A periodontal probe is a thin, ruler-like instrument that measures the depth of the space between your gums and teeth, in millimeters. Reading one comes down to understanding the markings on the instrument, knowing where measurements are taken, and interpreting what the numbers mean for gum health. Depths of 1 to 3 mm are generally healthy, 4 to 5 mm signal moderate disease, and 6 mm or more indicate severe periodontitis.
What the Markings Mean
Periodontal probes have millimeter markings etched or color-coded along their thin tip. The most common version used in clinical practice, the UNC-15 probe, has a black band at every millimeter up to 15 mm. The probe slides gently into the gum pocket, and the clinician reads the depth by noting where the gum tissue meets the probe’s markings, much like reading a dipstick.
A different design, the WHO probe (also called the CPITN probe), uses a slightly different system. It has a small 0.5 mm ball at the tip and two colored bands: one from 3.5 to 5.5 mm and another from 8.5 to 11.5 mm. The ball tip serves two purposes. It helps the clinician feel for calculus (hardened plaque) beneath the gumline, and it prevents the probe from puncturing inflamed tissue at the bottom of a pocket. If the black band disappears entirely below the gumline, that pocket is deeper than 5.5 mm, which is a red flag.
Where the Six Measurements Are Taken
A complete periodontal exam involves six measurements per tooth, not just one. Three readings are taken on the cheek side of the tooth (near the front corner, the middle, and the back corner) and three on the tongue side in the same pattern. This matters because disease doesn’t affect a tooth evenly. You could have a healthy 2 mm reading on the cheek side and a concerning 5 mm pocket on the tongue side of the same tooth. A full-mouth chart for an adult with 28 teeth produces 168 individual readings.
When these numbers are called out during an exam, they’re typically read in sets of three. You might hear something like “3, 2, 3” for one tooth, meaning the three sites on that surface are all within the healthy range. A call of “3, 5, 7” means the middle site is borderline and the back corner has a deep pocket that needs attention.
What the Pocket Depth Numbers Tell You
The depth of a periodontal pocket reflects how much the gum has separated from the tooth, or how much bone has been lost underneath. Here’s how to interpret the ranges:
- 1 to 3 mm: Healthy. A shallow sulcus (the natural space around every tooth) is normal. These depths are easy to keep clean with regular brushing and flossing.
- 4 to 5 mm: Moderate periodontitis. The pocket is deep enough that a toothbrush and floss can’t reach the bottom, allowing bacteria to accumulate. This range typically calls for deeper cleaning procedures.
- 6 mm or more: Severe periodontitis. Significant bone loss is likely. These pockets often require surgical treatment or ongoing specialized maintenance to prevent tooth loss.
A single deep reading doesn’t necessarily define your overall condition. Clinicians look at the pattern across all teeth. One isolated 4 mm pocket is very different from having 4 to 6 mm pockets throughout the mouth.
Bleeding on Probing: The Other Key Reading
Numbers aren’t the only thing recorded during probing. Whether the gum bleeds when the probe touches it is a critical diagnostic sign. Bleeding on probing indicates active inflammation beneath the gumline, even when pocket depths look acceptable.
The predictive value of bleeding is surprisingly precise. Research tracking patients over time found that sites bleeding at every single visit had a 30% chance of losing attachment (meaning the gum and bone receding further). That risk dropped sharply with less frequent bleeding: 14% for sites bleeding at three out of four visits, 6% for two out of four, and just 1.5% for sites that never bled. Patients with bleeding at 16% or more of their probing sites had a meaningfully higher risk of disease progression overall. So when your hygienist notes bleeding, it’s not a minor detail.
Clinical Attachment Level: The Deeper Calculation
Pocket depth alone can be misleading. If the gums have receded, the pocket might measure only 3 mm, but the actual loss of attachment to the tooth could be much greater. That’s where clinical attachment level (CAL) comes in.
CAL is calculated by adding two numbers together: the pocket depth plus the amount of visible gum recession. If your gum has receded 2 mm from its normal position and the pocket measures 3 mm deep, the clinical attachment level is 5 mm. That 5 mm figure more accurately captures how much support the tooth has lost. The current classification system from the American Academy of Periodontology uses CAL as the primary measure for staging periodontitis: 1 to 2 mm of attachment loss between teeth is Stage I, 3 to 4 mm is Stage II, and 5 mm or more is Stage III or IV.
What Can Throw Off a Reading
Probing looks simple, but several factors affect accuracy. The recommended force for inserting the probe is about 20 to 25 grams, roughly the weight of a AA battery resting on your fingertip. Too much pressure (above 45 grams) pushes the probe past the bottom of the pocket into healthy tissue, giving a falsely deep reading and causing unnecessary pain. Too little force stops the probe short and underestimates the true depth.
Physical obstacles also interfere. A chunk of calculus on the root surface can block the probe and produce a falsely shallow reading. Overhanging fillings or bulky crown margins can deflect the probe or prevent it from reaching the base of the pocket. Severe inflammation makes tissue soft and spongy, allowing the probe to penetrate further than it would in firm, healthy tissue. This is one reason why readings taken before and after initial treatment sometimes differ: once inflammation resolves and tissue firms up, pockets may measure shallower even before any surgical intervention.
How to Track Your Numbers Over Time
The real value of probing isn’t any single snapshot. It’s the trend. A site that goes from 3 mm to 5 mm over a year is more concerning than a site that has been stable at 4 mm for a decade. Ask for a copy of your perio chart at each visit so you can compare. Most charts display numbers in a grid, with each tooth showing six values. Some offices use color coding: green for healthy depths, yellow for moderate, and red for severe pockets.
When reviewing your chart, pay attention to which teeth have the deepest readings and whether bleeding was noted at those same sites. A deep pocket that also bleeds consistently is the highest-risk combination. A deep pocket with no bleeding and stable readings over multiple visits may simply be a residual defect that’s being managed well. Context, pattern, and change over time tell you far more than any single number on the probe.