diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index caa9eaf38503..a2600377e01c 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -81,6 +81,8 @@ Bug Fixes
 * GITHUB#14847: Allow Faiss vector format to index >2GB of vectors per-field per-segment by using MemorySegment APIs
   (instead of ByteBuffer) to copy bytes to native memory. (Kaival Parikh)
 
+* GITHUB#15120: Add estimatedByteSizes to merges kicked off by IndexWriter.addIndexes(CodecReader[]) (Craig Perkins)
+
 Changes in Runtime Behavior
 ---------------------
 * GITHUB#14187: The query cache is now disabled by default. (Adrien Grand)
diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
index 1224bebc9d06..a8c63c3000ea 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
@@ -21,6 +21,7 @@
 
 import java.io.Closeable;
 import java.io.IOException;
+import java.io.UncheckedIOException;
 import java.time.Instant;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
@@ -3285,6 +3286,11 @@ public AddIndexesMergeSource(IndexWriter writer) {
     }
 
     public void registerMerge(MergePolicy.OneMerge merge) {
+      try {
+        addEstimatedBytesToMerge(merge);
+      } catch (IOException e) {
+        throw new UncheckedIOException(e);
+      }
       synchronized (IndexWriter.this) {
         pendingAddIndexesMerges.add(merge);
       }
@@ -4777,6 +4783,21 @@ private void abortOneMerge(MergePolicy.OneMerge merge) throws IOException {
     closeMergeReaders(merge, true, false);
   }
 
+  /** Compute {@code estimatedMergeBytes} and {@code totalMergeBytes} for a merge. */
+  void addEstimatedBytesToMerge(MergePolicy.OneMerge merge) throws IOException {
+    assert merge.estimatedMergeBytes == 0;
+    assert merge.totalMergeBytes == 0;
+    for (SegmentCommitInfo info : merge.segments) {
+      if (info.info.maxDoc() > 0) {
+        final int delCount = numDeletedDocs(info);
+        assert delCount <= info.info.maxDoc();
+        final double delRatio = ((double) delCount) / info.info.maxDoc();
+        merge.estimatedMergeBytes += (long) (info.sizeInBytes() * (1.0 - delRatio));
+        merge.totalMergeBytes += info.sizeInBytes();
+      }
+    }
+  }
+
   /**
    * Checks whether this merge involves any segments already participating in a merge. If not, this
    * merge is "registered", meaning we record that its segments are now participating in a merge,
@@ -4868,17 +4889,7 @@ private synchronized boolean registerMerge(MergePolicy.OneMerge merge) throws IO
       mergingSegments.add(info);
     }
 
-    assert merge.estimatedMergeBytes == 0;
-    assert merge.totalMergeBytes == 0;
-    for (SegmentCommitInfo info : merge.segments) {
-      if (info.info.maxDoc() > 0) {
-        final int delCount = numDeletedDocs(info);
-        assert delCount <= info.info.maxDoc();
-        final double delRatio = ((double) delCount) / info.info.maxDoc();
-        merge.estimatedMergeBytes += (long) (info.sizeInBytes() * (1.0 - delRatio));
-        merge.totalMergeBytes += info.sizeInBytes();
-      }
-    }
+    addEstimatedBytesToMerge(merge);
 
     // Merge is now registered
     merge.registerDone = true;
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriterMerging.java b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriterMerging.java
index e3f47be432ab..a1c887e93b27 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriterMerging.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriterMerging.java
@@ -461,4 +461,35 @@ public void run() {
 
     directory.close();
   }
+
+  public void testAddEstimatedBytesToMerge() throws IOException {
+    try (Directory dir = newDirectory();
+        IndexWriter writer =
+            new IndexWriter(
+                dir,
+                newIndexWriterConfig(new MockAnalyzer(random()))
+                    .setMergePolicy(NoMergePolicy.INSTANCE))) {
+
+      Document doc = new Document();
+      doc.add(newTextField("field", "content", Field.Store.YES));
+
+      for (int i = 0; i < 10; i++) {
+
+        writer.addDocument(doc);
+      }
+      writer.flush();
+
+      // Create a merge with the segments
+      SegmentInfos segmentInfos = writer.cloneSegmentInfos();
+      MergePolicy.OneMerge merge = new MergePolicy.OneMerge(segmentInfos.asList());
+
+      writer.addEstimatedBytesToMerge(merge);
+
+      assertTrue(merge.estimatedMergeBytes > 0);
+
+      assertTrue(merge.totalMergeBytes > 0);
+
+      assertTrue(merge.estimatedMergeBytes <= merge.totalMergeBytes);
+    }
+  }
 }