Standing or supine

Posture is not usually on anyone's list of things that affect a reading, and it should be. Standing and lying down move three of the variables a bone-pressed length depends on, and they move them at the same time.

What changes between the two postures

The fat pad. Standing, gravity pulls the pre-pubic fat pad down and away from the base, which is part of why pressing the ruler firmly into the bone matters so much - it is compressing tissue that is already somewhat displaced. Lying down, the same tissue sits differently against the pubic bone, and how much it compresses under the same pressure can differ from the standing figure.

The angle. Standing, an erection points forward and slightly up along a fairly consistent axis. Lying flat changes what "up" means relative to the body, and can pull the shaft's natural resting angle closer to or further from the body than it sits standing.

The view. Standing, you are looking down and slightly forward at the ruler, roughly square to it. Lying down, especially reading over your own body, the sightline is worse and reading a ruler off-angle introduces parallax that a squarer view avoids.

None of these differences is large on its own. Together, across a session, they are enough that a standing reading and a supine reading are not guaranteed to land in the same place.

What the studies did

Clinical studies mostly measure standing, which is one more reason a standing reading is the one comparable to the published reference data. A supine reading is not wrong, but it is a different protocol, and a study-comparable figure is a standing one unless a study specifically used another posture.

Why three small effects add up

None of the three variables above is large enough, alone, to explain a noticeably different number. A slightly different fat-pad compression might shift a reading by a millimetre or two. A slightly different resting angle, likewise. A worse sightline adds its own small parallax error on top. The reason posture is worth naming explicitly, rather than folding it into "measure carefully," is that these three effects move together rather than independently - changing posture changes all three at once, in the same session, which is a different situation from a single technique slip that a careful re-read can usually catch. Stacked together, they are enough to explain why two otherwise careful readings, one standing and one lying down, can land close but not identical, without either one being wrong.

The fix costs nothing

Record which posture the reading was taken in, the same way you record bone-pressed, state, and instrument in the measurement log. If you want a figure to sit against Veale et al. (2015) or any other clinical reference, take it standing. If standing is impractical for some reason, a supine figure is still useful for tracking your own technique over a session, as long as it is labelled and never mixed with a standing figure as if the two were the same reading.

Not a question this site answers otherwise

This is entirely about which posture produces a number comparable to the literature. It has nothing to do with which posture looks a particular way to a viewer, which is a photography and presentation question that belongs elsewhere - a rating service like Rate Cock or a scoring tool built around submitted photographs, where framing and angle are explicitly part of what gets judged, is answering that different question. A human judge assessing a photo is doing the same kind of subjective read regardless of posture, and etiquette around what gets submitted is Rate Penis's territory, not this site's. An AI model scoring a photo is working from pixels rather than a ruler either way, which is a limitation AI Penis is straightforward about and one more reason a measurement protocol and a photo-scoring protocol never need to agree on posture for the same reasons.

The one-line rule

Stand up, use the standard method, and write "standing" next to the figure. It is a small thing to record, costs nothing to write down, and a real thing to get wrong if it is left off the page entirely.