The amplitude ratio of the susceptibility (or second size-moment) for two-dimensional percolation is calculated by two series methods and also by Monte-Carlo simulation. The first series method is a new approach based upon integrating approximations to the scaling function. The second series method directly uses low-and high-density series expansions of the susceptibility, going to unprecedented orders for both bond and site percolation on the square lattice. Putting all methods together we find a consistent value Γ − /Γ + = 162.5 ± 2, a significant improvement over previous results that placed the value of this ratio variously in the range of 14 to 220.Percolation is one of the fundamental problems in statistical mechanics [1,2] and is perhaps the simplest system exhibiting critical behavior. Through its mapping to the q-state Potts model (for q → 1) many theoretical predictions follow, such as exact critical exponents in two dimensions. Yet many unanswered questions remain. One of these is the value of amplitude ratios, which represent universal ratios of quantities related to integrals of the scaling function. A great amount of work has been done investigating amplitude ratios of various systems, both to demonstrate that systems expected to be in a given universality class have the same ratios, and to determine their values accurately [3]. The study of amplitude ratios remains an active area of research (e.g., [4,5,6,7,8,9,10,11]).In this paper we study specifically the universal amplitude ratio Γ − /Γ + for percolation, where Γ − and Γ + are the amplitudes of the second size-moment (also called the susceptibility) in the low-and high-density phases, respectively. This ratio has been especially difficult to estimate accurately and the status of the estimates is very controversial. In [3] values are quoted in a wide range from 14 to 220 based on numerical estimates from MonteCarlo simulations and series expansions for various percolation models. Furthermore, there are no reliable fieldtheoretical estimate for this quantity. Several years ago Delfino and Cardy [12] studied the q-state Potts model using methods from quantum field theory and predicted a value of 74.2 for percolation, by extrapolating the results for q = 2, 3, and 4 to q = 1. This was consistent with the numerical work of Corsten et al. [13], who gave the value 75 (+40, −26), but inconsistent with many other measurements [3]. In this work, we study bond and site percolation on the square lattice and use extensive exact enumerations to obtain estimates for Γ − /Γ + , using two different approaches: one a novel approach based upon directly integrating approximations to the scaling function, and the second a more conventional analysis of the high-and low-density series for the susceptibility. The estimates for the amplitude ratio are consistent with the value Γ − /Γ + = 162 ± 3. We also carried out a MonteCarlo calculation, which gave an almost identical value of 163 ± 2. These results are a significant improvement on previous published numerical est...