The Lens tool can also do some quick measurements, directly on the viewport. To do a quick measurement:

  1. Move your mouse inside the lens area
  2. Press and hold the Control key. You should see the cursor change to a crosshair.
  3. Drag a rectangle. You will see a sizing rectangle with some resize handles
  4. To resize, hold the Control key and drag on a resize handle

The size of the rectangle is shown in the bottom-left corner.

If you leave the Control key, you will be back in normal mode. Your sizing rectangle is not disturbed. To get back into the measurement mode, press the Control key. In short, the Control key is your way to toggle measurement mode.

Note As you zoom in and out, the rectangular area that you selected with snap to the pixel-grid and show you the size accordingingly. It’s also fun to see the dimensions change as you zoom in and out.