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Optimization of spinopelvic sagittal parameters in spinal deformity surgery have been shown to correlate with surgical outcomes. Commonly used parameters include pelvic incidence (PI), lumbar lordosis (LL) and PI–LL mismatch. Presence of lumbosacral transitional vertebra (LSTV) introduces variability regarding which endplate is considered the sacral endplate. This study aimed to determine the mathematical relationships between measurements using the upper transitional vertebra (UTV) versus the lower transitional vertebra (LTV). The property that internal angles of a triangle sum to 180° was used to create a system of equations to resolve the relationship between the PI–LL mismatch of the UTV and of the LTV. The ultimate relationship was employed on a lateral radiograph of a patient with LSTV for validation. It is possible to compute the PI–LL mismatch using either UTV or LTV and convert to the corresponding PI–LL when using the other vertebra simply by measuring one additional angle (PI–LL)L = (PI–LL)U + X. This angle X is defined by segments connecting the center of the femoral head to the midpoints of the superior endplates of the UTV and LTV. Using the LTV yields a larger PI–LL mismatch value. In patients with LSTV, it is controversial whether the UTV or LTV should be used as the sacral endplate for sagittal measurements. With this mathematical relationship, rather than completing two sets of measurements, the surgeon would only need to measure one set and the additional angle X to determine the resultant PI–LL mismatch for the other transitional vertebra.