Background: A lying prone position is recommended when scanning women’s breasts to ensure the entire breast can be visualised. However, several large databases contain three-dimensional scans of women’s breasts and torsos that were obtained while the women were standing. This study aimed to establish the error associated with calculating breast volume from three-dimensional breast scans taken when women were standing relative to lying prone. Methods: Breast volume was derived for 378 women with Small, Medium, Large and Hypertrophic breast sizes from scans taken while the women were standing and then lying prone. Results: The magnitude of error associated with breast volume derived from scans obtained while women stood compared to lying prone, ranged from ∼8–22% and increased with increasing breast size. Conclusion: Errors associated with breast volume data derived from breast scans collected while women stand must be accounted for, particularly for women with Medium, Large and Hypertrophic breast sizes. Practitioner summary: This original research provides evidence for bra designers and manufacturers on the degree of error associated when using breast scans obtained while women were standing, such as those scans readily available in large scanning databases. These errors increase with increasing breast size and must be accounted for when sizing and designing bra cups.