---
title: "Sellmeier Equation calculator — AperiOptic"
description: "n(λ) using a 3-term Sellmeier expansion (BK7-style coefficients by default). Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/sellmeier-equation"
---

# Sellmeier Equation calculator

n(λ) using a 3-term Sellmeier expansion (BK7-style coefficients by default).

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

Formula: `n² = 1 + Σ Bi·λ²/(λ² − Ci)`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Wavelength | `lam` | µm | 0.5876 |
| B1 | `B1` |  | 1.03961212 |
| C1 (µm²) | `C1` |  | 0.00600069867 |
| B2 | `B2` |  | 0.231792344 |
| C2 (µm²) | `C2` |  | 0.0200179144 |
| B3 | `B3` |  | 1.01046945 |
| C3 (µm²) | `C3` |  | 103.560653 |

## Result with the default inputs

- Refractive index n: 1.516798

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/sellmeier-equation?lam=0.5876`](https://aperioptic.com/calculators/sellmeier-equation?lam=0.5876). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Sellmeier equation](https://en.wikipedia.org/wiki/Sellmeier_equation)

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

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