Understanding fraction magnitudes is important for achievement and in daily life. However, adults' fraction reasoning sometimes appears to reflect whole number bias and other times reflects accurate reasoning. In the current experiments, we examined how contextual factors and individual differences in executive functioning (Experiment 1), knowledge of fraction equivalence (both experiments), and strategy use (Experiment 2) influenced adults' fraction reasoning. Adults were only biased by fraction components when reasoning about fractions as holistic magnitudes was difficult: when estimating under a time constraint, when estimating fractions with large components, or when comparing fractions close in decimal distance. However, adults' knowledge of fraction equivalence moderated the effects of whole number components on their fraction estimation performance: when modeled at low levels of equivalence knowledge, adults were biased by fraction components when estimating. Adults with more knowledge of fraction equivalence were able to reason about fractions as holistic magnitudes through adaptive strategy choices.