Photoshop web Crop Size does not match Image Size

Thanks - I can't see that option - I'm using the web version of PS

The crop tool will do that. Go to the box at the top left of the options bar and you can select Ratio or W x H x Resolution. Resolution can be in ppi or ppcm.
Dave

Thank you - I undestand the concept of PPI;  however there is no PPI value in the crop tool - it just shows an arbitrary number of centimetres.  I understand it is using a PPI to calculate the size in CM but what value is it using?  It's not the value used in the Image Size menu item (which would make sense to me).

I can work out how much to crop in pixels by taking the dimensions in CM from image size and the PPCM, using a calculator to work out the crop in pixels, going back to the crop tool and using pixels (not the random CM number) and then doing the crop. 

It would be far simpler if the crop tool had an explicit PPI/PPCM value - using a different number that the user can't see looks like a bug to me.

All of this becomes much easier to understand when you realize that Photoshop doesn't work with sizes. It only works with pixels.

The physical size of a document is determined by whatever "pixels per inch"-number (ppi) is assigned. The ppi number can be arbitrarily changed, thus changing the physical print size, but not actually changing the file itself. The pixels remain the same - unless the ppi value is used as a parameter for resampling the pixel data.

Always start with pixels. Then see what the ppi value ends up as at a given size, or what the size becomes at a given ppi value. One goes up, the other goes down.

Pixels per inch means exactly what it says. Read it literally: pixels per inch. It's just a simple formula that translates from pixels to physical size units whenever that's needed, such as for printing or calculating font sizes (a point is a physical size unit from the old days of lead typesetting).

On screen ppi is wholly irrelevant.