Purpose <p>Parathyroid weight is a simple, robust functional status indicator and cornerstone of gross/intraoperative assessment, though sometimes unobtainable. We evaluate models to estimate weight from size and create an online calculator.</p> Methods <p>Actual weights (AW), cellularity, prosector identity and size were prospectively collected for 124 parathyroids (111 hypercellular, 13 normocellular) in 76 patients (4-6/2023). Simple volumetric weight estimates (VWE) for: ellipsoid, capsular, the novel capsuloid, and box shapes were compared with AW. Multiple linear regression (MLR) was performed with internal (k-fold) validation and external validation on an archival cohort (263 adenomas, 2016–2022). Subsets with surgeon’s weight estimates, serum values, and microscopic surface area measured using the QuPath surface area tool were correlated with AW.</p> Results <p>The optimal MLR model included capsuloid VWE, cellularity, and prosector identity (R²:0.92, <i>p</i> &lt; 0.0001). A more generalizable model without including prosector identity (R²: 0.89, <i>p</i> &lt; 0.0001) was used for the online calculator weight estimate (CWE). The calculator was a good predictor of AW on the adenoma dataset (R²: 0.86, normalized root mean squared error (nRMSE): 0.058). Interestingly, the surgeon’s weight estimate (<i>n</i> = 31) was more favorable (R²: 0.97, nRMSE: 0.087) in this subset. QuPath assisted surface area-based weight estimates showed weaker correlation with AW. Neither AW, CWE, nor size correlated strongly with serum values.</p> Conclusion <p>An online calculator modeling capsuloid VWE and cellularity is a good predictor of AW. Variance in prosector measurements is important but impractical to model. Surgeon’s estimates were quite accurate, emphasizing the value of this skill.</p>