Precise Selection Size


Today’s Question: With the Crop tool I can draw a rectangle that is exactly 5 inches by 7 inches, for example. But what if I don’t want to crop? What if I just want to select an area with that span? Or draw a rectangle shape exactly that way? Can either of these tools be done to an exact size?

Tim’s Quick Answer: Yes, with most of the selection and shape tools (such as the Rectangular Marquee and Rectangle tools in this example) can be used to create a shape of specific dimensions in inches (or any other unit of measure). You’ll first want to confirm the image dimensions and resolution in the Image Size dialog, and then you can specify the shape size using the settings on the Options bar.

More Detail: When you specify dimensions for a shape, those dimensions are affected by the pixel per inch (ppi) resolution for the image. In other words, since Photoshop is really dealing with pixels, in order to communicate inches accurately you need to specify how many pixels should be counted as one inch. For example, when printing you will typically use a resolution of around 300 pixels per inch.

To review (or change) the ppi resolution for an image in Photoshop, choose Image > Image Size from the menu to bring up the Image Size dialog. There you’ll see a pixel per inch resolution, though you can also change this resolution to reflect centimeters per inch if you prefer.

If you want to change the resolution for the image, first turn off the Resample checkbox, so the number of pixels in the image can’t be changed. Then set the Resolution value to the desired number, and click OK to apply the change. Note that with this approach you are only changing the pixel per inch resolution for the image, without actually changing the size (the number of pixels) for the image.

Now you are ready to specify the specific size for a selection or shape using inches as your unit of measure. With the Rectangular Marquee tool, for example, you can choose the “Fixed Size” option from the Style popup, and then enter values for Width and Height. For the Rectangle tool you will always see the W (Width) and H (Height) fields on the Options bar.

The default unit of measure for the Width and Height values is pixels, which is abbreviated “px” after the numeric value. You can change the unit of measure by simply changing this text. So for five inches you could enter “5 in” for example. In addition to “px” for pixels and “in” for inches, you can also specify dimensions in centimeters using “cm”.

With the dimensions established, you can easily create the selection or shape at the desired size by clicking within the image. By default, that selection or shape will then be created automatically, with the point you clicked within the image becoming the top-left corner of the selection or shape.