We present a technique to measure, in situ, the diameter of an optical fibre during etching using a fibre Bragg grating (FBG). Differential shifts between the fundamental mode, and the higher-order Bragg resonances generated by the etching process are used to determine the diameter of a standard optical fibre (SMF28) with a precision of ~200nm. Numerical simulations are also carried out to investigate the overlap of the evanescent field of the fundamental mode and higher-order modes (LP11, LP02, LP21 and LP12). These simulations were used to find and calibrate the diameter of the etched-cladding fibre. Subsequently, the technique was used to experimentally determine the refractive index of two buffered hydrofluoric (BHF) acid solutions, (20:1) and (7:1), to be ~1.360 ± 0.005 and ~1.370 ± 0.005 respectively @ ~1550nm. The refractive index of both BHF solutions is calibrated against known indices of liquids and solvents, including deionised water, methanol, acetone, ethanol, isopropanol, and ethylene glycol. The numerical simulations and experimental results are in very good agreement. We believe the approach presented in this work provides a controlled technique to achieve precise target diameter of the etched fibres in real time.