Your monitor has a resolution (e.g., 1920x1080). That is irrelevant. Only the matter.
To recap:
import cv2 # Assume you have a focal length and distance # Or calibrate using a checkerboard pixel_mm_x = physical_width_mm / image_width_px area_per_pixel_mm2 = pixel_mm_x ** 2 print(f"Pixel Value: area_per_pixel_mm2 mm²/px") pixel value mm2