The purpose of this study was to investigate how the nature of metric estimation skill instruction affects prospective elementary and special education teachers' abilities to estimate metric length, area, and volume. Four types of estimation skills were identified by an estimation matrix. Three instructional strategies were selected: (1) a personal reference unit, (2) a cut or drawn unit of reference which was put out of sight before making an estimate, and (3) estimation without specifying an explicit strategy. The personal reference strategy was significantly more effective than the unit reference strategy for the estimation type that was the same as the instruction. (MK)