The proposed ICAL detector is designed to detect muons generated from interaction of ν_µ and anti-ν_µ with Iron. It is designed with a maximum magnetic field of about 1.5 Tesla (with 90% of its volume having > 1 Tesla magnetic field). The magnetic field is intended for charge identification and momentum reconstruction of the muons. The mini-ICAL is a fully functional 85-ton prototype detector. One of the main challenges for the mini-ICAL detector is to produce the required B-field and to measure it as accurately as possible to study muons. For the purpose of measuring the B-field in the detector, Hall sensor PCBs are used. Hall sensors provide real time measurement of B-field. Calibration and systematic study of characteristics of the Hall sensors which are used for the measurement are carried out. Out of 11 layers of iron, 3 layers (1, 6 and 11) have provision for measurement of B-field using Hall sensor. In the mentioned layers, the gap between the adjacent plates is kept 3-4 mm for the purpose of inserting of the Hall sensor PCBs. In the rest of the layers, the gap between the plates is kept 2 mm. Measurements of magnetic field in the air gap between the plates and measurement of fringe field is done with a precision of 3% for the top layer. This will help in completing the study on the final magnetic field configuration of ICAL.