The human body has 33 vertebrae in total: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal. That said, the number you’ll sometimes see is 26, because by adulthood the five sacral vertebrae fuse into one bone (the sacrum) and the four coccygeal vertebrae fuse into one bone (the tailbone). Both counts are correct depending on whether you’re counting individual vertebrae or functional bones.
The Five Regions of the Spine
Your spine is divided into five distinct sections, each with a different shape, range of motion, and job.
- Cervical (7 vertebrae): These sit in your neck and support your skull. They’re the smallest and most mobile vertebrae in the spine.
- Thoracic (12 vertebrae): These make up your mid and upper back. Each one connects to a pair of ribs, forming the protective cage around your heart and lungs.
- Lumbar (5 vertebrae): Your lower back. These are the largest, thickest vertebrae because they bear the weight of everything above them.
- Sacral (5 vertebrae, fused): These fuse together into a single triangular bone called the sacrum, which connects your spine to your pelvis.
- Coccygeal (4 vertebrae, fused): These form your coccyx, or tailbone. The number of coccygeal bones can range from 3 to 5 depending on the person.
Only the top 24 vertebrae (cervical, thoracic, and lumbar) remain separate and moveable throughout your life. Between each of these moveable vertebrae sits a cushioning intervertebral disc that absorbs shock and prevents the bones from grinding against each other.
Two Vertebrae That Don’t Follow the Rules
The top two cervical vertebrae are shaped completely differently from every other bone in the spine. The first, called the atlas (C1), is a simple ring of bone with no vertebral body at all. Instead, two thick sections on either side support the weight of your skull. This joint lets you nod your head up and down.
The second vertebra, called the axis (C2), has a bony peg that projects upward through the ring of the atlas. This peg acts as a pivot point and lets you turn your head from side to side, like shaking your head “no.” Together, these two bones give your head most of its range of motion.
When the Sacrum and Tailbone Fuse
Babies are born with all 33 vertebrae as separate bones. Fusion happens gradually over the first few decades of life. The lower sacral segments tend to fuse first, often before age 15, while the upper sacral joints (between S1-S2 and S2-S3) typically fuse between ages 18 and 25. The junction between the lowest sacral vertebra and the first coccygeal vertebra is especially variable, sometimes not fusing until the late twenties. The coccygeal segments themselves can fuse anywhere between ages 6 and 30.
This is why medical imaging of a teenager’s lower spine can look different from an adult’s. Not all the fusion is complete yet, which radiologists account for when reading scans.
Why the Lower Spine Bears the Most Stress
Each vertebra in the lumbar spine carries the cumulative weight of everything above it. Biomechanical models estimate that roughly 311 newtons of force (about 70 pounds) from the upper body presses down on the top of the lumbar spine at L1. Each level below adds a bit more from its own segment. This is why the lumbar vertebrae are progressively larger as you move downward, and why the lowest lumbar levels are the most injury-prone parts of the entire spine.
The two most common sites for a herniated disc are between L4-L5 and between L5-S1, right at the bottom of the moveable spine where compressive and shearing forces are greatest.
Not Everyone Has Exactly 33
The textbook count of 33 vertebrae is a standard, not a guarantee. A surprisingly common variation called a lumbosacral transitional vertebra (LSTV) affects roughly 1 in 5 people. In the most typical form, the lowest lumbar vertebra (L5) partially or fully fuses to the sacrum, a process called sacralization. Less commonly, the top sacral segment separates and behaves more like a lumbar vertebra, called lumbarization. Most people with these variations never know about them unless they happen to get a lower back X-ray or MRI for another reason.
These variations rarely cause problems on their own, but they can matter during spinal surgery. Surgeons need to count vertebrae carefully to make sure they’re operating on the correct level, and an extra or missing lumbar segment can complicate that process.