This article focuses on measurement uncertainty in the characterization of passive metal-insulator-metal (MIM) capacitors in the WR1.0 (0.75-1.1 THz) waveguide band. Two semiconductor processes were used to fabricate capacitors to characterize, and two designs of via-stitched grounded coplanar waveguide calibration kits were utilized for on-wafer S-parameter measurements. Uncertainty contributions from probe positioning, calibration kit process variation and vector network analyzer electrical repeatability were investigated. Probe positioning uncertainty was found to be the largest contributor to measurement uncertainty that could have a prediction interval as large as 30.2% of a capacitor's value at 900 GHz.