Method
Order of operations
Take length first because the girth site is defined from it, and because a tape wrap can change state; the sequence is small but it is part of the protocol.
Guides on Method: Bone-pressed length, start to finish, Stretched flaccid length, and why the clinics take it, Girth, done the way the studies do it, The measurement glossary
Measure length first, then girth, because the girth site - the midpoint of the shaft - is defined from the length, and handling the shaft to wrap a tape can shift firmness before a length reading. The order changes what your girth number means; it is part of the method, not housekeeping.
Midshaft is defined from length, not independently
The convention for where to wrap the tape is the midpoint of the shaft - halfway between the base and the tip. That point cannot be found until you know the full length, because it is defined as a fraction of it, not as a fixed distance from the base. Measure length first, mark or note the midpoint from that figure, then wrap the tape there. Do it the other way round and you are guessing at the midpoint, which quietly turns a defined measurement into an estimate. The site matters because studies do not all agree on it: Habous et al. (2018), International Journal of Impotence Research, took girth "at the base of the penis", so a girth figure only compares with another taken at the same stated point.
Wrapping a tape can change what you are measuring
The second reason is about state, not geometry. Handling the shaft to position and tension a tape is itself a stimulus, and for some men it is enough to shift firmness slightly, one direction or the other. If girth comes first, the length reading that follows may be taken at a state that has already been disturbed by the girth measurement, which is the opposite of the repeatable session the method is supposed to produce. Length first avoids the question entirely, because nothing about taking a bone-pressed length reading requires the same handling.
What this does not require
This is not a claim that girth needs a long recovery period or that the two measurements cannot happen in the same sitting - they can, and should, since a session should stay short enough that state does not drift across it. It is only an ordering rule: length, then the midpoint mark, then girth.
Write both down with the order implicit
A measurement log that records length and girth from the same short session, in that sequence, is more trustworthy than two numbers taken on different days in whatever order was convenient. Small procedural details like this one rarely change a figure by much on their own, but they are cheap to get right and they remove one more source of disagreement between your number and anyone else's.
None of this touches how a number gets judged once it exists. That is a separate question, handled elsewhere - by a photo-based score at Rate Cock, by the AI model behind it at AI Penis, by the aggregated scores at Penis Rater, or by a person's opinion at Rate Penis - and none of those depend on the order you took your own ruler readings in.