Two big questions in biological monitoring programs are: 1) How many samples should I take; and 2) How many organisms should I count per sample? Using a diatom-based case study with realistic costs for sampling, environmental analysis and enumeration, we provide a method for determining the most cost-effective sample configuration. For diatoms, the optimal sample configuration depends on the analysis. Although sample numbers and enumeration effort can be considerably reduced from typical levels for large-scale projects, there is no ‘magic number’ that works best for every analysis. Our paper can be found here.