Growth of a Mercury analogue at r = 0.4 AU by accretion of iron-rich planetesimals. We consider planetesimal sizes of either 100 km (nominal) or 10 km (small planetesimals) and two values for the accretion rate of the protoplanetary disc when the planetesimals form ( and ). The isolation mass by planetesimal accretion, with a feeding zone here assumed to be four Hill radii, is indicated by the dot-dashed lines. The protoplanets will grow to their isolation mass within approximately 1 Myr in all cases. For planetesimal formation in a dense protoplanetary disc, with , the isolation mass is similar to the mass of Mercury and no giant impacts are needed to explain its characteristic mass. The expected mass of planetesimals formed for each heating event of the protoplanetary disc is indicated by dash-triple-dotted lines.