<p>The deformation modulus of a rock mass is a critical parameter for geotechnical project design and is determined through field tests. The field test for determining a rock mass deformation modulus is tedious and costly, prompting the emergence of empirical methods. Several empirical approaches exist, each using different input parameters to achieve different outcomes. A comparative evaluation of empirical methods has highlighted the reliability of the empirical equations based on rock mass classification indices. The nature and characteristics of discontinuities greatly influence rock mass classification, and the deformation modulus of rock masses varies accordingly. This study has evaluated a range of empirical equations derived from rock mass classification systems to determine the deformation modulus of different rock types encountered in tunnelling within the Himalayan terrain of Nepal. The data for the analysis are obtained from three hydroelectric projects situated in the Sub-Himalayan and Higher Himalayan terrain. The geometric mean deformation modulus (<i>E</i>_m) for each lithological unit was evaluated and correlated with RMR, Q and GSI using regression analysis, yielding predictive equations. The performance of these equations was assessed using significant statistical metrics: R², RMSE, MAE and p-values. The predictive models demonstrated strong performance, with R² values ranging from 0.7605 to 0.9698. Prediction errors were generally low for sandstone, marble, schistose gneiss and banded gneiss, with RMSE &lt; 1.0401 and MAE &lt; 0.9682. Siltstone exhibited the weakest predictive performance (R²: 0.7605; RMSE &gt; 1; MAE &gt; 1). The p-values were below 0.05 for all lithologies except siltstone, indicating statistical significance. The proposed models offer a reliable framework for estimating lithology-specific deformation modulus for tunnelling projects in the Himalayan terrain.</p>