The National Science Education Standards emphasise the use of concepts and skills that cut across the science domains. One of these cross-cutting areas is measurement. Students should know measurement systems, units of measurement, tools and error in measurement as well as the importance of measurement to scientific endeavours. Even though measurement is an essential skill, little is known about how students estimate and use measurement in different contexts. This study examines the impact of teaching students to use their bodies as rough measurement tools (body rulers) on their ability to estimate linear measurements. Nineteen middle school students participated in metric intervention tasks and completed a pre-instruction and a post-instruction Linear Measurement Assessment. Results showed that teaching students to use rough body measures as tools (a body ruler) for estimation had a significant influence on their estimation accuracy. After instruction, students were better able to estimate the sizes of objects, use their body in making estimations of size, and estimate while touching an object or pacing a distance. Furthermore, proportional reasoning was significantly correlated with students' post-instruction scores on the Linear Measurement Assessment. (Contains 2 figures and 3 tables.)