What Is A Resolution Unit?

Resolution Unit is a label. On its own it says nothing about your file, but it gives meaning to the resolution figures sitting right next to it in the report, usually called X Resolution and Y Resolution. Those two numbers say how many pixels are packed into one unit of length, and Resolution Unit says which unit that is.

Which files carry it and who sets it

Resolution Unit is one of the oldest fields in image metadata. You will find it in JPEG photos, TIFF images, scanned documents, raw camera files and PDF pages. Video files can carry the same idea for their poster or preview images.

Nobody types this value in by hand. It is written automatically by whatever produced the file: a camera when it saves the photo, a scanner when it hands over the scan, or an image editor such as Photoshop or GIMP when you save your work. Cameras normally write a fixed pair of values here rather than measuring anything, which is why so many camera photos report inches with a resolution of 72.

What the value looks like

Only a small set of values is possible, and a metadata report usually spells them out in words:

• inches - the resolution figures are pixels per inch, often written ppi or dpi
• cm - the resolution figures are pixels per centimetre
• None - no unit is given, so the resolution figures have no physical meaning at all

Inside the file the unit is stored as a small number rather than a word: 1 for none, 2 for inches and 3 for centimetres. Read together, a report that shows Resolution Unit: inches and X Resolution: 300 is telling you 300 pixels per inch.

Why it matters

The unit only becomes important when a file has to turn into something physical. Pixel count divided by resolution gives the printed size: an image 3000 pixels wide at 300 pixels per inch prints 10 inches wide. Swap the unit to centimetres and the very same numbers now mean 10 centimetres, less than half the size. That is why a picture can land in a document or a print job at an unexpected size even though nothing about the image itself changed.

It helps to know what this field does not do. It does not change a single pixel, it does not affect quality, and it does not change how large the file is. Editing it only relabels the measurement, so raising a resolution figure never adds detail that was not captured in the first place. Layout and print software reads these three fields to decide the default size at which an image is placed, while web browsers ignore them and simply show the pixels.

There is no privacy concern with this field. It describes a measurement, not a person, a place or a device, although it does sit in the same metadata block as fields that can identify all three.