The UK weaning guidelines recommend the introduction of solid food at or around 6 months. The evidence suggests that knowledge of the guidelines is high, although only a small minority of parents wait until 6 months to wean. The aim of this study was to assess understanding of the UK weaning guidelines in a sample of UK parents and investigate the associations of this understanding with weaning timing, and in comparison to other influencing factors. This study conducted an online survey of UK parents. Eligible participants had weaned a child since the introduction of the current guidelines. Of 3607 participants, 86% accurately understood the guidelines. Eighty-seven per cent of health visitors were reported to have advised weaning at or around 6 months. Knowledge of the guidelines was associated with later weaning (independently of demographic factors) (P < 0.001) but did not ensure compliance: 80% of mothers who weaned before 24 weeks and 65% who weaned before 17 weeks were aware of the guidelines. Younger mothers (P < 0.001), those receiving benefits (P < 0.001), those educated only to 16 (P < 0.001) and minority ethnic groups (P < 0.001) had lower levels of awareness. Poor understanding of the guidelines was the most reliable predictor of early weaning (P = 0.021) together with young maternal age (P = 0.014). Following the baby-led weaning approach was the most reliable predictor of those weaning at 26 weeks, together with the Internet being the most influential source of advice. Understanding of the current weaning guidelines is high and is a key independent predictor of weaning age in this population.