Thulium fiber laser (TFL) lithotripsy has recently been introduced for clinical use. Previous TFL laboratory studies demonstrated high-power delivery through small (50- to 150-μm core) optical fibers. This preliminary study simulates forces on fibers during insertion into a flexible ureteroscope and determines mechanical feasibility of small fibers for a clinical setting. Simulations were conducted with commercially available fiber (core/cladding) sizes of 50/70, 72/108, 100/140, 150/165, 150/180, and 200  /  240  μm. Solidworks software integrating Euler’s buckling equation was used to calculate fiber buckling thresholds as a function of typical manual forces (0.15 to 2.0 N) applied near the proximal end of a ureteroscope. Forces on fibers were modeled assuming support from saline flow and resistance by the working channel wall. Simulation results were categorized based on force values previously reported in the literature, with smaller forces (<0.4 to 0.8 N) buckling fibers, mid-range forces (0.8 to 1.6 N) optimal for fiber manipulation, and higher forces (>1.6 to 2.0 N) at risk of damaging the ureteroscope working channel. Fiber sizes were simulated with two different types of holdings on each end to find a range of possible values that most closely simulate clinical behavior. Simulation results were confirmed using a force meter and a benchtop experimental setup. Numerical simulations predicted that optical fibers for TFL lithotripsy should be equal to or larger than 150  /  190  μm (core/cladding), for effective manual manipulation within flexible ureteroscopes.