What the term actually describes
Jaw ratio compares the height of the lower face — from the base of the nose to the chin — against the total height of the face, from forehead to chin. It's a proportion of vertical distances, not a measurement of jaw width, jaw angle, or bone structure directly. A higher ratio means the lower third of the face occupies more of the total height; a lower ratio means less. That's the whole definition — it's a geometry ratio, not a diagnosis.
Why a single photo is the wrong tool for this specific measurement
Because jaw ratio is built entirely from vertical distances up and down the face, it's directly sensitive to pitch — how far the chin is tilted up or down when the photo is taken. Tuck the chin down slightly and the nose-to-chin distance foreshortens in the frame; lift the chin and it stretches out. Of the six ratios Mogalytics tracks, jaw ratio is the one most exposed to this specific up-down head movement — see the full breakdown of why scores swing between scans for how the other five ratios get thrown off by different camera variables. A single uploaded photo has no way to correct for pitch; whatever angle the chin happened to be at is baked into the result.
What a repeatable read looks like instead
The fix isn't a better single photo, it's ruling out chin pitch as the variable before trusting the number. Keep the chin level and the camera at eye height, hold distance and expression steady, and take more than one reading — the practice guide covers framing and pose in more detail. If the ratio still moves between attempts once pitch is controlled for, that's a more meaningful signal than one photo can ever give you.
Where this fits in Mogalytics' composite
Mogalytics tracks jaw ratio as one of six weighted geometry ratios — alongside eye line, frame symmetry, midface, cheek ratio, and bridge — settled over a 30-frame trimmed-mean window so a single chin dip doesn't define the result. It's read live from your own camera on-device, not from one uploaded photo, so you can hold your chin level, check the read, adjust, and check again in the same session instead of guessing from a single shot. See what the full composite tier actually means once you have a stable reading, and how to track it across sessions rather than trusting any one scan.
Related reads
Jaw ratio is one piece of a six-measurement composite. If you're auditing the rest: canthal tilt is sensitive to head roll rather than pitch; the facial symmetry test is thrown off by head turn instead; and cheekbone ratio is a width-only measurement with its own angle sensitivity.