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A transmission-line type fluid-level sensor with a low-cost connected oscillator and detector is proposed. The designed physical sensor, applying microwave technology, is a simple fluid-level sensor that relies on changes in radio frequency (RF) signal attenuation based on the fluid’s height. The proposed sensor can offer real-time measurements of fluid height inside a closed metal pipe by inserting a microstrip line into the pipe. The fluid level inside the metal pipe can be accurately determined using a portable multimeter, eliminating the need for the complex calibration process typically associated with conventional RF-based measurement instruments. The prototype fluid-level sensor is designed to operate with a dynamic range of 13.1 dB in a 50-mm diameter metal pipe filled with purified water condition. The sensor accommodates microstrip line with a superstrate and a superstrate-to-substrate thickness ratio of 0.236, suitable for sensing water height. The measurement results show a linear relationship between the insertion loss of the transmission line and fluid height at 3.9 GHz, operating within the maximum error range of 2.4 dB.