Access to woodlands is very important for individuals to support their mental health and well-being. However, these benefits have yet to be included in natural capital accounts at national level. This study is the first attempt to provide national estimates of the natural capital value of the mental health benefits provided by UK woodlands. Values are estimated using an avoided cost approach that avoids potential double counting with values for other types of benefits. It is based upon an association between regular visits to natural environments and a reduced prevalence of common mental illnesses, combined with societal costs of depression and anxiety, including lost working days and NHS costs.