While research on informal digital learning of English (IDLE) increases in the fields of teaching English to speakers of other languages and computer-assisted language learning, few studies have examined the relationship between quantity and diversity of IDLE practices and different language learning outcomes. To address this gap, data were collected through one questionnaire, six English learning outcomes, and a semi-structured interview from 71 Korean English-as-a-foreign-language (EFL) university students. Hierarchical linear regression analyses showed that IDLE Quantity, Age, and Major were significant predictors of two affective variables (Confidence and Enjoyment), while IDLE Diversity and Major were significantly predictive of productive language outcomes (Speaking and Productive Vocabulary Knowledge), scores in a standardized English test (TOEIC), and one affective variable (Lack of Anxiety). These results revealed how IDLE quantity and diversity can make a unique contribution to EFL learners' English outcomes.