The Spearman-Karber method can be used to estimate the threshold value or difference limen in two-alternative forced-choice (2AFC) tasks. This method yields a simple estimator for the difference limen and its standard error, such that both can be calculated with a pocket calculator. In contrast to previous estimators, the present approach does not require any assumptions about the shape of the true underlying psychometric function. The performance of this new nonparametric estimator is compared with the standard technique of probit analysis. The Spearman-Karber method was found to have excellent statistical properties and thus to be a valuable addition to the toolbox of psychophysical methods for the analysis of 2AFC tasks.