Children’s ability to compare symbolic (e.g., Arabic numerals) and nonsymbolic (e.g., dot arrays) numerical magnitudes has been found to correlate with their math achievement. Most research, however, has focused on computerized paradigms, which may not always be suitable for quick application in classrooms. Consequently, we designed a two-minute paper-and-pencil assessment to measure kindergarten children’s ability to compare symbolic and nonsymbolic numerical magnitudes and assessed the degree to which performance on this measure explains individual differences in achievement. Children were required to cross out the larger of two, single-digit numerical magnitudes. Results from 250 kindergartners revealed that symbolic and nonsymbolic number comparison accuracy scores correlated with individual differences in arithmetic achievement. Results also demonstrated that participants’ scores on the paper-and-pencil test in kindergarten was a significant predictor of math performance in a later grade. These findings suggest the important role of symbolic and nonsymbolic processing in children’s higher-level math abilities and the importance of assessing this very basic skill in children, highlighting the potential of this tool for the assessment of early, foundational numerical abilities.