Fibre–resin composites are widely used for their excellent strength-to-weight ratio. Predicting their mechanical properties is crucial but often involves laborious calculations. This study introduces a web-based tool that automates the calculation of composite modulus and density using the Rule of Mixtures for parallel loading conditions. Developed with HTML and JavaScript, the tool allows users to input fibre and resin properties along with fibre volume fraction to obtain real-time results. This simplifies the design process and aids both educational and industrial applications.