---
title: "Lensmaker's Equation calculator — AperiOptic"
description: "Focal length of a thin lens from radii and refractive index. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/lensmakers-equation"
---

# Lensmaker's Equation calculator

Focal length of a thin lens from radii and refractive index.

Category: [Geometric Optics](https://aperioptic.com/calculators)

Formula: `1/f = (n-1)·(1/R₁ - 1/R₂)`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Index n | `n` |  | 1.5 |
| R₁ | `R1` | mm | 100 |
| R₂ | `R2` | mm | -100 |

## Result with the default inputs

- Focal length: 100 mm
- Power: 10 D

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/lensmakers-equation?n=1.5`](https://aperioptic.com/calculators/lensmakers-equation?n=1.5). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Lens, Lensmaker's equation](https://en.wikipedia.org/wiki/Lens#Lensmaker's_equation)

[All optics calculators](https://aperioptic.com/calculators)

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The interactive page: https://aperioptic.com/calculators/lensmakers-equation (AperiOptic runs in the browser; free, no account). Every page of the site as Markdown: https://aperioptic.com/llms.txt