The 2020 global spatial targets for protected areas set by the Convention on Biological Diversity have almost been achieved, but management effectiveness remains deficient. Personnel shortages are widely cited as major contributing factors but have not previously been quantified. Using data from 176 countries and territories, we estimate a current maximum of 555,000 terrestrial protected area personnel worldwide (one per 37 km2), including 286,000 rangers (one per 72 km2), far short of published guidance on required densities. Expansion by 2030 to 30% coverage of protected areas and other effective area-based conservation measures is widely agreed as a minimum for safeguarding biodiversity and ecosystem services. We project that effective management of this expanded system will require approximately 3 million personnel (one per 13 km2), including more than 1.5 million rangers or equivalents (one per 26 km2). Parallel improvements in resourcing, working conditions and capacity are required for effective, equitable and sustainable management.