# How to calculate horizontal decentration

For a symmetric frame model, geometric centre distance is A + bridge. Nasal decentration per eye is (A + bridge) / 2 minus monocular PD.

## Example in millimetres

A = 52 and bridge = 18 give 70 mm between centres. Right PD = 31 and left PD = 33 give 4 mm and 2 mm nasal shifts.

## Convention

Positive means nasal displacement from the geometric centre of each eyewire; negative means temporal displacement. Zero means horizontal coincidence.

## Model limits

This assumes a symmetric frame and a central nasal reference. It does not calculate fitting height, thickness, minimum diameter, prescribed prism or design compensation. Verify measurements and conventions with the laboratory.

## References

- [Laramy-K / OpticianWorks — ABO Study Guide](https://opticianworks.com/wp-content/uploads/2024/01/Laramy-K-OpticianWorks-Ultimate-ABO-Study-Guide-v6.23.pdf)

## Related topics

- [Frame geometry: A, B and bridge](https://lensas.com.br/en/knowledge/frame-geometry/)
- [Pupillary distance: which measurements should you record?](https://lensas.com.br/en/knowledge/pupillary-distance/)
- [Prentice’s rule: approximate prism magnitude](https://lensas.com.br/en/knowledge/prentice-rule/)

Educational content · Fictional examples · Automated technical review, no claimed clinical validation.

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Version: 2026-09-15.1