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A passive resonant sensor with kirigami patterning is presented to wirelessly report material deformation in closed systems. The sensors are fabricated from copper‐coated polyimide by etching a conductive Archimedean spiral and then laser cutting kirigami patterns. The sensor response is defined as the resonant frequency in the transmission scattering parameter signal (S₂₁), which is captured via a benchtop vector network analyzer. The sensors are tested over a 0–22 cm range of extension and show a significant shift in resonant frequency (e.g., 90 MHz shift for 10 cm stretch). Furthermore, the effect of resonator coil pitch on the extension sensor gain (MHz cm⁻¹) and linear span of the sensor is studied. The repeatability of the sensor gain is confirmed by performing hysteresis cycles. The sensors is coated with polydimethylsiloxane films to protect from electrical shorting in aqueous environments. The coated resonators are placed in a pipe to report flow rates. The sensor with 1 mm coating is found to have the largest gain (0.17 MHz⋅s mL⁻¹) and linear span (10–100 mL s⁻¹). Thus, flexible resonant sensors with kirigami‐inspired patterns can be tuned via geometric and coating considerations to wirelessly report a large range of extension lengths for potential uses in health monitoring, motion tracking, deformation detection, and soft robotics.