Cephalometric Normal Values: SNA 82°, ANB 2°, FMA 25°

A cephalometric value means little until you know which norm it is compared with: which analysis, which age, which population. This page gathers in one table the norms of our site and free tools (Steiner and Tweed, from D.U. course handouts), the values our course base gives for Downs and Ricketts, and the variants between sources. Two measurements get their own section: the increased interincisal angle and Merrifield’s Z angle. Where we found no verified source, we say so instead of giving a number.

Reference table of cephalometric norms

Values in the Steiner and Tweed rows are those of our free cephalometric analysis tool. "Boileau", "Bassigny" and "EMC" refer to the textbooks cited in our D.U. knowledge base. McNamara and Björk-Jarabak norms are not in this table: we found none in a source we could verify (see below).

How to read a norm

Four rules prevent most misreadings.

• A norm belongs to one analysis. SN-GoGn (Steiner, Go-Gn) and FMA (Tweed, tangent to the lower border) both describe the mandibular plane, but their numbers are not interchangeable. The same goes for the occlusal plane, measured to S-N by Steiner (14°) and to Frankfort by Tweed (10°). Our mandibular plane angle guide shows the three constructions.
• The standard deviation is a range, not a verdict. A value one degree outside it is a signal to explain, not an indication for treatment.
• Millimeters need calibration. Angles do not depend on scale, but U1-NA and L1-NB in mm, the Wits and the facial heights do. On an uncalibrated tracing, only angles and ratios (such as the facial height index) can be compared with a norm.
• Age matters for some analyses. Ricketts’ norms in the EMC are given at 9 years with an expected change per year. Applying a 9-year value to an adult, or the reverse, creates false abnormalities.

Sagittal norms and their disagreements

ANB at 2° ± 2° is the most quoted number in cephalometrics, and the one that misleads most often: it changes with the position of Nasion, the inclination of S-N, jaw rotation and facial height. The usual cross-check is the Wits appraisal, for which two traditions coexist: 0 ± 2 mm in Steiner materials and 2 ± 2 mm on Tweed sheets. Neither is wrong; always write which one you use. A teaching example from our courses: ANB 4° with a Wits of −1 mm remains a borderline Class I, not a Class II.

The beta angle avoids both the cranial base and the occlusal plane: in its original description, a value between 27° and 35° was considered a Class I skeletal pattern, more acute pointing to Class II and more obtuse to Class III. Our ANB angle guide details the reading by SNA and SNB.

Vertical norms: read them in groups

No vertical value is enough alone. FMA (25° ± 3°) is cross-checked with SN-GoGn (32° ± 5°) and with the facial height index (0.69). The courses give cases where they disagree, for example an SN-GoGn of 41° (hyperdivergent) with an index of 0.80 (hypodivergent tendency): the decision then returns to the face and to Björk’s structural signs.

Two warnings from our base:

• Schudy’s averages, as reported by Boileau, give a posterior/anterior height ratio of about 69%, 63% and 57% for hypo-, meso- and hyperdivergent faces, with the posterior height measured differently from Tweed’s index. Do not compare them with the 0.65-0.75 range of the facial height index.
• A steep cranial base raises SN-GoGn without excess facial height, and Frankfort depends on two fragile landmarks: measure both angles.

Increased interincisal angle: what it means

The interincisal angle is the angle between the long axes of the upper and lower central incisors. Its norm depends on the analysis: 131° ± 5° for Steiner, the value our tool uses, 135° ± 5° attributed to Tweed and 130° ± 5° to Ricketts by Boileau. Use the target of the analysis you work with.

An increased angle (above about 136° with Steiner’s norm) means that one or both incisors are retroclined. When it is very open and the incisor distances are also reduced, the courses speak of bimaxillary dentoalveolar retrusion. The angle alone does not say which incisor is responsible: read U1-NA, L1-NB, IMPA and the incisors’ position on their bases.

Situations where it is typically increased:

• Class II division 2. In our course base the interincisal angle is almost always above 140°. In 144 Class II patients aged 9 to 17, the increased interincisal angle of division 2 subjects was due to reclination of both upper and lower incisors (Kirschneck et al., 2013, Head Face Med). Our Class II division 2 guide covers the diagnosis.
• Anterior mandibular rotation (hypodivergent pattern). Among Björk’s structural signs, the courses and the EMC describe an open interincisal angle, with an intermolar angle above 180°.
• Missing upper lateral incisors. In 79 patients, agenesis of the maxillary lateral incisors was associated with an increased interincisal angle and reduced total divergence (Guarnieri et al., 2022, Minerva Dent Oral Sci).
• After excessive incisor retraction. The base warns that an overcorrected incisor target flattens the profile, and uprighting both incisors opens the angle.

A decreased angle (below about 126°) points to proclination of one or both incisors and, with increased distances, to bimaxillary protrusion. In division 2, which always comes with a deep bite, closing the interincisal angle is one of the classic treatment objectives (Bassigny).

Merrifield’s Z angle

Merrifield’s Z line runs from soft-tissue pogonion through the most prominent lip, whichever it is; the Z angle is the angle between that line and the Frankfort plane. Merrifield described the profile line in 1966 ("The profile line as an aid in critically evaluating facial esthetics", Am J Orthod, PubMed); PubMed has no abstract for it, so the values below come from our course base.

• Norm: 75° ± 5° (70° to 80°) on the D.U. Tweed sheet; 78° for Boileau, inside the D.U. range but higher.
• Low Z angle: protrusive or convex profile (lips ahead, chin behind, or both).
• High Z angle: flat or retrusive profile; a course case shows 84° with a flat soft-tissue profile.
• When the line is oblique, find out whether an advanced upper lip or a retruded chin is responsible, and compare upper lip thickness (UL) with total chin thickness (TC), which is expected to be equal or larger.
• It changes with growth: in a teaching case, one patient’s Z angle went from 64° at 12 years to 70° at 12.9 years, 70° at 14.3 years and 69° at 21 years.

The Z angle also enters the cranial-facial part of Merrifield’s difficulty index. One recent study on 135 white adolescents measured FMA, IMPA, FMIA and the Z angle before and after treatment: final FMIA converged at about 65° in skeletal Class I and 60° in Class II, and the authors suggested that the horizontal skeletal relationship suits the choice of the FMIA target better than the vertical one (Kang et al., 2023, AJODO). This differs from Tweed’s rule taught in the D.U., which sets the target from FMA; we report the divergence without settling it. Our Ricketts E-line guide compares the Z angle with the other esthetic lines.

Ricketts: norms corrected for age

The EMC resident’s manual, cited in our base, gives Ricketts’ simplified analysis at 9 years with the expected change for each value (see the table). Two more are worth knowing: the mandibular arc, 26° ± 3.5° (+0.5° per year), and the upper first molar to the pterygoid vertical (6-PTV), equal to age + 3 ± 3 mm (+1 mm per year). Boileau adds cranial values at 9 years: cranial deflection 27° ± 3°, porion location 39 ± 3 mm and cranial length (CC-Na) 55 ± 3 mm.

The facial angle illustrates the problem: 87° at 9 years in the EMC, 90° without age in Boileau, 88° in Bassigny’s older values. Part of the gap is the age correction. If you use Ricketts, compute the norm at the patient’s age.

McNamara and Björk-Jarabak: why no numbers here

McNamara’s 1984 paper states that it documents standards for each measure (McNamara, 1984, Am J Orthod), but the abstract does not give them, and our course base does not reproduce them. The same applies to the Björk polygon sum and the Jarabak ratio: current studies use them to classify vertical patterns, but none of the abstracts we checked states the reference values. Rather than copy numbers from unverified tables, we leave them out. Use the handout of your program for these analyses and write its name next to the value.

Norms and populations

Most classic norms come from North American samples. Our base quotes Boileau, who considers population-specific norms indispensable and lists the Maghreb population among those whose incisors are normally more proclined. Two studies point the same way:

• In 102 Moroccan adults with normal occlusion in Rabat, eight angular and six linear measurements differed significantly from Caucasian norms, with bimaxillary protrusion; the authors concluded that those norms cannot be applied to Moroccan faces (Lahlou et al., 2010, Eur J Orthod).
• In 34 African American adults with normal occlusion in the Philadelphia region, SN-GoGn, FMA and the Y-axis were smaller and the interincisal angle reduced compared with previously published African American norms; the authors conclude that norms vary by ethnicity and by region (Kim et al., 2025, J Orofac Orthop).

A value outside a North American norm is a starting point for discussion, not a diagnosis; the courses recall that strict use of norms from small samples once led to too many premolar extractions.

In practice: a checklist

1. Write the analysis and the source next to every norm you report.
2. Calibrate the radiograph before comparing any millimeter value.
3. Read sagittal values in pairs (ANB and Wits) and vertical values in groups (FMA, SN-GoGn, facial height index).
4. For the interincisal angle, identify which incisor is responsible before concluding.
5. For the Z angle, check which lip defines the line and whether the chin explains the value.
6. For Ricketts, compute the norm at the patient’s age.
7. Keep population differences in mind, especially for incisors and lips.

Frequently asked questions

What are the normal cephalometric values?

For the Steiner analysis: SNA 82° ± 2°, SNB 80° ± 2°, ANB 2° ± 2°, SN-GoGn 32° ± 5°, U1-NA 22° ± 2° and 4 mm ± 1, L1-NB 25° ± 2° and 4 mm ± 1, interincisal angle 131° ± 5°. For Tweed: FMA 25° ± 3°, IMPA 88° ± 3°, FMIA 67° ± 3°, with Merrifield’s Z angle at 75° ± 5°. Other analyses have their own norms, and standard deviations vary between sources.

What does an increased interincisal angle mean?

That one or both incisors are retroclined. With Steiner’s norm (131° ± 5°), a value above about 136° is increased; it is typical of Class II division 2, where it is almost always above 140°, and of hypodivergent faces with anterior mandibular rotation. Measure each incisor separately to know which one is responsible.

What is the Z angle in orthodontics?

The angle between Merrifield’s profile line (soft-tissue pogonion to the most prominent lip) and the Frankfort plane. The D.U. Tweed sheet gives 75° ± 5°; Boileau gives 78°. A low value means a protrusive or convex profile, a high value a flat one.

Why do cephalometric norms differ between textbooks?

Because each analysis uses its own planes, each norm comes from a specific sample (age, sex, population, era), and some authors give a descriptive norm while others give a treatment target. The interincisal angle (125° to 135° depending on the source) and the Wits (0 or 2 mm) are classic examples.

Are there cephalometric norms for Moroccan patients?

A Rabat study of 102 adults with normal occlusion found bimaxillary protrusion compared with Caucasian norms, but its abstract does not list the values. Read incisor and lip values with that difference in mind.

With GS Ortho

In GS Ortho, AI places 29 landmarks on the lateral cephalogram and the Steiner and Tweed analyses are calculated straight away. You check each landmark, correct it if needed, then validate: the decision stays yours. The measurements feed the 12-panel guided diagnostic studio, and from any panel, "Request an opinion" sends your question, with the panel attached, to a colleague or supervisor you choose; the answer stays in the record. Without an account, our free cephalometric analysis tool calculates 12 Steiner and Tweed measurements and our Steiner’s chevrons calculator turns ANB, U1-NA, L1-NB and Pog-NB into incisor targets. The GS Ortho interface is currently available in French.

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