diff --git a/uploader/s3.go b/uploader/s3.go
index 542827be1..ea3f37bac 100644
--- a/uploader/s3.go
+++ b/uploader/s3.go
@@ -24,6 +24,21 @@ const (
 	defaultS3PartSize    = 64 * 1024 * 1024 // 64MB per part
 )
 
+// CountingReader is a wrapper of io.Reader to count number of bytes read
+type CountingReader struct {
+	reader io.Reader
+	BytesRead int
+}
+
+// Read aggregates number of bytes read
+func (r * CountingReader) Read(p []byte) (n int, err error) {
+	n, err = r.reader.Read(p)
+	if err == nil {
+		r.BytesRead += n
+	}
+	return
+}
+
 // S3Uploader uploads logs to S3
 type S3Uploader struct {
 	log        logrus.FieldLogger
@@ -90,11 +105,6 @@ func (u *S3Uploader) Upload(ctx context.Context, local string, remote string, ct
 		}
 	}()
 
-	// Record number of bytes being uploaded.  At this point, file must exist.
-	if stat, err := os.Stat(local); err == nil {
-		u.metrics.Counter("titus.executor.S3Uploader.uploadMB", int(stat.Size()/1024/1024), nil)
-	}
-
 	return u.uploadFile(ctx, f, remote, contentType)
 }
 
@@ -105,17 +115,21 @@ func (u *S3Uploader) uploadFile(ctx context.Context, local io.Reader, remote str
 		contentType = defaultS3ContentType
 	}
 
+	// wrap input io.Reader with a counting reader
+	reader := &CountingReader{local, 0}
+
 	result, err := u.s3Uploader.UploadWithContext(ctx, &s3manager.UploadInput{
 		ACL:         aws.String(defaultS3ACL),
 		ContentType: aws.String(contentType),
 		Bucket:      aws.String(u.bucketName),
 		Key:         aws.String(remote),
-		Body:        local,
+		Body:        io.Reader(reader),
 	})
 	if err != nil {
 		return err
 	}
 
+	u.metrics.Counter("titus.executor.S3Uploader.uploadMB", int(reader.BytesRead/1024/1024), nil)
 	u.log.Printf("Successfully uploaded file from: %s to: %s", local, result.Location)
 
 	return nil