Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 km, if its reflectivity is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects (KBOs). At the time of discovery, Hydra was about 25 percent brighter than its sister moon Nix, which led to the assumption that its diameter was some 10 percent bigger. Pre-discovery data from Hubble observations in 2002-3 inferred that Nix was the brighter moon. However, Hubble observations in 2005-6, specifically targeting the dim moons, once again showed Hydra to be a little brighter. Hydra appears to be grey like Charon and Nix, though Pluto is reddish.