diff --git a/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java b/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java
index d14c0f3a59..615276e1bb 100644
--- a/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java
+++ b/libraries/transformer/src/main/java/androidx/media3/transformer/Transformer.java
@@ -66,6 +66,7 @@ import androidx.media3.extractor.DefaultExtractorsFactory;
 import androidx.media3.extractor.mp4.Mp4Extractor;
 import com.google.common.collect.ImmutableList;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import java.io.File;
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
 import java.lang.annotation.Documented;
@@ -593,6 +594,8 @@ public final class Transformer {
 
   @Nullable private MuxerWrapper muxerWrapper;
   @Nullable private ExoPlayer player;
+  @Nullable private String outputPath;
+  @Nullable private ParcelFileDescriptor outputParcelFileDescriptor;
   private @ProgressState int progressState;
   private boolean isCancelling;
 
@@ -705,6 +708,8 @@ public final class Transformer {
       throw new UnsupportedEncodingException(
           "Clipping is not supported when slow motion flattening is requested");
     }
+    this.outputPath = path;
+    this.outputParcelFileDescriptor = null;
     startTransformation(mediaItem, muxerFactory.create(path, containerMimeType));
   }
 
@@ -733,6 +738,8 @@ public final class Transformer {
   @RequiresApi(26)
   public void startTransformation(MediaItem mediaItem, ParcelFileDescriptor parcelFileDescriptor)
       throws IOException {
+    this.outputParcelFileDescriptor = parcelFileDescriptor;
+    this.outputPath = null;
     startTransformation(mediaItem, muxerFactory.create(parcelFileDescriptor, containerMimeType));
   }
 
@@ -876,6 +883,26 @@ public final class Transformer {
     }
   }
 
+  /**
+   * Returns the current size in bytes of the current/latest output file, or {@link C#LENGTH_UNSET}
+   * if unavailable.
+   */
+  private long getCurrentOutputFileCurrentSizeBytes() {
+    long fileSize = C.LENGTH_UNSET;
+
+    if (outputPath != null) {
+      fileSize = new File(outputPath).length();
+    } else if (outputParcelFileDescriptor != null) {
+      fileSize = outputParcelFileDescriptor.getStatSize();
+    }
+
+    if (fileSize <= 0) {
+      fileSize = C.LENGTH_UNSET;
+    }
+
+    return fileSize;
+  }
+
   private static final class TransformerRenderersFactory implements RenderersFactory {
 
     private final Context context;
@@ -1064,6 +1091,7 @@ public final class Transformer {
                 .setAverageAudioBitrate(muxerWrapper.getTrackAverageBitrate(C.TRACK_TYPE_AUDIO))
                 .setAverageVideoBitrate(muxerWrapper.getTrackAverageBitrate(C.TRACK_TYPE_VIDEO))
                 .setVideoFrameCount(muxerWrapper.getTrackSampleCount(C.TRACK_TYPE_VIDEO))
+                .setFileSizeBytes(getCurrentOutputFileCurrentSizeBytes())
                 .build();
 
         listeners.queueEvent(