Studies are emerging documenting the experience of fatigue, exhaustion and loss of functioning that has long been described by autistic adults as autistic burnout. New assessment tools are needed to enable identification and diagnosis. Here, we sought to identify factors associated with severity, develop an autistic burnout assessment and test the prepublication AASPIRE Autistic Burnout Measure tool. A co-produced survey of 141 autistic adults with experience of autistic burnout (98% above cut-off for depression) was subjected to exploratory factor analysis and scale reduction to identify a grouping of Autistic Burnout Severity Items. Autistic Burnout Severity Items showed strong overall internal consistency and acceptable internal consistency across four factors. Masking and depression were associated with the Autistic Burnout Severity Items, once variation in alexithymia, interoception, repetitive behaviours, sensory sensitivities and autism severity had been adjusted for. There is some suggestion that the Autistic Burnout Measure may not be as robust as the Autistic Burnout Severity Items, particularly as it showed a significant relationship with depression but not masking. Our findings alongside recent literature highlight a core phenomenon, comprising exhaustion, withdrawal and cognitive overload, associated with stressors potentially unique to autistic people. Further disambiguation from autistic shutdown and other conditions is needed in work towards the measurement of autistic burnout.