Mindfulness shows promise for improving health, but unbiased measurement of mindfulness is lacking. Currently, the only way to measure how mindful someone is is to ask, and answers may be biased. In contrast, breath counting, a longtime adjunct of mindfulness training, offers an objective measure of accuracy resistant to bias. Here, we show breath counting accuracy is a reliable and valid measure associated with greater meta-awareness, less mind wandering, better mood, and greater nonattachment. In addition, we developed an online breath counting game that increased players’ mindfulness more than randomization to control training conditions. These findings suggest breath counting is a rigorous method for studying mindfulness skill and its correlates, as well as a training tool with potential clinical relevance.