Tooth 15: Your Upper Left Second Molar

Reviewed by the toothcheck Dental Team Independent dentist providing online second opinions.

If your dentist, your treatment plan or an X-ray report mentions "tooth 15" or "tooth #15", they are talking about one specific tooth: the upper left second molar. It is a large back tooth that does a lot of the heavy chewing, and because of where it sits it is one of the teeth most often filled, crowned or treated.

This guide explains where tooth 15 is, how the different numbering systems label it, what it does, and the problems that affect it most. If a plan has been recommended for this tooth, you will be better placed to understand it and ask good questions.

Quick Answer

Tooth 15 is the upper left second molar in the US Universal Numbering System. It sits at the back of the upper left jaw, just in front of the wisdom tooth (tooth 16). In the international FDI system it is tooth 27, and in Palmer notation it is the upper left 7. It is a molar, built for grinding food, and it usually has three roots.

Where Tooth 15 Is Located

Count from the middle of your upper teeth towards the left. The two front teeth are the central incisors, then the lateral incisor, canine, two premolars, and then three molars. The first molar is tooth 14, the second molar is tooth 15, and the third molar (wisdom tooth) is tooth 16.

So tooth 15 is the second tooth from the back on your upper left, or the very last tooth if the wisdom tooth was removed or never developed. For more on how the whole system works, see our guide to dental tooth numbering explained. The wisdom tooth directly behind it is covered in tooth 16: the upper left wisdom tooth.

How Tooth 15 Is Numbered in Each System

Dentists in different countries use different labels for the same tooth. This is why an X-ray report from one clinic can look unfamiliar at another.

If you are comparing paperwork from a clinic abroad with a US plan, check that the tooth is the same one. Tooth 27 on an FDI chart and tooth 15 on a US chart are the same tooth.

What a Molar Like Tooth 15 Does

Molars are the grinding teeth. Tooth 15 has a wide, flat top with several cusps and deep grooves that crush and break down food before you swallow it. Together with the first molar in front of it, it carries a large share of your chewing force on that side.

Upper molars normally have three roots, two on the cheek side and one on the tongue side, which spread out to anchor the tooth firmly in the bone. Those roots sit close to the floor of the maxillary sinus, the air space behind your cheek. That closeness is why upper molar problems can occasionally feel like sinus pressure, and why sinus problems can sometimes feel like a toothache.

Why Tooth 15 Is Prone to Problems

Tooth 15 sits far back in the mouth. It is harder to brush and floss thoroughly, and the deep grooves on its chewing surface trap food and plaque. Over time this makes it a common site for decay. The back teeth also take the most biting force, so once a tooth has a large filling, it is more likely to crack.

Common issues include:

• Cavities in the grooves or between teeth. Decay often starts in the pits of the chewing surface, or on the side touching tooth 14 or tooth 16 where floss does not always reach.
• Large or failing fillings. An old filling can leak at the edges, letting decay form underneath.
• Cracks and fractures. Heavy chewing, grinding at night, or a big filling that weakens the tooth can lead to a crack.
• Crowns. When much of the tooth is gone or cracked, a crown is often used to protect what is left. Our guide on crown vs filling explains how that decision is usually made.
• Root canal treatment. If decay or a crack reaches the nerve, the tooth may need a root canal. Because upper molars have several canals, these can be more complex than treatment on a front tooth.
• Gum disease. Bone loss around a molar can loosen it or expose the area where the roots divide.
• Crowding from the wisdom tooth. An impacted or tilted wisdom tooth behind it can trap food and cause decay on the back of tooth 15.

How Problems With Tooth 15 Are Found

Most issues are found at a routine exam, with an X-ray and a visual check. Bitewing X-rays show the crowns of the upper and lower molars side by side and are particularly good at showing decay between teeth. A periapical X-ray shows the full tooth including the roots and the surrounding bone, which is how infection at the root tip is found.

On an X-ray, decay appears as a darker area in the enamel or dentin. If you want to learn how to read that, see what a cavity looks like on an X-ray. The American Dental Association (https://www.ada.org/) notes that regular exams and X-rays are how many problems are caught before they cause pain.

If your X-rays are on file, you can also have them looked at independently. Our dental X-ray review page explains how that works.

Does a Cavity on Tooth 15 Always Need a Filling?

Not necessarily. This is an honest point worth knowing. A very early cavity that has only started to soften the outer enamel can often be watched over time, and in some cases the area can be strengthened with fluoride and better cleaning. A cavity that has clearly reached the dentin, or one that is growing between check-ups, generally needs a filling to stop it spreading.

The question to ask is how deep the cavity is, whether it has changed since earlier X-rays, and what happens if you wait. Also ask whether a filling or a crown is being proposed. A crown is a much bigger treatment than a filling, and it makes sense only when the tooth has lost enough structure that a filling would not hold.

When to See a Dentist Promptly

Do not wait for your next routine visit if you notice:

• Pain when biting down or when you release a bite
• Lingering sensitivity to hot or cold that lasts after the stimulus is gone
• Swelling in the gum near the back upper left, or a small bump on the gum
• A bad taste, or a broken edge or missing piece of the tooth
• Throbbing pain, especially at night, or facial swelling or fever

Facial swelling together with a fever needs urgent care the same day.

FAQ

What is tooth 15? Tooth 15 is the upper left second molar in the US Universal Numbering System, the large back tooth in front of the upper left wisdom tooth.

Is tooth 15 a molar? Yes. It is a molar with a broad chewing surface and usually three roots.

What is tooth 15 called in other numbering systems? It is tooth 27 in the FDI system and the upper left 7 in Palmer notation.

Does a cavity on tooth 15 always need a filling right away? No. Very early cavities can sometimes be monitored. Deeper ones usually need treatment.

Can a problem with tooth 15 cause sinus symptoms? Sometimes. The roots of upper molars sit near the sinus, so infection can create pressure that feels like a sinus problem.

Final Thoughts

Tooth 15 does a lot of work and sits where it is hard to keep clean, so it is no surprise that it often ends up with fillings, crowns or root canals. Knowing that it is the upper left second molar, tooth 27 in FDI, helps you follow what your dentist is proposing.

If you have been told that tooth 15 needs a crown, a root canal or a bigger filling and you are not sure it is necessary, an independent opinion can help. toothcheck is an online dental second opinion: a licensed dentist reviews your X-rays and treatment plan and tells you in plain English what is needed, what could wait, and what to ask. It costs 59 dollars and you get your answer in about 72 hours.

Last medically reviewed: November 2026