Despite the importance of understanding error associated with sampling, there has been a paucity of discussion in many analyses within sea turtle biology. We investigated sampling and non-sampling errors for size measurements (straight carapace length, straight carapace width, and mass) of hatchling Flatback Turtles, Natator depressus, from two study sites in tropical Australia using a priori and post hoc methods. A priori and post hoc methods produced similar error estimates (max difference  =  0.19 g and 0.24 mm), highlighting methodologies that may be transferred for other species without requiring a census of every hatchling in a clutch. The standard practice of sampling ten hatchlings per nest resulted in reasonably low sampling error (upper 95% CI  =  0.9 g for mass, 0.7 mm for straight carapace length, and 1.0 mm for straight carapace width), but sample size may be reduced if less precise size measurements are required. Effort analysis showed that sampling error from a nesting population was minimized when sampling effort was spread over the maximum number of clutches rather than concentrating effort within fewer clutches.