Millions of people are tested for COVID-19 daily during the pandemic, and a lack of evidence to guide optimal nasal swab testing can increase the risk of false-negative test results. This study aimed to determine the optimal insertion depth for nasal mid-turbinate and nasopharyngeal swabs. The measurements were made with a flexible endoscope during the collection of clinical specimens with a nasopharyngeal swab at a public COVID-19 test center in Copenhagen, Denmark. Participants were volunteer adults undergoing a nasopharyngeal SARS-CoV-2 rapid antigen test. All 109 participants (100%) completed the endoscopic measurements; 52 (48%) women; 103 (94%) white; mean age 34.39 (SD, 13.2) years; and mean height 176.7 (SD, 9.29) cm. The mean swab length to the posterior nasopharyngeal wall was 9.40 (SD, 0.64) cm. The mean endoscopic distance to the anterior and posterior end of the inferior turbinate was 1.95 (SD, 0.61) cm and 6.39 (SD, 0.62) cm, respectively. The mean depth to nasal mid-turbinate was calculated as 4.17 (SD, 0.48) cm. The optimal depths of insertion for nasal mid-turbinate swabs are underestimated in current guidelines compared with our findings. This study provides clinical evidence to guide the performance of anatomically correct nasal and nasopharyngeal swab specimen collection for virus testing.