RESEARCH QUESTION Can a reliable measure of precise harm levels for the 100 most harmful and frequently occurring offences be developed in Western Australia (WA) based on analysis of actual court penalties for first-time offenders? DATA Criminal and traffic court sentences in 2.2 million records over 6.5 years were analysed to extract the number of days of imprisonment actually imposed in sentencing decisions for approximately 52,000 first-time offenders (see House 2017). METHODS Sentences for all first offenders in a sample of the 102 most common offence categories were analysed to compute for the median number of days of imprisonment to which each first offender was sentenced in each of the categories. Monetary penalties and conditional community sentences were converted to equivalent ‘prison days’ and added to the computation of the median of days of imprisonment per offence category. The number of reported offences in WA in the study period for each of the 102 categories was then multiplied by the median prison days sentenced per category. The sum of the products of median prison days times offence count was then tallied across all offence categories to form a weighted index of crime harm, which we define as the Western Australian Crime Harm Index (WACHI). Applying a minimum requirement of at least five separate court cases for each crime category, a total of 88 offence categories survived the reliability threshold for inclusion in the index. FINDINGS The 88 offence categories in the WACHI contain both high-harm and highvolume offences, permitting 95% of all offences reported for over 5 years to be assessed for WACHI scores. The counts for these offences moved in different directions from the WACHI total in two of the four year-to-year comparisons. Changes in WACHI were shown to have been highly sensitive to increased reporting of historical sex crimes, isolated in one district each of both Metropolitan Perth and one Regional centre. CONCLUSIONS Carefully implemented use of the West Australian CHI could improve both public safety and policing by adding precision to resource allocation decisions, assessments of priorities and evaluations of policing initiatives. The WACHI would be even more reflective of the changing level of harm to victims if all crime trends were to be based on crimes that occurred in the year under analysis, with separate reporting of crimes that happened many years ago. With that key adjustment, police professionals, department of justice officials, citizens and local governments can use a WACHI to make better decisions about how to prioritise policing in a wide range of contexts.