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Study design National cross-sectional study. Background Thoracic kyphosis (TK) is related to sagittal parameters as pelvic tilt (PT), lumbar lordosis (LL) and pelvic incidence (PI). The equation TK =2 (PT+LL–PI) was validated for adolescents. Objective: to investigate if this equation correctly predicts TK regardless of age. Methods Sagittal alignment parameters were assessed on full spine radiographs of 2599 individuals without spine pathology (1488 females, 1111 males). Calculated TK (CTK) = 2 (PT+LL–PI) and measured TK (MTK) were compared by calculating the gap and using a linear regression between both parameters. Subgroup analyses were performed for gender, age, TK groups (≤20°, 21°-40°, 41°-60°, >60°), and PI groups (<45°, 45°-60°, >60°). Results Average values in the total population were: MTK 45.0°, CTK 36.9°. Average TK gap was 8.1°, 5.2° in females (intercept 11.7, slope .61) and 11.9° in males (intercept 7.1, slope .58). The mean gap was 3.6° for 15-34 years, 5.7° under 15 years and it increased progressively after 35 years with a maximum of 19.9° over 80 years. The gap also increased with the amount of MTK: -3.5° for TK<20° up to 17.3° for TK >60°. Differences in gaps were minor between PI groups. The intercept was smallest and slopes >.6 for PI <45° and TK ≤20°. Conclusion The formula TK=2 (PT+LL-PI) yielded moderate accuracy for adolescents and young adults, but did not fit for over 35 years and under 15. The amount and variance in TK increased in elderly subjects, which made the formula less accurate.