The Water Uptake and Loss Code provides a computational framework for modeling the dynamic behavior of binary aqueous particles in response to changes in their surrounding environment. It offers implementations in both Mathematica and Matlab, enabling researchers to calculate the time-dependent radius and characteristic equilibration time of these particles following stepwise alterations in relative humidity (RH). This code is a critical resource for understanding the hygroscopic properties of atmospheric aerosols. This versatile code is developed based on the theoretical and experimental work by B. J. Wallace and T. C. Preston (2019), specifically addressing water uptake and loss in viscous aerosol particles where diffusivity can be concentration-dependent. The availability in both Mathematica and Matlab provides researchers with flexibility in their computational environment, allowing for seamless integration into existing workflows. It is designed to accurately simulate complex physical processes, including water diffusion and phase transitions within aerosol particles, which are often non-ideal and highly sensitive to environmental conditions. The primary application of this code lies within atmospheric aerosol science, where it is used to investigate hygroscopic growth and the transport of water within highly viscous, amorphous, or 'glassy' aerosol particles. Understanding these processes is fundamental to predicting how aerosols behave as cloud condensation nuclei (CCN) and how they influence heterogeneous chemical reactions in the atmosphere. The code's ability to model time-dependent changes is particularly valuable for studying the kinetics of water exchange and the evolution of particle properties under dynamic atmospheric conditions. A key feature of this implementation is its capacity to account for concentration-dependent diffusivities, which is essential for accurately representing the non-ideal and often complex behavior observed in real-world viscous aerosols. This code serves as an invaluable tool for interpreting experimental data, such as those obtained from optical trapping experiments that monitor changes in particle size and composition as a function of relative humidity. It facilitates a deeper understanding of the microphysical properties that govern aerosol-climate interactions.