# Diopter Calculator

URL: https://speedycalculator.com/calculators/physics/diopter/
Last updated: 2026-10-30

> The diopter calculator converts between a lens's focal length and its optical power in diopters.

## Inputs

- Focal length f (cm, mm, m, in, ft)

## Results

- Optical power P

## Formula

```
P (D) = 1 / f (m)
```

## Explanation

Optometrists and lens makers measure lens strength in diopters: the reciprocal of the focal length in metres. A +2 D reading lens has a focal length of 50 cm; a −4 D lens for short-sightedness diverges light as if from 25 cm.

### How to use the Diopter Calculator

- **Focal length f**
- Every result field is editable: type into **Optical power P** and **Focal length f** plus every other field update to match.

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the focal length, or type a power into the result to find the focal length. Negative values describe diverging lenses.

### Example values

How the main result changes when **Focal length f** changes and everything else stays at the example values above:

Focal length f (cm)Optical power P (D)25437.52.666750262.51.6751.33331001

## Worked example

Inputs — Focal length f: 50 cm.

Results — **Optical power P = 2 D**.

## Key terms

- **Diopter**: Unit of optical power, 1/m.

## FAQ

**What is a diopter?**
The reciprocal of focal length in metres (m⁻¹).

**What does a negative diopter mean?**
A diverging (minus) lens, used for myopia.

**How do powers combine?**
Thin lenses in contact add their powers.

**What is the power of the human eye?**
About 60 D.


## Sources

- [OpenStax – University Physics Volume 3](https://openstax.org/details/books/university-physics-volume-3)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)