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Coupling effects between on-chip spiral inductors operating in the millimeter-wave (mm-wave) regime have been investigated. The trend of the coupling factor as a function of inductor spacing and number of turns is found, and the implications of this for radio-frequency (RF) and mm-wave circuit layout are discussed. A series of coupled circular spiral inductors with different number of turns and distance was fabricated in a 0.18-??m CMOS process and measured up to 65 GHz. A new SPICE-compatible scalable equivalent circuit model for coupled inductors on silicon is also developed and verified by measurements. The model can help RF and mm-wave circuit designers to characterize the coupling effects of neighboring inductors, optimize layout to save expensive chip area, and achieve first-pass silicon success.