Measuring Size within a Photo


Today’s Question: I recently took a headshot for my passport renewal. The requirements are that the head be between 1” and 1.5” from top of head to chin, while the overall picture be 2”x2”. I set the canvas and image size to 2×2, but I could not find an easy way to measure the head size in Photoshop. The only option seemed to be to set guides and calculate the difference between them. Is there an easier way?

Tim’s Quick Answer: While guides can be helpful for establishing dimensions within an image, you can also measure an object within a photo using the Ruler tool or with the Rectangular Marquee or Elliptical Marquee selection tools.

More Detail: There are several ways to measure an area within a photo in Photoshop, though none of them are quite perfect.

In this scenario you could place guides in the image and use them as the basis of resizing the image layer to fit within the dimensions defined by the guides. One of the easiest ways to precisely position guides is to choose View > Guides > New Guide from the menu. In the New Guide dialog specify if you want the guide to be a horizontal or vertical line and enter a value for the Position field. The position can be defined in various units of measure, including inches by entering a number followed by “in”, such as “0.25 in”.

With the example of a passport photo, you could add horizontal guides at 0.25 inches and 0.5 inches representing the top limits, and at 1.5 inches and 1.75 inches for the lower limits. Then resize the image layer representing the passport photo to fit within the dimensions.

If you just need to measure an area of the image, adding guides isn’t likely to be the most convenient approach. Instead, the Ruler tool provides a good solution. You’ll want to make sure the pixel-per-inch (PPI) resolution of the image is set for the intended output resolution and size, which you can adjust by choosing Image > Image Size from the menu. Then make sure the rulers are displayed around your image, which you can toggle by choosing View > Rulers from the menu. Right-click on the ruler at the top or left of the image and select Inches (or the intended unit of measure) from the popup.

The Ruler tool can be found on the toolbar on the button associated with the Eyedropper tool. So, you can right-click on the button for the Eyedropper tool to bring up a popup menu where you can select the Ruler tool. Then drag on the image to measure, holding the Shift key to constrain the measurement to perfectly horizontal or vertical. The measurements are shown on the Options bar, so you could reference the “W” (width) value for a horizontal line or the “H” (height) value for a vertical line.

With the Rectangular Marquee tool, you can choose Fixed Size from the Style popup on the Options bar, and then set a value for Width and Height, using “in” after the numbers to designate inches as the unit of measure. You can then click within the image to place the selection at the designated size.