Overbite and overjet are measured in millimeters using two different planes: overbite is the vertical overlap of your upper front teeth over your lower front teeth, while overjet is the horizontal distance your upper teeth stick out in front of the lower ones. Both measurements use the central incisors (your two front teeth, top and bottom) as reference points, and ideal values for each fall between 1 and 2 mm.
Overbite vs. Overjet: The Key Difference
These two terms get mixed up constantly, but they describe completely different things. Overbite measures how much your top teeth cover your bottom teeth when you look straight at them, like a curtain dropping down. Overjet measures how far forward your top teeth sit compared to your bottom teeth, like a shelf sticking out.
Think of it this way: if you bite down and look in a mirror, overbite is the up-and-down overlap you see. Overjet is the gap you’d notice from the side, between the back surface of your upper front teeth and the front surface of your lower ones.
How Overbite Is Measured
Overbite is the vertical distance between the biting edges of your upper and lower central incisors when your teeth are fully together in their natural bite position. A dentist or orthodontist will have you bite down normally, then measure how many millimeters the upper incisor edge drops below the lower incisor edge.
In a clinical setting, this is done with a small ruler called a periodontal probe. The examiner identifies the biting edge of the upper right central incisor and measures straight down to the biting edge of the lower right central incisor. Some clinicians use a pencil-mark technique: they mark the point on the lower tooth where the upper tooth overlaps, then have you open your mouth and measure from that mark to the biting edge of the lower tooth.
A measurement greater than 4 mm is classified as a deep overbite. A measurement below 0 mm means your front teeth don’t overlap at all when you bite down, which is called an open bite.
How Overjet Is Measured
Overjet is the horizontal distance between the biting tip of the upper central incisor and the front-facing surface of the lower central incisor, measured parallel to the biting plane. In other words, it captures how far forward your top teeth protrude beyond your bottom teeth.
To measure this, a clinician places a probe or ruler horizontally from the outer surface of the lower front tooth to the tip of the upper front tooth. The measurement runs straight forward, not at an angle. Overjet is categorized as mild when it falls between 0 and 4 mm, moderate between 4 and 6 mm, and severe above 6 mm.
What Counts as Normal
An ideal bite has both an overbite and an overjet of 1 to 2 mm. At this range, your upper teeth slightly overlap and sit just in front of your lower teeth, which allows for proper chewing and protects both sets of teeth from excessive wear. Most people fall somewhere outside this ideal without it being a serious problem, but the further you get from that range, the more likely you are to experience complications.
For overjet specifically, anything under 4 mm is considered mild and often doesn’t require treatment. Once you cross into the 4 to 6 mm range, the protrusion becomes more noticeable and can affect function. Above 6 mm, the overjet is severe and typically needs orthodontic correction. Deep overbite, where the upper teeth cover more than 4 mm of the lower teeth vertically, can cause the lower teeth to bite into the roof of the mouth or wear down the backs of the upper teeth over time.
How to Estimate at Home
You can get a rough idea of your measurements using a mirror, good lighting, and a small millimeter ruler (the kind found in a sewing kit or on a phone screen ruler app).
For overbite: bite down naturally and look in a mirror. Notice how much of your lower front teeth is hidden behind your upper front teeth. If your upper teeth cover about a third of your lower teeth, that’s roughly 2 to 3 mm. If they cover half or more, you’re likely in the 4 mm and above range. If they cover the lower teeth entirely, you have a significant deep bite.
For overjet: this one is trickier to see yourself because it’s a side-view measurement. Bite down, then try to feel or see the gap between where your upper front teeth end and where your lower front teeth begin. You can gently place a thin ruler horizontally from the front surface of a lower incisor to the tip of the corresponding upper incisor. Having someone else look from the side while you bite down makes this easier.
These home estimates give you a general sense of where you fall, but they’re not precise. Clinical measurements use standardized instruments and controlled head positioning that’s hard to replicate on your own.
Why These Measurements Matter
Overbite and overjet aren’t just cosmetic numbers. Excessive values in either direction can lead to real functional problems. A deep overbite can cause jaw pain and stiffness, difficulty chewing, accelerated tooth decay from abnormal wear patterns, and temporomandibular joint disorders. Speech can also be affected, particularly with sounds that require your tongue to contact your front teeth.
Excessive overjet makes the upper front teeth more vulnerable to trauma, especially during sports or falls, because they protrude beyond the protective cushion of the lips. It can also make it difficult to close your lips comfortably at rest, leading to mouth breathing and dry mouth.
Orthodontists use these measurements alongside a broader classification system to plan treatment. A Class II division 1 malocclusion, for example, involves both excessive overjet and a deep overbite, with the upper incisors flaring forward. A Class II division 2 features a deep overbite but with the upper front teeth tipped backward rather than forward. Each pattern calls for a different treatment approach, which is why precise measurement matters more than a general impression of “my bite looks off.”
What Happens During a Professional Assessment
If you visit an orthodontist for a bite evaluation, they’ll measure overbite and overjet as part of a broader exam. You’ll be asked to bite down in your natural position (called maximum intercuspation) while they take measurements with a periodontal probe. Many offices now also use digital scanning or 3D models of your teeth, which allow for precise measurements without a physical ruler.
Beyond the numbers, the orthodontist will assess how your back teeth fit together, whether your jaw shifts when you close, and how your teeth align with your facial midline. Overbite and overjet are two data points in a larger picture, but they’re often the starting point for understanding what’s going on with your bite and what, if anything, needs to change.