# Prentice’s rule: approximate prism magnitude

Approximate prism magnitude is the absolute meridional power multiplied by displacement in centimetres: |F × c|.

## Example

A meridional power of −5.00 D and 4 mm displacement give |−5 × 0.4| = 2 prism dioptres (Δ). Divide millimetres by ten before multiplying.

## Which power to use

For a lens with cylinder, use power in the relevant meridian; do not automatically substitute sphere. The tool receives an already determined meridional power.

## Limits

This is an educational approximation without base direction, vector combination or binocular balance. It does not calculate prescribed prism or approve fitting. Off-axis behaviour of a real lens depends on its geometry.

## References

- [National Academy of Opticianry — Prentice’s Rule](https://www.nao.org/wp-content/uploads/2020/12/Prentices-Rule.pdf)
- [Technical notes on prism-dioptre and Prentice’s rule (2021)](https://pubmed.ncbi.nlm.nih.gov/34862626/)

## Related topics

- [How to calculate horizontal decentration](https://lensas.com.br/en/knowledge/horizontal-decentration/)
- [Transposition: another notation for the same prescription](https://lensas.com.br/en/knowledge/prescription-transposition/)

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

Canonical: https://lensas.com.br/en/knowledge/prentice-rule/
Version: 2026-09-15.1