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. Lay abstract Autistic burnout has been talked about by autistic adults for some time on blogs and in social media. Now, research describes fatigue, exhaustion and other related symptoms experienced by autistic people. We need new ways to help identify autistic burnout. In this study, we tested a new questionnaire called the AASPIRE Autistic Burnout Measure, and we investigated things that are linked to worse autistic burnout. We also trialled a group of Autistic Burnout Severity Items that we made. Working with an autistic researcher, we made the Autistic Burnout Severity Items based on published definitions of autistic burnout. Autistic adults ( n = 141) who had experienced autistic burnout completed an online survey. We found that autistic burnout was connected to masking and depression. The Autistic Burnout Measure tool was associated with depression but not with masking. It was not very accurate in telling apart participants who were currently experiencing burnout versus those who were reporting on their past experience. The Autistic Burnout Severity Items might have problems with subscales adding together to measure autistic burnout. More work is needed on how to measure autistic burnout. Our research and other recent studies show autistic people experience a combination of exhaustion, withdrawal and problems with their concentration and thinking. Burnout seems to be linked to the stress experienced by autistic people in their daily lives. We need more research to understand the difference between autistic burnout and other conditions and experiences. We need to develop assessment tools that can help identify this burnout.