What is the Cobb angle?

The Cobb angle is the angle between perpendicular lines drawn to the end-plates of the most tilted upper and lower vertebrae in a scoliotic curve. It is the international standard for measuring and classifying scoliosis severity.

How do I measure end-plate tilt angles from an X-ray?

On an AP (front-to-back) X-ray, draw a line along the top end-plate of the uppermost tilted vertebra and another along the bottom end-plate of the lowermost tilted vertebra. Measure each line's angle relative to horizontal. These two values are the inputs for this calculator.

Why is the Cobb angle the sum of the two tilts?

Because both end-plates slope toward the apex of the curve, their perpendiculars diverge. The total angle between those perpendiculars equals the sum of the individual tilts from horizontal — identical to the classic geometric construction.

At what Cobb angle is surgery typically recommended?

Surgical intervention is generally discussed when the Cobb angle reaches 45–50° in skeletally mature patients, or 40–45° in adolescents still growing. The exact threshold depends on curve location, patient age, and progression rate — always consult a spine specialist.

Is this tool a substitute for a clinical evaluation?

No. This is an educational reference tool. Diagnosis, bracing decisions, and surgical planning require in-person clinical assessment, full imaging review, and professional judgment by a qualified clinician.

Cobb Angle Calculator

Name: Cobb Angle Calculator
Availability: InStock
Author: Nham Vu

Enter the tilt angles of the upper and lower vertebral end-plates to calculate the Cobb angle and see the scoliosis severity classification.

Vertebral End-Plate Angles

Enter each end-plate's tilt from horizontal in degrees.

Upper end-plate tilt (°)

Angle of the top vertebra's end-plate from horizontal

Lower end-plate tilt (°)

Angle of the bottom vertebra's end-plate from horizontal

Formula

Cobb angle = upper tilt + lower tilt

Both tilts are toward the curve apex, so they add.

Enter both end-plate angles and click Calculate.

Summary

Enter the tilt angles of the upper and lower vertebral end-plates to calculate the Cobb angle and see the scoliosis severity classification.

How it works

On an anterior-posterior spinal X-ray, identify the most tilted vertebra at the top of the curve (upper end vertebra) and the most tilted vertebra at the bottom (lower end vertebra).
Measure the angle each end-plate makes with the horizontal. Record both values in degrees.
Enter the upper end-plate angle and the lower end-plate angle into the calculator.
The Cobb angle equals the sum of the two tilt values (they lean toward the apex of the curve, so they add).
The result is classified from Normal variation through Very Severe to indicate observation, bracing, or surgical thresholds.

Use cases

Orthopedic and radiology trainees verifying manual Cobb-angle measurements during X-ray reading.
Physical therapists tracking treatment progress over serial X-rays.
Patients wanting to understand the clinical significance of their reported Cobb angle.
Medical educators demonstrating the relationship between end-plate tilt and curve magnitude.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu