How do I convert pixels to real-world size on a flat plane at a known distance?

If the plane is perpendicular to the camera and you know the horizontal field of view, you can estimate scale with simple trigonometry:

mm per pixel ≈ 2 × distance × tan(horizontal FOV / 2) ÷ image width in pixels

Then: real width ≈ pixel count × mm per pixel

This gives a useful approximation, especially when the subject distance is much larger than the focal length. At close focus, effective focal length can shift, so accuracy drops.

Undistortion does not guarantee that every pixel corresponds to exactly the same physical area. Depending on the lens, calibration, and how the undistorted image is remapped or cropped, the pixel/mm ratio may vary somewhat across the frame.

For best accuracy, especially with noticeable distortion, the practical recommendation is to calibrate experimentally: place a ruler or tape measure on the plane at the same distance and measure the pixel-to-mm ratio directly in the corrected image. That will account for focus distance, distortion correction, and any non-uniformity better than relying only on intrinsics.