Background 2D digital subtraction angiography (DSA) images are the gold standard for neuroradiological vascular assessment and the basis of interventional procedures such as mechanical thrombectomy and cerebral aneurysm coiling. However, length measurements in projected DSA images are affected by the distance between the x-ray source, the object, and the detector. Precise coordination between all integrated parts of a novel biplane system makes it possible to accurately measure DSA distances without manual calibration. The aim of this study was to compare vascular diameter measurements in uncalibrated DSA images with computed tomography angiography (CTA). Methods Consecutive patients undergoing interventional neuroradiological procedures were retrospectively included. Vascular diameter measurements in the image isocenter and periphery were performed. These measurements were repeated in picture archiving and communication system (PACS) on DSA images and maximum intensity pixel (MIP) CTA images. Results Forty-two (42) consecutive patients with adequate DSA and CTA images were included in the final analysis. The correlation between vessel diameter measurements in the image isocenter (R² = 0.81/0.85, p < 0.0001/p < 0.0001 [Reader1/Reader2]), periphery (R² = 0.85/0.82, p < 0.0001/p < 0.0001 [Reader1/Reader2]), and all measurements combined (R² = 0.87/0.87, p < 0.0001/p < 0.0001 [Reader1/Reader2]) on DSA and CTA were strong and statistically significant. The interclass correlation coefficient for measurements performed by two independent reviewers was strong (ICC = 0.96, 95% CI 0.92–0.98). Conclusions The correlations between uncalibrated DSA measurements and CTA for vessel diameter were strong. In addition, there were strong correlations between these image types for repeated measurements in the image isocenter as well as image periphery for vessel diameter. Consequently, endovascular devices can be sized correctly without the need for pre-operative non-invasive imaging.