We experimentally determine tensile force-elongation diagrams of tapered optical fibers with a nanofiber waist. The tapered optical fibers are produced from standard silica optical fibers using a heat and pull process. Both, the force-elongation data and scanning electron microscope images of the rupture points indicate a brittle material. Despite the small waist radii of only a few hundred nanometers, our experimental data can be fully explained by a nonlinear stress-strain model that relies on material properties of macroscopic silica optical fibers. This is an important asset when it comes to designing miniaturized optical elements as one can rely on the well-founded material characteristics of standard optical fibers. Based on this understanding, we demonstrate a simple and non-destructive technique that allows us to determine the waist radius of the tapered optical fiber. We find excellent agreement with independent scanning electron microscope measurements of the waist radius.Glass fibers are among the most versatile inventions of the last century with applications reaching from fiberreinforced materials as used in construction [1] to optical data transmission in global telecommunication networks [2]. In recent years, so-called tapered optical fibers (TOFs) have received growing attention [3], both as optical components, e.g., for coupling light into microand nano-optical components, for sensing, as well as for the controlled coupling of light and matter at or near the TOF surface [4][5][6][7]. Beyond their optical properties, which have been extensively studied in the past, it is important to understand and control the mechanical properties of these devices for many of these applications.Here, we study the mechanical response of silica TOFs with a nanofiber waist when exposed to tensile stress. Qualitatively, we observe a brittle stress-strain behavior and no constriction of the two fiber ends when the fiber has been ruptured apart. Previous studies on the mechanical properties of such ultra-thin silica structures are divided over the importance of size effects in the nanoscopic domain, e.g., suggesting a decrease or an increase of Young's modulus compared to bulk values [8][9][10][11]. In our study, even for the smallest investigated waist radius of only 160 nm, the recorded stress-strain diagrams match those of macroscopic optical fibers [12][13][14]. As a consequence, in the parameter range considered here, it is justified to assume the well-studied mechanical properties of standard optical fibers when designing nano-optical elements. Based on this understanding, we provide a practical, non-destructive in-situ method to determine the TOF waist radius. The results obtained in this way are in very good agreement with independent radius measurements using a scanning electron microscope.The TOFs we study in our experiments are produced from commercial optical fibers (SM800, Fibercore) using a heat and pull process [15,16]. A schematic of such a cylindrically symmetric TOF is shown in Fig. 1(a). The TOF con...