This tutorial provides a comprehensive guide to performing quantitative image analysis using ImageJ, with a special focus on length measurements, phase quantification, and grain size analysis. It is intended for researchers, engineers, and students working with microscopic images in materials science and related fields. New in Version 1.2:A dedicated section on Grain Size Analysis has been added, including detailed steps for manual grain boundary tracing, thresholding, and using ImageJ’s “Analyze Particles” tool to determine grain sizes (e.g. Feret’s diameter). The tutorial outlines image preparation techniques and includes recommendations for accurate analysis and troubleshooting. Topics Covered: Installing and setting up ImageJ (v1.54g) Performing distance and length measurements Phase quantification via greyscale thresholding Manual and semi-automated grain size analysis Annotating and exporting images and data Best practices for reproducibility and reliability This guide is openly shared to support accessible, reproducible image analysis workflows in materials characterisation. Prior experience in microscopy is recommended.