PURPOSE: To compare the Lenstar LS 900 (Haag-Streit AG) and the Nidek AL-Scan (Nidek Co) optical biometry devices in intraocular lens (IOL) calculation and refractive error prediction in cases with uncomplicated phacoemulsification surgery. METHODS: In this study, axial length (AL), flat (K1), and steep (K2) axis, and the average (K-avg) keratometry values measured with the Lenstar LS 900 and the Nidek AL-Scan in patients who underwent uncomplicated phacoemulsification and the mean absolute error (MAE) of both devices were compared. RESULTS: One hundred five eyes of 105 patients were examined. AL was measured shorter with the Lenstar LS 900 than with the Nidek AL-Scan, and K1, K2, and K-avg were measured steeper ( CONCLUSIONS: The Nidek AL-Scan produced more predictable outcomes than the Lenstar LS 900 in selecting IOL power. Despite the clinical similarity between AL, K1, and K-avg readings, there may be notable differences in choosing the IOL power between these two devices.