifying strategies for minimizing blood loss are important for operative planning. Previous literature utilizes estimated blood loss (EBL), which may underestimate bleeding, and are limited in their usage of new anti-fibrinolytic medications and electrocautery technology. We sought to investigate blood loss difference in adolescent idiopathic scoliosis (AIS) and neuromuscular scoliosis (NMS) utilizing both EBL and calculated blood loss (CBL) based on height, weight, and hemoglobin values.</p> Methods <p>Retrospective review was conducted of children who underwent PSF for scoliosis correction from 2013 to 2021. EBL was determined from estimated intraoperative blood loss and measured postoperative drain output. CBL was computed as described by Foss et al. All values were further normalized based on vertebral levels fused and patient weight.</p> Results <p>Final cohort included 224 children with AIS and 76 with NMS. EBL significantly underestimated blood loss compared to CBL (882.2 ± 447.1&#xa0;cc vs. 1315.0 ± 375.1&#xa0;cc, <i>P</i> &lt; 0.001 in AIS; 1132.9 ± 562.1&#xa0;cc vs. 1455.2 ± 482.7&#xa0;cc, <i>P</i> &lt; 0.001 in NMS). CBL in patients with NMS was significantly higher than those with AIS (1455.2 ± 482.7&#xa0;cc vs. 1215.0 ± 375.1&#xa0;cc, <i>P</i> &lt; 0.001). Per-level CBL, however, was lower in the NMS cohort, particularly in patients requiring fusion to pelvis.</p> Conclusion <p>EBL significantly underestimates blood loss compared to CBL. Patients with NMS have significantly greater blood loss than AIS. These differences appear to be driven by the extent of fusion. Updated guidelines would be valuable for more accurate determination of when to cross-match blood products prior to PSF for pediatric scoliosis.</p> Level of evidence <p>IV—Case series, diagnostic.</p>