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Background We sought to compare the accuracy of standard and novel echographic methods for computing intraocular tumor largest basal diameter (LBD). Design Multicenter, retrospective cohort study. Subjects All patients presenting with new diagnosis of uveal melanoma (UM). Methods Ultrasounds were obtained for all patients, and axial length (AL) was measured for a subset of patients. LBD was calculated as: (1) a single chord measured on B scan ultrasound (one-chord method [1CM]), or (2) by subdividing the basal diameter into two chords, which were summated (two-chord method [2CM]), or (3) by a mathematically-derived formula (MF) based on geometric relationships. The accuracy of each method was then compared, and sensitivity of each technique to factors such as tumor size and AL were analyzed. Main outcome measures Accuracy, robustness, correctness of predicted plaque size. Results 116 UMs were analyzed; 1CM-calculated LBD underestimated 2CM-calculated LBD by 7.5% and underestimated LBD by MF by 7.8%; 2CM and MF were tightly correlated (average LBD difference = 0.038%). At larger LBDs, 1CM underestimated 2CM and MF by a much greater percentage (p 12 mm, where a single chord on ultrasound is more likely to lead to incorrect, undersized plaque selection. Our MF can be applied with great accuracy even in cases where the AL of the eye is not measured, using the population average AL (23.7 mm), and the formula \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{LBD}} = 23.7\sin^{ - 1} ({{{\text{chord}}\;{\text{length}}} \mathord{\left/ {\vphantom {{{\text{chord}}\;{\text{length}}} {23.7}}} \right. \kern-0pt} {23.7}}) $$\end{document}LBD=23.7sin-1(chordlength/23.7).