How to Calculate Plaque Index: Step-by-Step

Updated 10 November 2026 · Standard formula, shown below

This guide explains how to calculate the plaque index, a standardized method for scoring the amount of dental plaque along the gumline. You will learn the two most common indices (Silness-Löe and O’Leary), how to score each tooth surface, and how to interpret the final number. By the end, you’ll be able to perform a plaque index assessment accurately for clinical or research purposes.

What You’ll Need

Dental explorer or probe (for detecting plaque)
Disclosing agent (tablets or liquid) to stain plaque, if desired
Dental mirror and good lighting
Periodontal probe (for measuring, if using the Silness-Löe method)
Recording sheet or chart with tooth numbers and surfaces
Gloves and appropriate PPE

Step-by-Step

Choose your index. Decide between the Silness-Löe Plaque Index (scores 0–3 per surface, no disclosing agent needed) or the O’Leary Plaque Control Record (dichotomous: present/absent, typically with disclosing agent). This guide covers both.
Prepare the patient. Have the patient rinse with water to remove loose debris. If using disclosing agent, apply it according to the product instructions and have the patient rinse again.
Identify the teeth to score. For the Silness-Löe index, you typically score all teeth, or a selected subset (e.g., Ramfjord teeth: 16, 21, 24, 36, 41, 44). For O’Leary, score all teeth present.
For each tooth, examine four surfaces: mesial (toward midline), distal (away from midline), buccal/labial (cheek/lip side), and lingual/palatal (tongue side).
Score each surface (Silness-Löe):
- 0 = No plaque.
- 1 = A film of plaque adhering to the free gingival margin and adjacent area, not visible to the naked eye. Detect only by running a probe across the surface.
- 2 = Moderate accumulation of soft deposits within the gingival pocket, or on the tooth and gingival margin, visible to the naked eye.
- 3 = Abundant soft matter within the gingival pocket and/or on the tooth and gingival margin.
Score each surface (O’Leary): After disclosing, record whether plaque is present (1) or absent (0) on each of the four surfaces. No grading of thickness.
Calculate the Silness-Löe index: Add the scores for all surfaces of all teeth, then divide by the total number of surfaces examined (4 × number of teeth). For example, if you examined 6 teeth (24 surfaces) and the sum of scores is 18, the index is 18 ÷ 24 = 0.75.
Calculate the O’Leary index: Count the number of surfaces with plaque, divide by the total number of surfaces examined, and multiply by 100 to get a percentage. For example, 15 plaque-positive surfaces out of 60 total surfaces = (15 ÷ 60) × 100 = 25%.
Interpret the result: For Silness-Löe, a score of 0 is excellent, 0.1–0.9 is good, 1.0–1.9 is fair, and 2.0–3.0 is poor. For O’Leary, a score below 10% is considered good oral hygiene; above 20% indicates inadequate plaque control.
Record the data. Note the index value and the date. For research, also record the examiner and any calibration details.

Common Mistakes

Scoring the wrong surfaces: Forgetting to score all four surfaces (mesial, distal, buccal, lingual) leads to inaccurate totals.
Using the wrong denominator: In Silness-Löe, divide by the number of surfaces, not the number of teeth. In O’Leary, divide by surfaces, not teeth, and multiply by 100.
Not using a probe for grade 1: Grade 1 plaque is invisible without a probe; relying on visual inspection alone will under-score.
Inconsistent pressure when probing: Press too hard or too softly, and you may miss or over-detect plaque.
Applying disclosing agent unevenly: This can cause false positives or negatives in the O’Leary index.
Mixing up indices: Using the 0–3 scale for O’Leary or the percentage for Silness-Löe will give meaningless results.

Tips

Calibrate with another examiner before starting a study to ensure consistent scoring.
Use a consistent order (e.g., always start at the upper right, then move around the arch) to avoid missing teeth.
For the Silness-Löe index, run the probe gently along the gingival margin to detect thin plaque.
If using disclosing agent, take photos before and after for documentation.
Practice on a few patients first; the difference between grade 1 and 2 is the most challenging to master.

Troubleshooting

If the patient has heavy calculus: Calculus is not plaque; it should not be scored. Remove calculus before the assessment, or note it separately.
If a tooth is missing: Simply exclude that tooth from both the numerator and denominator. Do not substitute another tooth unless you are using a fixed subset like Ramfjord teeth.
If the patient has crowns or restorations: Score the surface as you would a natural tooth; plaque can accumulate on restorations.
If you get a very high score (e.g., 2.5+): Re-examine a few surfaces to ensure you are not over-scoring. It may be helpful to have a second examiner verify.
If you get a score of 0 but see visible debris: You may be missing the buccal/lingual surfaces; check that you are actually scoring all four surfaces.

Frequently Asked Questions

What is the difference between plaque index and gingival index?

The plaque index measures the amount of plaque on the tooth surface, while the gingival index (e.g., Löe-Silness) measures inflammation of the gums. They are often used together but assess different things.

Do I need to use a disclosing agent for the Silness-Löe index?

No, the Silness-Löe index is designed to be used without disclosing agent. You detect plaque by running a probe along the gingival margin. Disclosing agent is optional but can help visualize thin plaque.

Can I use the plaque index on children?

Yes, but you may need to adapt the teeth selected. For primary teeth, the same scoring criteria apply. The O’Leary index is commonly used in pediatric dentistry.

What is a good plaque index score?

For Silness-Löe, a score below 1.0 is generally considered good. For O’Leary, a score below 10% is excellent, and below 20% is acceptable for most patients.

How long does a plaque index assessment take?

For a full mouth with 28 teeth, it takes about 5–10 minutes, depending on the examiner’s experience and the patient’s cooperation.

Can I calculate the plaque index from photos?

Yes, if you have standardized intraoral photos showing all surfaces after disclosing, you can score them later. However, you may miss thin plaque that is not visible in photos.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.