Systems and methods for calibrating a gas pycnometer utilizing a custom volume reference standard are disclosed. The custom volume reference standard may include a material. The material may include a low GTE and high-accuracy dimensions. The material may have a high-aspect ratio reference shape corresponding to an inner area of a custom made sample cup. The custom volume reference standard may include a specified number of inclusions of the material, a high purity, and/or an accurately known density. The custom volume reference standard may include a known volume.