35mm eq.
 

I have seen ads for a 6-55mm lens saying it's 35mm equiv. is 37 - 450mm
some ads have different 35mm equivelents, my xl1s has a 16x zoom what is the 35mm equiv.
and does anyone know how to measure the ratio?

While I don't know exact formula, it really has to do with size of chip/image.

So a 1/4 chip camera would be different from a 1/3 or 1/2 inch camera. As the chip increases in size, the focal length of the lens must increase to get the same field of view. Looks like your example comes out just over 6 to 1. the 16 x does not tell you enough about the lens, to use an equivalence, because you need the starting focal lengt to calculate. 16x means lens will zoom in to give maximum 16 x image from the extreme wide.

Ratio is 7.2 in 16:9 and a bit more in 4:3 something like 9 I think.

The stock XL1 lens is 5.5mm - 88mm (16x) zoom, with a 3x4 image aspect ratio for video. Because 35 mm has a 2:3 aspect ratio, any equivalent is only approximate. With that in mind, it is described as "approx. 39mm - 624mm for a 35mm camera" in Canon's literature.

OF course, focal length is focal length. The equivalent is based on the resulting image, and that depends on the image sensor size, either film frame in 35mm, or effective CCD size in the camcorder. BTW, as I understand it, the nominal published CCD size is not its actual physical size, but for consistency with legacy gear is based on the nominal size of the imaging tubes used in early TV cameras that would produce the same image with the same lens. Kind of like the old tube-based monitor size was usually an inch or two larger that the viewable image.