You can estimate it in theory, but for real measurements the reliable answer is: calibrate the camera setup.

Why theory falls short:

• Reported focal length may not be exact, especially if focus distance changes.
• Typical photographic cameras/lenses are not precision measuring instruments.
• Small errors in focal length, focus, distance, or lens distortion can cause noticeable size errors.

Best practical method:

1. Place a known-size object at the same distance and position as your subject.
2. Photograph it with the same camera/lens/settings.
3. Measure how many pixels that known object spans.
4. Compute a scale factor: real size per pixel.
5. Use that ratio to convert pixel measurements in later images into meters.

If your setup is fixed and repeatable, this calibration method is usually the most useful. Many editors can also store a custom pixel-to-distance scale so measurement tools report directly in physical units.

So: yes, a geometric calculation is possible in principle, but for dependable real-world dimensions, use a calibration shot with a ruler, tape, or other known reference.