Top: inferred bulk metallicities and their uncertainties for the 11 key targets from the Ariel reference mission sample. Bottom: Relative bulk-metallicity uncertainties in percent for the same planets. The grey dots and error bars represent the metallicities and uncertainties when the atmospheric metallicity is unknown between 0.5 and 8 Z_⊙. The purple hexagons are the results for when the atmospheric metallicity is known to be three times solar. See text for details.