Estimation of process rates (e.g. growth rate, glucose consumption rate, target molecule production rate) is a key part of data analysis for bioprocess experiments. Common approaches for rate calculation assume batch systems with no liquid addition or sampling drawdowns. These approaches, when not used carefully, can suffer from estimation errors when addition or sampling does in fact take place in a process, especially in small-scale bioreactors. This white paper describes a simple method to correct for volume changes in fed-batch systems, allowing for accurate estimation of process rates. Culture Biosciences carefully keeps track of volume changes throughout bioreactor experiments and provides all the data needed to perform these calculations correctly.