This paper introduces a design methodology for optimizing a micro nuclear magnetic resonance (NMR) planar coil to ensure maximum signal-to-noise ratio (SNR) in NMR systems. Although the SNR is influenced by multiple factors, some remain nearly impossible to model and simulate using traditional electromagnetic simulators. To address this challenge, we propose the concept of the coil performance factor (CPF). The CPF exclusively considers coil-related parameters, such as magnetic flux density, inhomogeneity factor, coil temperature, and coil resistance. Utilizing this methodology, we evaluated multiple planar coils, each with varying turn numbers, trace widths, and shapes but maintaining a diameter consistent with the standard 5-mm NMR tubes, within an identical environment. Notably, unlike coils designed for other applications that prioritize high-quality factor (Q), NMR coils necessitate a balance between high homogeneity and other factors. Comparative analysis revealed that the optimized octagonal coil, comprising 3 turns and a 0.27 mm trace width, exhibited a CPF of 0.0214 millitesla per the square root of Kelvin ohms. In contrast, the high-Q coil demonstrated a CPF of 0.0102 millitesla per the square root of Kelvin ohms. Our findings underscore the potential of CPF as an insightful metric for micro-NMR planar coil design and optimization.