<p>Bulk density, which characterizes the compactness of a substrate, influences not only the proportions of air, water, and solids but also the dielectric permittivity, thereby affecting soil moisture sensor measurements. While calibration of frequency domain reflectometry (FDR) sensors has been extensively studied, the effect of bulk density in soilless substrates remains underexplored. This study evaluated the impact of bulk density on FDR calibration in a peat-based substrate at four bulk density levels (0.125, 0.140, 0.155, and 0.170&#xa0;g·cm^− 3). Bulk density significantly affected the slope of the calibration equation (<i>p</i>_{slope × bulk density} &lt; 0.001). Higher bulk density produced steeper slopes and lower intercepts, leading to a volumetric water content (VWC) error of up to 0.08 m³·m^− 3 when unadjusted calibration equations were applied. While errors may be minor within the plant–available water range (0.30–0.40 m³·m^− 3), they can become substantial under drier or wetter conditions. Therefore, careful attention to bulk density during calibration is necessary to ensure accurate VWC measurement and, in turn, reliable irrigation scheduling.</p>