This letter presents a convenient approach based on the two-port kQ-product theory to analyze the influence of interwinding capacitive coupling on the efficiency of the transformer. It is demonstrated that a transformer with proper size can benefit from the interwinding capacitive coupling to maximize its efficiency at a desired frequency. The proposed design approach is used in a W-band low-noise amplifier (LNA) fabricated with the 40-nm CMOS process to optimize the insertion loss of the input transformer-based balun. Thanks to the approach, the W-band LNA achieves a minimum noise figure of 5.7 dB, a maximum gain of 18.5 dB, and a 3-dB bandwidth of 76.5-92.6 GHz, while consuming 23.4 mW from a 0.9-V supply.