Why do we use HDF5? We moved to HDF5 for our simulation data in 2016 from using our own proprietary file format. HDF5 had been on our radar for some time and we spent a couple of years investigating it and other file formats before deciding which we should switch to. HDF5 met all the criteria we had at the time. Amongst the criteria were: performance in speed and size, an accepted standard for scientific data, being open source, providing additional tools.