ABSTRACT: Non-intrusive and non-destructive acquisition of length measurements for marine megafauna is increasingly valuable given growing threats to many species’ long-term survival. Stereo videography provides a means of obtaining length data with minimal impact on the organism and minimal observer bias. However, for many researchers, there are still significant financial barriers to employing stereo videography. Small-action cameras have reduced costs, but camera calibration still requires significant investment in software and equipment. Here, we trial open source calibration procedures using the R package StereoMorph and a simple 2D checkerboard as a calibration object to test if this approach yields accurate length data. We used a stereo-video system comprising 2 GoPro™ cameras to estimate the lengths of known targets in a pool and the lengths of oceanic whitetip sharks Carcharhinus longimanus in situ. Sharks were restrained in water alongside a boat and measured with a tape measure for comparison with the stereo-video lengths. Both pool and field trials yielded accurate results comparable to previous studies using 3D calibration cubes. Stereo-video measurements of lengths >1 m had proportional errors of <1% in the pool and <3.0% (64.2 mm) in comparison with tape-measured lengths of sharks in the field. Our open source calibration methods and affordable GoPro™-based stereo-video system yielded measurements that are comparable to other systems that use 3D calibration techniques and more expensive cameras. Additionally, we improve on previous attempts of 2D calibration approaches.