In this work, we describe an optical setup to determine the internal diameter of narrow bore fused silica capillary used in capillary electrophoresis and Taylor dispersion analysis (TDA). Indeed, fluctuations up to about ±3–4 µm on the capillary I.D. can generate important inaccuracy on the hydrodynamic radius determination by TDA. Calibration of the optical set‐up, impact of the operator and of the placement of the capillary in the focal plane, and the influence of the way to cut the capillary were investigated and discussed. This optical set‐up was next used to determine capillary I.D. on a 60 m long capillary spool. Relatively small variations were observed along a 60 m capillary spool (0.3 µm maximum variation), while important I.D. fluctuations can be observed from capillary batch to batch. Taking three capillaries of three different nominal I.D. values, R h values of sodium benzoate obtained by TDA were not significantly different if the capillary I.D. were optically measured, while significant variations were observed with the nominal I.D. values. A protocol based on TDA of sodium benzoate was proposed for calibrating narrow bore fused silica capillary I.D. without the use of optical measurements for researchers that would not have access to such optical equipment.