How do I convert pixels to centimeters?

I've been using this code below to convert from pixels to centimeters by measuring a line known to be 1 cm in the image. I recently discovered my results are off and the code is not converting properly because the area I get later on is way too small. Does anyone have suggestions?

p2cm = 1/norm(hline.Position(1:2) - hline.Position(3:4));

CalibrationLine = hline.Position;

disp(['Length per pixel = ' num2str(p2cm) ' cm/pixel'])

** Result was .000759 cm/pixel and the area of the polygon was way, way smaller when it was supposed to be around 1.73 cm^2.

Answer 1

[Image Analyst]

Sine the line gives you the number of pixels, your p2cm is actually centimeters per pixel, not pixels per cm like you thought. So it's something like this

cmPerPixel = distanceInCm / distanceInPixels;
% Now to convert a distance.
lengthInCm = lengthInPixels * cmPerPixel
% And to convert an area in pixels to square cm:
areaInSquareCm = areaInPixels * cmPerPixel ^ 2

See my attached demo.