Research Findings: In this research we explore the relationship between young children's number knowledge and their measurement of length. First, we examined 4- to 5-year-olds' (kindergartners') understanding of and preference for using standard or nonstandard units to measure length. Second, we investigated whether the following tasks were related to children's understanding of using standard (i.e., rulers) and nonstandard (i.e., blocks) units to measure length: (a) counting and written number identification knowledge, (b) symbolic or nonsymbolic number magnitude comparison ability, and (c) approximate number line estimation ability. Third, we examined whether understanding these number tasks predicted understanding how to measure length for both standard and nonstandard units. Our results show that young children prefer to use standard units of measurement when given a choice, and some of these children use a ruler correctly. Our results also show an important relationship between children's understandings of numbers and measurement. Practice or Policy: Given children's preference for rulers, introducing both nonstandard and standard units in early learning settings concurrently rather than consecutively is recommended for practice.