RN
Ask this question and you will be given a number, usually to two decimal places, usually with no indication of who was measured, in what state, or from which landmark to which. That precision is fake, and chasing it is the reason the question so rarely gets a satisfying answer.
Here is the more useful framing: the vaginal canal is not a cavity of fixed dimensions. At rest its walls lie against each other. It is a potential space — closed until something opens it — and asking how deep it is resembles asking how long a sleeve is when the arm is out of it. Once that clicks, the rest of this makes sense.
What is actually being measured
Measurement studies on this exist, and they disagree with each other for structural reasons rather than because anyone got it wrong.
The canal runs from the vaginal opening up to the cervix, and the cervix moves. It sits lower in the canal at some points in the menstrual cycle and higher at others, so the same person measured on two different days gives two different figures. Whether the measurement is taken at rest or under arousal changes it substantially. And the tissue is soft and distensible, meaning the number partly reflects how hard the instrument was pushed.
So the range is wide, individual variation is real, and any figure quoted without those caveats is being presented as more solid than it is. That is why you will not find one on this page.
Arousal changes the geometry
The thing worth knowing instead is what happens during arousal, because that is the part with practical consequences.
Blood flow to the pelvis increases. The upper portion of the canal balloons outward and the uterus draws upward, lifting the cervix out of the way — the change usually described as tenting. The effect is that the space available is considerably greater when someone is aroused than when they are not, and it develops over time rather than instantly.
The practical reading of that is simple and worth stating plainly: discomfort at depth is very often a timing problem rather than an anatomy problem. If penetration feels like it is hitting something, the most likely explanations are that arousal has not had time to do this, or that the angle is directing pressure onto the cervix, or that lubrication is short. All three are adjustable in the moment. If you want the specifics on the last one, the guide to lubricants covers it.
The lower third behaves differently — it is the part that grips rather than expands, and it is also where the majority of the nerve endings are. Sensation is not distributed evenly along the canal, which is the anatomical reason why depth is a poor predictor of how anything feels.
Age, childbirth, and what actually changes
Both do change things, and both are surrounded by more folklore than fact.
After vaginal birth, the tissue has been stretched considerably and takes time to recover. Some change persists for some people; for many the difference is smaller than expected, and pelvic floor recovery matters more to how things feel than any change in dimensions. The tissue itself is not the whole story — muscle tone is.
Around and after menopause, falling oestrogen thins the vaginal lining and reduces natural lubrication, which can make the canal feel less accommodating and make sex uncomfortable. This is common, it is not something to endure quietly, and it is one of the more treatable things in this article. A GP or gynaecologist has straightforward options.
What neither of these does is make anyone permanently "loose". That word is doing cultural work rather than anatomical work, and the elasticity it implies has been lost is largely a property of muscle, which responds to training.
Depth as a partner concern
If you arrived here worried about whether you fit, the anatomy above answers it: the space adapts, the sensitive part is not deep, and the range of normal is wide on both sides of any encounter.
What is worth attention is the reverse case. Repeated contact with the cervix is genuinely painful for many people and is a common reason sex stops being enjoyable. It is easily fixed by changing angle, changing position, or slowing the start — and much less easily fixed if nobody says anything, which is the actual failure mode.
When it is a medical question
Anatomy questions are not the same as symptom questions, and this is the part to be direct about.
See a clinician for pain during or after sex that persists, bleeding outside your period or after sex, a change in discharge that comes with itching, burning, or an unfamiliar smell, sores or lumps, or a persistent feeling of pressure. None of these are self-diagnosable from a web page, several have simple treatments, and a few matter more if left.
Two general points that are not diagnoses: the canal maintains its own bacterial balance and washing inside it disrupts that, so douching is best avoided; and pain is a signal rather than something to push through.
Everything else on this page is background. The appointment is the advice.