How to Find the Tooth Number Using an FMX: A Complete Guide for Dental Billers

This guide will change that. By the time you finish reading, you'll be able to look at an FMX, identify which quadrant you're looking at, match each image to the Universal Numbering System, and fill out your ADA claim form with confidence.

Start Here: What Is a Tooth Number?

In the United States, dental professionals use the Universal Numbering System — established and endorsed by the American Dental Association (ADA) — to identify every tooth in the mouth. Every permanent tooth has a unique number from 1 to 32.

This system is:

• The standard used on all ADA claim forms (Box 27: Tooth Number)
• Required for CDT code billing for any tooth-specific procedure
• Built into every major Practice Management System (PMS): Dentrix, Eaglesoft, Curve Dental, Open Dental
• Used by virtually every insurance carrier in the United States

If a claim is submitted with an incorrect tooth number, the procedure may be rejected, denied for frequency limitations that don't actually apply to the correct tooth, or flagged for audit.

The Universal Numbering System: Every Tooth's Address

Think of the mouth as four neighborhoods — four quadrants — and each tooth as a house with an address.

Here is how the Universal Numbering System works:

• Numbering starts at the upper right wisdom tooth (#1) and travels across the top of the mouth
• Reaches the upper left wisdom tooth (#16)
• Drops down to the lower left wisdom tooth (#17)
• Travels back across the bottom of the mouth
• Ends at the lower right wisdom tooth (#32)

Complete Permanent Tooth Reference Chart

The 4 teeth most commonly billed: #3 and #14 (upper first molars), #19 and #30 (lower first molars) — these are the most common crown, root canal, and extraction sites in dental billing.

What Is an FMX and What Does It Show?

An FMX (Full Mouth Series) is a complete set of intraoral dental radiographs that captures every tooth and the surrounding bone in a single diagnostic session. The CDT code for billing a full mouth series is D0210.

According to the FDA, an FMX is defined as "a set of intraoral radiographs usually consisting of 14 to 22 periapical and posterior bitewing images intended to display the crowns and roots of all teeth, periapical areas, and alveolar bone crest."

In 2023, the ADA updated the D0210 descriptor from "complete series" to "comprehensive series", removing the rigid image count requirement. The updated standard is that the series must display the crowns, roots, periapical areas, interproximal areas, and alveolar bone of all teeth — however many images that requires for that specific patient.

The Two Types of Images in an FMX

A standard FMX contains:

• 14 periapical images (showing all teeth in sections)
• 4 bitewing images (showing posterior contact areas and bone levels)
• Total: 18 images (though this varies by patient anatomy)

How to Read an FMX and Identify Tooth Numbers

This is where most non-clinical billing staff get stuck. Here is a step-by-step system for reading an FMX and matching each image to the correct tooth number.

Step 1: Orient Yourself — Maxillary (Upper) vs. Mandibular (Lower)

The single most important first step is identifying whether you are looking at the top or the bottom of the mouth.

How to tell:

• Maxillary (upper) teeth: Roots point upward. You will often see the ghostly shadow of the maxillary sinus (a large, radiolucent/dark empty space) above the roots of the upper premolars and molars.
• Mandibular (lower) teeth: Roots point downward. The bone below the roots is denser (more radiopaque/white). You may see the mental foramen (a small oval dark spot) near teeth #28 and #29.

Simple memory trick: "Roots go home." Upper teeth roots go up toward the skull. Lower teeth roots go down toward the chin. Once you see this, you cannot unsee it.

Step 2: Identify Right vs. Left

Dental radiographs are always mounted and viewed as if you are looking at the patient face-to-face. This means:

• Patient's RIGHT appears on YOUR LEFT side of the mount
• Patient's LEFT appears on YOUR RIGHT side of the mount

This is opposite to how you might intuitively read it, but it mirrors how you would look at a patient sitting across from you.

Practical test: If a patient is missing their upper left first molar (tooth #14), that missing space should appear in the upper right area of the FMX mount as you view it.

Step 3: Identify Tooth Type by Shape and Root Pattern

Once you know upper or lower, and right or left, identify the tooth type:

Step 4: Count From the Midline

When you've confirmed upper or lower and right or left, count teeth from the midline (the center of the mouth where the two central incisors meet):

• Upper right central incisor = #8
• Upper left central incisor = #9
• Lower right central incisor = #25
• Lower left central incisor = #24

From the central incisor, count outward. In the upper right: 8 (central), 7 (lateral), 6 (canine), 5 (premolar), 4 (premolar), 3 (first molar), 2 (second molar), 1 (third molar/wisdom).

How an FMX Is Mounted: Understanding the Standard Layout

When you look at an FMX mounted in a standard film mount or on your PMS screen, it is arranged in a specific, standardized layout that maps directly to the Universal Numbering System:

[Upper Right Posteriors] [Upper Anteriors] [Upper Left Posteriors] Teeth 1–5 Teeth 6–11 Teeth 12–16 [Right Bitewing] [Left Bitewing] [Lower Right Posteriors] [Lower Anteriors] [Lower Left Posteriors] Teeth 28–32 Teeth 22–27 Teeth 17–21

The midline of the mount = the midline of the patient's mouth. Upper images = top half of mount. Lower images = bottom half. Patient's right = your left when facing the mount.

Why Correct Tooth Numbers Matter in Dental Billing

Frequency Limitations Are Tooth-Specific

Insurance carriers apply frequency limits on a per-tooth basis for certain procedures. If you bill a crown on tooth #30 but the procedure was actually performed on tooth #31, the claim may:

• Be paid correctly (if #30 has no claim history) — creating a false record
• Be denied (if #30 had a crown billed previously for a different reason)
• Trigger a request for records during an audit

The record in the insurance database is wrong either way, which can cause problems for that patient's future claims at your office or another.

The ADA Claim Form Requires It

Box 27 of the 2024 ADA Dental Claim Form requires the tooth number for any procedure affecting a specific tooth. Box 26 requires the tooth surface (mesial, distal, buccal, lingual, occlusal). Getting these right requires correctly identifying the tooth from the clinical notes and, when in doubt, confirming against the FMX.

Missing Tooth Documentation

When billing for an FMX (D0210), the claim form and clinical notes should document which teeth are present and which are missing. This creates a radiographic record of the dentition at that date of service — which is directly relevant to missing tooth clause determinations. An FMX taken at a new patient appointment establishes the "baseline" record of which teeth were present when coverage began.

D0210 Billing: What to Include on the Claim

When billing a full mouth series (D0210), here is what the ADA claim form requires:

For individual periapicals (D0220 and D0230):

• D0220 can only be billed once per claim
• D0230 is used for each additional periapical image beyond the first
• Enter the tooth number(s) covered by each image in Box 27
• Enter the quantity of D0230 images in Box 29b

Important rule from eAssist: Always double-check the number of images on the claim against what was actually taken. It is easy to lose count when billing multiple PAs. Verify before submitting.

FMX Frequency Limits: What Every Dental Biller Needs to Know

Most insurance plans limit D0210 to once every 3–5 years. The exact limitation varies by carrier and plan:

• Some plans allow an FMX every 36 months from date of service
• Others reset by calendar year
• Some plans have separate frequency rules for D0220/D0230 that don't align with the D0210 limit

Before billing an FMX, verify:

• When the patient's last FMX was billed (check prior authorization or benefits history)
• The specific plan's frequency limitation (not just the "standard" 3–5 years)
• Whether a panoramic image (D0330) taken previously might be counted against the FMX frequency — some carriers bundle these

If the patient received a panoramic X-ray recently and you now take an FMX, some payers may consider these cumulatively as an FMX equivalent, triggering the frequency limit. Always verify carrier-specific rules.

Primary (Baby) Teeth: A Different System

Primary teeth use letters instead of numbers. This is relevant for pediatric billing:

When billing for a pediatric patient, always confirm whether you are billing for a primary or permanent tooth, as the codes and claim form entries differ.

Quick Reference: Most Commonly Billed Tooth Numbers in Dental Practice

Keep this near the billing workstation:

Common Mistakes in Tooth Number Billing — and How to Avoid Them

Mistake 1: Transposing Left and Right

The most common error. Remember: patient's right is YOUR left on the FMX mount.

Fix: Before entering any tooth number, verbally confirm: "Upper left molar — that would be #14." Cross-reference with the clinical chart. If the doctor charted the procedure on #19 and the clinical notes say lower left, confirm the lower left first molar is indeed #19 before submitting.

Mistake 2: Confusing #14 and #15 (or #3 and #2)

Posterior teeth in the same quadrant are easy to mix up when counting.

Fix: Anchor your count from a known landmark — the midline central incisors. Count outward rather than guessing by position in the image.

Mistake 3: Billing D0220 More Than Once Per Claim

D0220 is only billable once per claim (for the first periapical). Every additional PA is D0230.

Fix: Set this as a rule in your billing software. Any claim with two D0220 codes on the same date of service will be rejected.

Mistake 4: Using a Tooth Number on D0210 (Full Mouth Series)

D0210 is a series covering all teeth. Entering a specific tooth number in Box 27 for D0210 is incorrect.

Fix: For D0210, leave Box 27 blank or use the carrier's preferred notation for "all teeth."

Outsourcing Your Dental Billing Means Tooth Number Accuracy Every Time

Tooth number errors are one of the most common contributors to claim rejections that don't get caught until the AR aging report shows a claim has been sitting untouched for 45 days. When your front desk team is managing phones, scheduling, and patient check-in simultaneously, it's easy for a transposed tooth number to slip through.

At Verimedix, our dental billing specialists review every claim before submission — verifying tooth numbers against clinical notes, checking FMX documentation against claim form entries, and catching errors before they become denials.

Frequently asked questions