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Crosstalk noise between signal wires has become a prominent source of failures in high-speed VLSI systems. This paper describes an accurate and efficient method to estimate the crosstalk noise caused by multiple aggressor nets. Traditionally, the noise injected by each individual aggressor net is computed while the coupling capacitances from the victim net to non-switching aggressor nets are grounded. This approach, however, can significantly underestimate the maximum noise voltage. We therefore propose a more accurate approach where the noise generated by an aggressor is calculated while each quiet aggressor is replaced with an effective load capacitor, the value of which is obtained by multiplying the coupling capacitance with a load factor. A formula is derived to calculate the load factor based on parameters of the aggressor, the coupling capacitance and the effective slew rate of the victim net. Using HSPICE simulation, we demonstrate that the proposed approach results in an average error of 1% and 3σ error of 9%, while the traditional approach that used grounded coupling capacitors consistently underestimates the actual noise voltage and has an average error of 9% and 3σ error of 42%. Based on the new reduction method, a crosstalk noise estimation flow is proposed and is shown to produce promising results.