Objectives: This paper explores how changes in demography, service use and the aged care system have implications for the appropriateness of the denominator in the aged care provision ratio. Methods: Official statistics were analysed to create five alternative scenarios for residential care planning in a 2000-2027 time series. Results: The four age-based denominators and a revised denominator incorporating age- and sex-specific disability rates generated diverse levels of absolute supply. The 75+ denominator produced 1400 fewer beds than the 70+ denominator by 2018, but over 14 000 additional beds by 2027. Conclusion: The 70+ based ratio developed almost forty years ago has served its purpose reasonably well. However, changes in the nature of the aged care system, different patterns of use by age and sex, and demographic changes suggest that long-term aged care planning requires careful review in choosing a more robust version of the provision ratio for the future.