einundzwanzig-verein/videos/.opencode/skills/remotion-best-practices/rules/calculate-metadata.md

---
name: calculate-metadata
description: Dynamically set composition duration, dimensions, and props
metadata:
  tags: calculateMetadata, duration, dimensions, props, dynamic
---

# Using calculateMetadata

Use `calculateMetadata` on a `<Composition>` to dynamically set duration, dimensions, and transform props before rendering.

```tsx
<Composition id="MyComp" component={MyComponent} durationInFrames={300} fps={30} width={1920} height={1080} defaultProps={{videoSrc: 'https://remotion.media/video.mp4'}} calculateMetadata={calculateMetadata} />
```

## Setting duration based on a video

Use the `getMediaMetadata()` function from the mediabunny/metadata skill to get the video duration:

```tsx
import {CalculateMetadataFunction} from 'remotion';
import {getMediaMetadata} from '../get-media-metadata';

const calculateMetadata: CalculateMetadataFunction<Props> = async ({props}) => {
  const {durationInSeconds} = await getMediaMetadata(props.videoSrc);

  return {
    durationInFrames: Math.ceil(durationInSeconds * 30),
  };
};
```

## Matching dimensions of a video

```tsx
const calculateMetadata: CalculateMetadataFunction<Props> = async ({props}) => {
  const {durationInSeconds, dimensions} = await getMediaMetadata(props.videoSrc);

  return {
    durationInFrames: Math.ceil(durationInSeconds * 30),
    width: dimensions?.width ?? 1920,
    height: dimensions?.height ?? 1080,
  };
};
```

## Setting duration based on multiple videos

```tsx
const calculateMetadata: CalculateMetadataFunction<Props> = async ({props}) => {
  const metadataPromises = props.videos.map((video) => getMediaMetadata(video.src));
  const allMetadata = await Promise.all(metadataPromises);

  const totalDuration = allMetadata.reduce((sum, meta) => sum + meta.durationInSeconds, 0);

  return {
    durationInFrames: Math.ceil(totalDuration * 30),
  };
};
```

## Setting a default outName

Set the default output filename based on props:

```tsx
const calculateMetadata: CalculateMetadataFunction<Props> = async ({props}) => {
  return {
    defaultOutName: `video-${props.id}.mp4`,
  };
};
```

## Transforming props

Fetch data or transform props before rendering:

```tsx
const calculateMetadata: CalculateMetadataFunction<Props> = async ({props, abortSignal}) => {
  const response = await fetch(props.dataUrl, {signal: abortSignal});
  const data = await response.json();

  return {
    props: {
      ...props,
      fetchedData: data,
    },
  };
};
```

The `abortSignal` cancels stale requests when props change in the Studio.

## Return value

All fields are optional. Returned values override the `<Composition>` props:

- `durationInFrames`: Number of frames
- `width`: Composition width in pixels
- `height`: Composition height in pixels
- `fps`: Frames per second
- `props`: Transformed props passed to the component
- `defaultOutName`: Default output filename
- `defaultCodec`: Default codec for rendering