According to the 1-in-X effect, 1-in-X ratios (e.g., 1 in 12) trigger a higher subjective probability than do numerically equivalent N-in-XN ratios (e.g., 3 in 36). Here we tested the following (a) the effect on objective measures, (b) its consequences for decision-making, (c) whether this effect is a form of bias by measuring probability accuracy, and (d) its amplification in people with lower health literacy and numeracy. In parallel-designed experiments, 975 participants from the general adult population participated in 1 of 5 experiments following a 2(format 1-in-X or N-in-XN) × 4(scenarios) mixed design. Participants assessed the risk of contracting a disease on either a verbal probability scale (Experiment 1) or a numerical probability/frequency scale with immediate (Experiments 2–3) or delayed presentation (Experiments 4–5). Participants also made a health-related decision and completed a health literacy and numeracy scale. The 1-in-X ratios yielded higher probability perceptions than did the N-in-XN ratios and affected relevant decisions. Critically, the 1-in-X ratios led to a larger objective overestimation of numerical probabilities than did the N-in-XN ratios. People with lower levels of health literacy and numeracy were not more sensitive to the bias. Health professionals should use 1-in-X ratios with great caution when communicating to patients, because they overestimate health risks.