C# Image class extension methods for physical print size in millimetres

We had a business need for the actual print size of an image in millimetres.

The PhysicalDimension property on System.Drawing.Image was unfortunately not what I had hoped for.

Here’s a couple of extension methods that use the width and height in pixels, along with the DPI (as HorizontalResolution and VerticalResolution) to return an Image object’s width and height in mm.

public static class ImageExtensions
    {
        public static double WidthInMm(this Image img)
        {
            const double mmPerInch = 25.4;

            return Convert.ToDouble(img.Width / img.HorizontalResolution * mmPerInch);
        }

        public static double HeightInMm(this Image img)
        {
            const double mmPerInch = 25.4;

            return Convert.ToDouble(img.Height / img.VerticalResolution * mmPerInch);
        }
    }

Usage example for those new to using extension methods:

var bitmapImage = new Bitmap(filename);
var bitmapWidthInMm = bitmapImage.WidthInMm();