Methods and systems for measuring the volume of a liquid in a subject chamber, such as the mixing chamber in a system for mixing and delivering drugs, for example. A volume of a control gas in communication with the subject chamber is isolated, such that the total volume of the control gas and the liquid is held constant, and the initial pressure of the control gas is measured. Then, a known volume of liquid is moved into or out of the subject chamber. This known volume may be exchanged between the subject chamber and a measurement chamber, in order to determine this volume. The final pressure of the control gas is also measured after the reference volume has been removed from or added to the chamber and while the volume of control gas remains otherwise isolated. The volume of liquid in the chamber may then be measured based on (a) the initial pressure, (b) the final pressure, (c) the reference volume and (d) the volume of gas in the subject chamber when the subject chamber is empty of liquid.