Research exploring nonbelievers’ reasons for not believing in the existence of god(s) has focused on theory development. Such efforts are valuable, but may not capture the lived experiences of nonbelievers. The current two studies quantitatively examined nonbelievers’ self-reported reasons for nonbelief through developing the Reasons of Atheists and Agnostics for Nonbelief in God’s Existence Scale (RANGES). We developed an initial pool of 64 items using prior published research, revised by a panel of experts including researchers and thought leaders in nonbelief communities. Both studies included participants from Amazon’s Mechanical Turk (Study 1 & 2 Ns = 520 & 369), all of whom reported not believing in god(s). In Study 1, our exploratory factor analysis suggested nine factors across 35 items. In Study 2, we confirmed the nine-factor structure using 38 items (35 from Study 1 plus three new items for better coverage of factors with few items) with adequate fit. Across both studies, the RANGES subscales showed good reliability, convergent validity (e.g., positive correlations with previous lists of reasons for religious doubt), predictive validity (e.g., positive and negative feelings toward God and religion), and discriminant validity (e.g., subscales were not unexpectedly associated with other measures). Our 1-year follow-up with a subset of Study 2 participants (N = 132) found different levels of stability among the RANGES subscales. This measure can promote further understanding the motivations, identities, and experiences of nonbelievers across cultures.