diff --git a/internal/datacoord/index_inspector.go b/internal/datacoord/index_inspector.go
index 2870b96d8e..49335825c9 100644
--- a/internal/datacoord/index_inspector.go
+++ b/internal/datacoord/index_inspector.go
@@ -179,8 +179,9 @@ func (i *indexInspector) createIndexForSegment(ctx context.Context, segment *Seg
 	indexParams := i.meta.indexMeta.GetIndexParams(segment.CollectionID, indexID)
 	indexType := GetIndexType(indexParams)
 	isVectorIndex := vecindexmgr.GetVecIndexMgrInstance().IsVecIndex(indexType)
-	segSize := segment.getSegmentSize()
-	taskSlot := calculateIndexTaskSlot(segSize, isVectorIndex)
+	fieldID := i.meta.indexMeta.GetFieldIDByIndexID(segment.CollectionID, indexID)
+	fieldSize := segment.getFieldBinlogSize(fieldID)
+	taskSlot := calculateIndexTaskSlot(fieldSize, isVectorIndex)
 
 	// rewrite the index type if needed, and this final index type will be persisted in the meta
 	if isVectorIndex && Params.KnowhereConfig.Enable.GetAsBool() {
@@ -219,7 +220,9 @@ func (i *indexInspector) createIndexForSegment(ctx context.Context, segment *Seg
 	log.Info("indexInspector create index for segment success",
 		zap.Int64("segmentID", segment.ID),
 		zap.Int64("indexID", indexID),
-		zap.Int64("segment size", segSize),
+		zap.Int64("fieldID", fieldID),
+		zap.Int64("segment size", segment.getSegmentSize()),
+		zap.Int64("field size", fieldSize),
 		zap.Int64("task slot", taskSlot))
 	return nil
 }
@@ -237,7 +240,9 @@ func (i *indexInspector) reloadFromMeta() {
 			indexParams := i.meta.indexMeta.GetIndexParams(segment.CollectionID, segIndex.IndexID)
 			indexType := GetIndexType(indexParams)
 			isVectorIndex := vecindexmgr.GetVecIndexMgrInstance().IsVecIndex(indexType)
-			taskSlot := calculateIndexTaskSlot(segment.getSegmentSize(), isVectorIndex)
+			fieldID := i.meta.indexMeta.GetFieldIDByIndexID(segment.CollectionID, segIndex.IndexID)
+			fieldSize := segment.getFieldBinlogSize(fieldID)
+			taskSlot := calculateIndexTaskSlot(fieldSize, isVectorIndex)
 
 			i.scheduler.Enqueue(newIndexBuildTask(
 				model.CloneSegmentIndex(segIndex),
diff --git a/internal/datacoord/segment_info.go b/internal/datacoord/segment_info.go
index 516c726fd5..561cb37e8e 100644
--- a/internal/datacoord/segment_info.go
+++ b/internal/datacoord/segment_info.go
@@ -532,6 +532,29 @@ func (s *SegmentInfo) getSegmentSize() int64 {
 	return s.size.Load()
 }
 
+func (s *SegmentInfo) getFieldBinlogSize(fieldID int64) int64 {
+	var size int64
+	for _, binlogs := range s.GetBinlogs() {
+		if binlogs.GetFieldID() == fieldID {
+			for _, l := range binlogs.GetBinlogs() {
+				size += l.GetMemorySize()
+			}
+		} else {
+			for _, childFieldID := range binlogs.GetChildFields() {
+				if childFieldID == fieldID {
+					for _, l := range binlogs.GetBinlogs() {
+						size += l.GetMemorySize()
+					}
+				}
+			}
+		}
+	}
+	if size <= 0 {
+		return s.getSegmentSize()
+	}
+	return size
+}
+
 // Any edits on deltalogs of flushed segments will reset deltaRowcount to -1
 func (s *SegmentInfo) getDeltaCount() int64 {
 	if s.deltaRowcount.Load() < 0 || s.GetState() != commonpb.SegmentState_Flushed {
diff --git a/internal/datacoord/segment_info_test.go b/internal/datacoord/segment_info_test.go
index cd42ec092e..887bb0c67f 100644
--- a/internal/datacoord/segment_info_test.go
+++ b/internal/datacoord/segment_info_test.go
@@ -110,6 +110,7 @@ func TestGetSegmentSize(t *testing.T) {
 		SegmentInfo: &datapb.SegmentInfo{
 			Binlogs: []*datapb.FieldBinlog{
 				{
+					FieldID: 1,
 					Binlogs: []*datapb.Binlog{
 						{
 							LogID:      1,
@@ -143,6 +144,9 @@ func TestGetSegmentSize(t *testing.T) {
 
 	assert.Equal(t, int64(3), segment.getSegmentSize())
 	assert.Equal(t, int64(3), segment.getSegmentSize())
+	assert.Equal(t, int64(1), segment.getFieldBinlogSize(1))
+	// field 2 has no binlogs, fallback to getSegmentSize
+	assert.Equal(t, int64(3), segment.getFieldBinlogSize(2))
 }
 
 func TestIsDeltaLogExists(t *testing.T) {
diff --git a/internal/datacoord/util.go b/internal/datacoord/util.go
index 4a9bf35898..383f711712 100644
--- a/internal/datacoord/util.go
+++ b/internal/datacoord/util.go
@@ -373,17 +373,17 @@ func getSortStatus(sorted bool) string {
 	return "unsorted"
 }
 
-func calculateIndexTaskSlot(segmentSize int64, isVectorIndex bool) int64 {
+func calculateIndexTaskSlot(fieldSize int64, isVectorIndex bool) int64 {
 	defaultSlots := Params.DataCoordCfg.IndexTaskSlotUsage.GetAsInt64()
 	if !isVectorIndex {
 		defaultSlots = Params.DataCoordCfg.ScalarIndexTaskSlotUsage.GetAsInt64()
 	}
-	if segmentSize > 512*1024*1024 {
-		taskSlot := max(segmentSize/512/1024/1024, 1) * defaultSlots
+	if fieldSize > 512*1024*1024 {
+		taskSlot := max(fieldSize/512/1024/1024, 1) * defaultSlots
 		return max(taskSlot, 1)
-	} else if segmentSize > 100*1024*1024 {
+	} else if fieldSize > 100*1024*1024 {
 		return max(defaultSlots/4, 1)
-	} else if segmentSize > 10*1024*1024 {
+	} else if fieldSize > 10*1024*1024 {
 		return max(defaultSlots/16, 1)
 	}
 	return max(defaultSlots/64, 1)