A ruler and a pixel loupe inside the capture reticle.

DX, DY, and D: the horizontal, vertical, and straight-line distance from the anchor to the cursor, in points.

A square loupe with a hard grid, so single device pixels can be counted, with the color under the cursor as hex and its pixel coordinates.

Enter copies the values as text. Measure mode produces measurements, not files: no capture file is saved.

DX and DY are each rounded to a whole point, and D is worked out from those two whole numbers to one decimal place, so the three values you copy agree with each other.

The ruler counts points, the units macOS lays the screen out in. On a Retina display one point covers more than one device pixel, so single pixels are counted on the loupe, not the ruler.

Measure mode saves no file and adds nothing to the list of recent captures. The editor's rail has a measure tool too, for a capture you already took, and a measurement there never becomes part of the image.

The first click sets the anchor. A later click does not move it: it reads like the cursor passing through that point. The measurement stays live until Enter copies it or Escape clears it.

The loupe shows 15 by 15 device pixels, each drawn as an 8 point cell with no smoothing, and hairlines cross at the center pixel. It trails the cursor without covering the crosshair.

Under the loupe, a tag gives the center pixel's color as uppercase hex and its coordinates in device pixels. The tag widens for four-digit coordinates, so it never clips.

The reticle freezes the screen when it opens, and the loupe and the hex both read that frozen image, so a color that changes under you is read as it was.

A measurement stays on the display where its anchor is. Cross to another display and it holds its last reading; a click there starts a separate measurement.