---
title: "Resize an image in C#"
url: https://recursiveai.org/oneimage/cookbook/resize-image/
description: "Resize an image in C# and keep the aspect ratio: to a width, into a box, cropped to fill or padded, at the quality you choose. Runnable code with OneImage, on any OS."
version: "1.10.4"
---

# Resize an image in C#

Load the image, call `Resize` with the width you want, and save it. The height follows the aspect ratio. The program below resizes NASA’s “Blue Marble” photograph (4579×4579, public domain) three ways. OneImage is pure C#, so the same code runs on Windows, Linux, macOS and in containers.

```shell
dotnet new console -o ResizeImage && cd ResizeImage
dotnet add package RecursiveAI.OneImage --version 1.10.4
curl -L -o photo.jpg \
  https://images-assets.nasa.gov/image/as17-148-22727/as17-148-22727~orig.jpg
```

OneImage needs the .NET 10 SDK. Put this in `Program.cs` and run `dotnet run`:

```csharp
using OneImage;

Image photo = Image.Load("photo.jpg");   // reads the headers only

// 800 px wide; the height follows the aspect ratio.
photo.Resize(width: 800).Save("photo-800.jpg");

// Fill an exact 800x450 box and crop what overflows, keeping the center.
photo.Resize(width: 800, height: 450, fit: Fit.Cover).Save("banner.jpg");

// Fit inside 800x450 and pad the rest with white.
photo.Resize(width: 800, height: 450, fit: Fit.Pad,
             background: new Rgba32(255, 255, 255, 255))
     .Save("padded.jpg");

foreach (string file in new[] { "photo-800.jpg", "banner.jpg", "padded.jpg" })
{
    ImageInfo info = Image.Identify(file);
    long kb = new FileInfo(file).Length / 1024;
    Console.WriteLine($"{file}: {info.Width}x{info.Height}, {kb} KB");
}
```

```text
photo-800.jpg: 800x800, 90 KB
banner.jpg: 800x450, 84 KB
padded.jpg: 800x450, 43 KB
```

The output format follows the file extension, so `.png`, `.webp`, `.gif`, `.tif`, `.bmp` and `.ico` work the same way. `Image` is immutable, so one loaded photo feeds all three. Nothing is decoded until `Save`, and a large JPEG is decoded at 1/2, 1/4 or 1/8 scale when that is enough for the target, so the full-size photo is never held in memory.

## Fit modes

Give both a width and a height, and `fit:` decides how the image fills the box:

| `Fit` | Result |
|---|---|
| `Contain` (default) | Fits inside the box, aspect ratio kept; one side may be smaller. |
| `Cover` | Fills the box, aspect ratio kept; the overflow is cropped. `anchor:` picks the part kept (nine positions, or `Anchor.Smart` for the busiest area). |
| `Pad` | Fits inside the box, then pads to the exact size with `background:`. |
| `BoxPad` | Like `Pad`, but never enlarges. |
| `Min` | Covers the box without cropping; never enlarges. |
| `Stretch` | Exactly the box; aspect ratio not kept. |

## Quality and several sizes at once

The default filter already uses Lanczos3 when shrinking and Catmull-Rom when enlarging, in linear light, so the setting that matters most is the encoder’s quality. This writes JPEG at quality 85 and the three widths a `srcset` needs, as WebP:

```csharp
using OneImage;

Image photo = Image.Load("photo.jpg");

var jpeg = new SaveOptions
{
    EncoderOptions = new JpegEncoderOptions { Quality = 85 },
};
photo.Resize(width: 1200).Save("photo-1200.jpg", jpeg);

foreach (int width in new[] { 320, 640, 1280 })
{
    // upscale: false treats the width as a maximum for smaller sources.
    photo.Resize(width: width, upscale: false).Save($"photo-{width}.webp");
}

foreach (string file in new[] { "photo-1200.jpg", "photo-320.webp", "photo-640.webp",
                                "photo-1280.webp" })
{
    Console.WriteLine($"{file}: {new FileInfo(file).Length / 1024} KB");
}
```

```text
photo-1200.jpg: 216 KB
photo-320.webp: 24 KB
photo-640.webp: 53 KB
photo-1280.webp: 140 KB
```

JPEG and WebP default to quality 75. `filter: ResamplingFilter.Nearest` keeps hard pixel edges for pixel art; the other filters are listed in the [guide](https://recursiveai.org/oneimage/docs/guide/#operations).

## Limits

- **`Resize` enlarges when asked to.** Pass `upscale: false` when the width is a maximum, or use [`Image.Thumbnail`](https://recursiveai.org/oneimage/cookbook/create-thumbnail/), which never enlarges.
- **Metadata is kept.** A plain load and save keeps EXIF, which can hold the GPS position of a phone photo. For anything public, use `Thumbnail` or [remove the metadata](https://recursiveai.org/oneimage/cookbook/remove-exif-metadata/).
- **Formats.** The core package reads and writes PNG, JPEG, WebP, GIF, TIFF, BMP and ICO. SVG, PSD and PDF come in add-on packages; HEIC and AVIF aren’t supported.
- **.NET 10 only.** On older .NET, `System.Drawing` is Windows-only since .NET 6: see [the “not supported on this platform” error](https://recursiveai.org/oneimage/articles/system-drawing-common-not-supported/).

For web copies, see [Create a thumbnail](https://recursiveai.org/oneimage/cookbook/create-thumbnail/); to cut a region at full resolution, [Crop an image](https://recursiveai.org/oneimage/cookbook/crop-image/). Every option is in the [guide](https://recursiveai.org/oneimage/docs/guide/#operations).