How this tool works
The Plaque Index (Silness and Löe, 1964) grades the thickness of plaque at the gum margin, not how much of the crown it covers. It uses the same teeth and areas as the Gingival Index: four areas per tooth (buccal, lingual, mesial and distal), each scored from 0 to 3.
- Tooth PlI = sum of its four scores ÷ 4.
- Patient PlI = sum of the tooth scores ÷ number of teeth scored.
The paper divides by six and notes that missing teeth were not substituted. So when a tooth is missing, tick Missing and the calculator averages the teeth you scored. The result is the mean of all areas examined, which is how the paper describes the patient index. You also get results for incisors, premolars and molars, which the paper allows.
Worked example (the pre-filled one): tooth sums of 6, 3, 3, 7, 5 and 3 give 27 over 24 areas. PlI = 27 ÷ 24 = 1.13, which rounds to 1.1 and falls in the fair band.
Scoring criteria (paraphrased)
| Score | What you see |
|---|---|
| 0 | No plaque |
| 1 | A thin film at the gum margin, seen only with disclosing solution or by running a probe over the surface |
| 2 | Moderate soft deposits in the pocket or on the tooth and gum margin, visible to the naked eye |
| 3 | Heavy soft deposits in the pocket and/or on the tooth and gum margin |
In the 1964 study the teeth were dried with air first. The authors found that running an explorer along the surfaces, above and below the gum line, worked better than disclosing solution, and used that method.
The six index teeth (Ramfjord teeth)
The paper examined the same six teeth used for the Gingival Index:
| FDI | Universal | Tooth |
|---|---|---|
| 16 | #3 | Upper right first molar |
| 12 | #7 | Upper right lateral incisor |
| 24 | #12 | Upper left first premolar |
| 36 | #19 | Lower left first molar |
| 32 | #23 | Lower left lateral incisor |
| 44 | #28 | Lower right first premolar |
Choose Whole mouth to score every tooth instead. Third molars start as not scored; untick them to include them.
Interpreting the score
The 1964 paper gives no cut-offs. This interpretation is common in teaching material and is a convention:
| Patient PlI | Interpretation (convention) |
|---|---|
| 0.0 | Excellent |
| 0.1–0.9 | Good |
| 1.0–1.9 | Fair |
| 2.0–3.0 | Poor |
The printed bands leave gaps (a score of 0.95 fits none of them). The calculator rounds to one decimal first, so 0.95 becomes 1.0, which is fair.
Plaque Index or OHI-S?
- The Plaque Index looks at plaque thickness where it matters for the gums. Use it alongside the Gingival Index in gingivitis studies.
- The OHI-S records how much of six surfaces is covered by soft debris, and scores calculus separately.
- The two indices measure different things, so their scores don't convert into each other.
Limits
Plaque builds up and is brushed away every day, so a single score reflects that visit only. Scores also depend on the examiner and on whether you use a probe or disclosing solution, so keep the method the same between visits and calibrate examiners in studies.
Frequently asked questions
How is the Plaque Index calculated?
What is the difference between the Plaque Index and the OHI-S?
Do I need disclosing solution?
What is a good Plaque Index score?
Sources
- Silness J, Löe H. Acta Odontologica Scandinavica 22:121-135. Periodontal disease in pregnancy. II. Correlation between oral hygiene and periodontal condition. 1964. Accessed October 26, 2026.
- Löe H, Silness J. Acta Odontologica Scandinavica 21:533-551. Periodontal disease in pregnancy. I. Prevalence and severity. 1963. The six index teeth were first set out here, for the Gingival Index. Accessed October 26, 2026.
- Löe H. Journal of Periodontology 38(6 Suppl):610-616. The Gingival Index, the Plaque Index and the Retention Index Systems. 1967. Accessed October 26, 2026.
- Mandal SK (SlideShare teaching slides, secondary source). Periodontal indices final. Convention only. Accessed October 26, 2026.
Last reviewed by the Dental Health Time editorial team, against the sources above. How we review.