Single-walled carbon nanotube (CNT) based thin-film transistors (TFTs) are promising candidates for future electronic devices because of their excellent properties. However, device-to-device performance variability of TFTs can pose challenging problems, which is rooted in the randomness of CNT networks, the variation in individual CNT properties due to change in chirality, and the fabrication imperfections. This paper presents a systematic study of variability in I - V characteristics through a combination of experimental and theoretical analysis of the major sources that cause performance variation. The sources of variation including the percentage of metallic CNTs (m-CNTs), threshold voltage, CNT mean length, and CNT network density have been studied separately. The analysis shows that the presence of m-CNTs is a major source contributing to the performance variation for short channel TFTs, but its effect reduces for large channel length transistors. The threshold voltage is found to be the major source of variation for long channel TFTs. A better consistency in performance can be guaranteed for TFTs with larger channel area, which ensures a smaller variation in CNT network density and CNT mean length. These results provide key insights into the variability estimation of I - V characteristics of CNT-based devices, which is vital for reliability studies of CNT-TFTs based circuits for different electronic applications.