diff --git a/docs/guide/writing-media-files.md b/docs/guide/writing-media-files.md
index 384ce9a..5e58081 100644
--- a/docs/guide/writing-media-files.md
+++ b/docs/guide/writing-media-files.md
@@ -197,7 +197,18 @@ output.state; // => 'pending' | 'started' | 'canceled' | 'finalizing' | 'finaliz
 
 ## Output targets
 
-The _output target_ determines where the data created by the `Output` will be written. This library offers two targets:
+The _output target_ determines where the data created by the `Output` will be written. This library offers a couple of targets.
+
+---
+
+All targets have an optional `onwrite` callback you can set to monitor which byte regions are being written to:
+```ts
+target.onwrite = (start, end) => {
+	// ...
+};
+```
+
+You can use this to track the size of the output file as it grows. But be warned, this function is chatty and gets called *extremely* frequently.
 
 ### `BufferTarget`
 
diff --git a/src/source.ts b/src/source.ts
index a262bf6..1b3d41a 100644
--- a/src/source.ts
+++ b/src/source.ts
@@ -63,7 +63,7 @@ export abstract class Source {
 		return result;
 	}
 
-	/** Called each time data is retrieved from the source. Will be called with the retrieved range. */
+	/** Called each time data is retrieved from the source. Will be called with the retrieved range (end exclusive). */
 	onread: ((start: number, end: number) => unknown) | null = null;
 }
 
diff --git a/src/target.ts b/src/target.ts
index 91d9960..03366bd 100644
--- a/src/target.ts
+++ b/src/target.ts
@@ -19,6 +19,14 @@ export abstract class Target {
 
 	/** @internal */
 	abstract _createWriter(): Writer;
+
+	/**
+	 * Called each time data is written to the target. Will be called with the byte range into which data was written.
+	 *
+	 * Use this callback to track the size of the output file as it grows. But be warned, this function is chatty and
+	 * gets called *extremely* often.
+	 */
+	onwrite: ((start: number, end: number) => unknown) | null = null;
 }
 
 /**
@@ -113,6 +121,6 @@ export class StreamTarget extends Target {
 export class NullTarget extends Target {
 	/** @internal */
 	_createWriter() {
-		return new NullTargetWriter();
+		return new NullTargetWriter(this);
 	}
 }
diff --git a/src/writer.ts b/src/writer.ts
index 5f2b8f7..30de003 100644
--- a/src/writer.ts
+++ b/src/writer.ts
@@ -6,7 +6,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
-import { BufferTarget, StreamTarget, StreamTargetChunk } from './target';
+import { BufferTarget, NullTarget, StreamTarget, StreamTargetChunk } from './target';
 import { assert } from './misc';
 
 export abstract class Writer {
@@ -156,8 +156,9 @@ export class BufferTargetWriter extends Writer {
 		this.ensureSize(this.pos + data.byteLength);
 
 		this.bytes.set(data, this.pos);
-		this.pos += data.byteLength;
+		this.target.onwrite?.(this.pos, this.pos + data.byteLength);
 
+		this.pos += data.byteLength;
 		this.maxPos = Math.max(this.maxPos, this.pos);
 	}
 
@@ -250,6 +251,8 @@ export class StreamTargetWriter extends Writer {
 			data: data.slice(),
 			start: this.pos,
 		});
+		this.target.onwrite?.(this.pos, this.pos + data.byteLength);
+
 		this.pos += data.byteLength;
 
 		this.lastWriteEnd = Math.max(this.lastWriteEnd, this.pos);
@@ -463,8 +466,13 @@ export class StreamTargetWriter extends Writer {
 export class NullTargetWriter extends Writer {
 	private pos = 0;
 
+	constructor(private target: NullTarget) {
+		super();
+	}
+
 	write(data: Uint8Array) {
 		this.maybeTrackWrites(data);
+		this.target.onwrite?.(this.pos, this.pos + data.byteLength);
 		this.pos += data.byteLength;
 	}