Rheology describes the flow of fluids from food and plastics, to coatings, adhesives, and 3D printing inks, and is commonly denoted by viscosity alone as a simplification. While viscometers adequately probe Newtonian (constant) viscosity, most fluids have complex viscosity, requiring tests over multiple shear rates, and transient measurements. As a result, rheometers are typically large, expensive, and require additional infrastructure (e.g., gas lines), rendering them inaccessible for regular use by many individuals, small organizations, and educators. Here, we introduce a low-cost (under USD$200 bill of materials) Open Source Rheometer (OSR), constructed entirely from thermoplastic 3D printed components and off-the-shelf electromechanical components. A sample fluid rests in a cup while a micro stepping motor rotates a tool inside the cup, applying strain-controlled shear flow. A loadcell measures reaction torque exerted on the cup, and viscosity is calculated. To establish the measurement range, the viscosity of four Newtonian samples of 0.1-10 Pa.s were measured with the OSR and compared to benchmark values from a laboratory rheometer, showing under 23% error. Building on this, flow curves of three complex fluids - a microgel (hand sanitizer), foam (Gillette), and biopolymer solution (1% Xanthan Gum) - were measured with a similar error range. Stress relaxation, a transient test, was demonstrated on the biopolymer solution to extract the nonlinear damping function. We finally include detailed exposition of measurement windows, sources of error, and future design suggestions. The OSR cost is ∼1/25th that of commercially available devices with comparable minimum torque (200 µN.m), and provides a fully open-source platform for further innovation in customized rheometry. Rheological properties similarly influence the design, validation, and use of concrete, paint, plastic feedstock for injection molding and polymer film shaping 12 , food 13 , and personal care products like hand sanitizer. Hand sanitizers are carefully engineered to be both shear-thinning and to have a yield stress 11 . An ideal behavior is to have a high viscosity or yield stress to reduce phase separation during storage and transportation, as well as sufficient yield stress when dispensed onto a surface or hand, while allowing dispensing without requiring excessive force. When hand sanitizer is pumped, the narrow nozzle inlet applies a high shear rate, effectively lowering the viscosity of the shear-thinning fluid 11 . Therefore, the formulation must tailor the rheology while maintaining medical efficacy 14 . Understanding of viscosity is also key to successful thermoplastic 3D printing by fused filament fabrication. If the viscosity of the filament is too high, the nozzle of the 3D printer can clog, and if the viscosity is too low, then material can leak out of the nozzle at a rate different than desired 15 , leading to defects like stringing, surface roughness, or internal voids. Ideally, a 3D printed material would be shear thinning 15 so it stays in place after being deposited. The material used to make the prototype rheometer, poly-lactic acid (PLA), is shear thinning 16 . These properties remain important in applications of bioprinting, where often yield stress and thixotropy are used rather than temperature-based solidification to retain shape. These and other rheological properties of fluids are essential to everyday life. However, the ability to make accurate and robust rheological measurements is restricted for schools, hobbyists, and professionals, and even within research labs due to cost and requirements for facilities (e.g., air lines), space, and expertise, often precluding familiarity with rheology in general. A low-cost and open-source rheometer would allow students to explore and gain a hands-on understanding of these concepts, and would allow hobbyists to incorporate fluid properties locally and immediately into their playful designs of new materials e.g., for homemade hand sanitizers, foods, and locally-sourced recyclables 17 . Three non-Newtonian fluids were tested: a hand sanitizer, shaving cream, and 1% Xanthan gum solution in water showing, in order, ( a , c , The hand sanitizer and shaving cream OSR values were within 93% of those of the DHR and the Xanthan gum OSR estimate was within 260% of the DHR estimate, due to operating near the noise floor. The hand sanitizer sample was also tested with an 8-arm vane geometry which prevented slip (teal data on Fig. 7 a). This improved accuracy with the yield stress improving from 1.6 (cylinder) to 32.6 Pa (vane) compared to the DHR value of 24.3 Pa. Fluid appearance and measurability The physical appearance of fluids can give a qualitative idea of how easy they are to measure. Among the fluids we measured, the shaving cream was foamy and stiff, forming a pile when directly sprayed from the can, which indicates a high yield stress 21 , 44 . In contrast, the hand sanitizer formed a much smaller mound if put onto a surface and had a slightly more runny appearance. In practice, both fluids had similar yield stress, and the foam appeared more stiff simply because it is much less dense than hand sanitizer, reducing the force due to gravity. Finally, the Xanthan gum mixture was watery with hardly perceptible solid nature, looking more like a “thick” fluid. This was matched by measurements showing a very low, but present, yield stress, with predominantly shear-thinning behavior. Measurement limits due to inertia Rheological measurements typically must operate under conditions in which machine inertia (e.g., of the testing cylinder) is negligible. In order to identify this regime, we develop Eqs. ( 7 )-( 8 ) based on previous literature for inertia in oscillatory rheometry 45 to determine when the torque due to true material measurement is larger than torque due to inertia of the constantly rotating testing cylinder, or T material > T inertia . Using Eq.