image size

I´m new to this forum and I´m new to Photoshop, and I have a problem.. I´m sure someone here can impress me.. When I make a new image and define the size ( in this case it was width: 35,6 cm and height: 16 cm ) Then I click the OK-button.. When I go to image size it shows width: 35,61 and height: 16,01.. And if I try to change the numbers and click the OK- button again it doesn´t change.. Why is that? Do I need to change anything in preferences?? It makes no difference if I constrain proportions or not??? Please help me!! I need to deliver an advertise in certain messurements.. Thanks, Luna

Don't worry about it. One tenth of a millimeter is rarely anything to bother with.

First thing you must remember, always:
In Photoshop, EVERY unit of measurement is converted to pixels. That's the only unit the Photoshop uses internally, and it's the only measurement it understands. Measurement readouts are converted to Inches or Centimeters solely for the convenience of the user.

What you're seeing is Photoshop adjusting the height and width so the edges fall right on the closest pixel edge. This is because in Photoshop, you can't split a pixel. So the height and width, as measured in cm & mm, is rounded off to the edge of the closest pixels near your desired height and width.

If you were to raise the resolution of your starting "New Image" size, there will be more pixels per inch, or perhaps in your case, pixels per centimeter. Since centimeters are a fixed size, by dividing them into smaller increments (by raising the resolution of your new image) you will be able to get closer in pixels to an actual physical dimension in centimeters. If plus or minus 1/10 of a millimeter is not accurate enough for your needs, raise the starting resolution of your new image.