---
title: "f-Number / Aperture calculator — AperiOptic"
description: "Relationship between focal length, aperture diameter and f/#. Free online optics calculator, with the formula and code snippets."
url: "https://aperioptic.com/calculators/f-number-aperture"
---

# f-Number / Aperture calculator

Relationship between focal length, aperture diameter and f/#.

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

Formula: `N = f / D`

## Inputs

| Input | Parameter | Unit | Default |
| --- | --- | --- | --- |
| Focal length | `f` | mm | 50 |
| Aperture | `D` | mm | 25 |

## Result with the default inputs

- f-number: f/2
- Rel. aperture: 0.5

Open it with your own values: put each input in the address as a parameter, for example [`/calculators/f-number-aperture?f=50`](https://aperioptic.com/calculators/f-number-aperture?f=50). A unit other than the listed one goes in `<parameter>_unit`.

Read more: [Wikipedia: F-number](https://en.wikipedia.org/wiki/F-number)

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

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