---
title: "Mode Field Diameter calculator — AperiOptic"
description: "Marcuse approximation of MFD for a step-index single-mode fiber. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/mode-field-diameter"
---

# Mode Field Diameter calculator

Marcuse approximation of MFD for a step-index single-mode fiber.

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

Formula: `MFD/2a ≈ 0.65 + 1.619·V⁻³ᐟ² + 2.879·V⁻⁶`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Core radius a | `a` | µm | 4.1 |
| λ | `lam` | nm | 1550 |
| NA | `NA` |  | 0.14 |

## Result with the default inputs

- V: 2.3268
- MFD: 9.2192 µm

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/mode-field-diameter?a=4.1`](https://aperioptic.com/calculators/mode-field-diameter?a=4.1). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: Mode field diameter](https://en.wikipedia.org/wiki/Mode_field_diameter)

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

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