Background Intravenous cannulation is usually the first procedure performed in modern healthcare, although establishing peripheral intravenous access is challenging in some patients. The impact of the ratio between venous diameter and the size of the inserted catheter (catheter to vein ratio, CVR) on the first attempt success rate can be of added value in clinical. This study tries to give insight into the consideration that must be made when selecting the target vein and the type of catheter, and proved the null hypothesis that an optimal CVR would not be associated with increased first attempt cannulation success. Methods This was a post-hoc analyses on adult patients admitted for peripheral intravenous cannulation. Intravenous cannulation was performed according to practice guidelines, by applying the traditional landmark approach. The CVR was calculated afterwards for each individual patient by dividing the external diameter of the inserted catheter by the diameter of the target vein, which was multiplied by 100%. Results In total, 610 patients were included. The median CVR was 0.39 (0.15) in patients with a successful first attempt, whereas patients with an unsuccessful first attempt had a median CVR of 0.55 (0.20) (P<0.001). The optimal cut-off point of the CVR was 0.41. First attempt cannulation was successful in 92% of patients with a CVR<0.41, whereas as those with a CVR>0.41 had a first attempt success rate of 65% (P<0.001). Conclusion This first introduction of the CVR in relation to cannulation success should be further investigated. Although, measuring the venous diameter or detection of a vein with a specific diameter prior to cannulation may increase first attempt cannulation success.