diff --git a/util/chunk/chunk.go b/util/chunk/chunk.go
index 86c6613cc2071..1ecac28a7234d 100644
--- a/util/chunk/chunk.go
+++ b/util/chunk/chunk.go
@@ -131,7 +131,7 @@ func newFixedLenColumn(elemLen, cap int) *column {
 	return &column{
 		elemBuf:    make([]byte, elemLen),
 		data:       make([]byte, 0, cap*elemLen),
-		nullBitmap: make([]byte, 0, cap>>3),
+		nullBitmap: make([]byte, 0, (cap+7)>>3),
 	}
 }
 
@@ -147,7 +147,7 @@ func newVarLenColumn(cap int, old *column) *column {
 	return &column{
 		offsets:    make([]int64, 1, cap+1),
 		data:       make([]byte, 0, cap*estimatedElemLen),
-		nullBitmap: make([]byte, 0, cap>>3),
+		nullBitmap: make([]byte, 0, (cap+7)>>3),
 	}
 }
 
diff --git a/util/chunk/chunk_test.go b/util/chunk/chunk_test.go
index 704ad3fe93fdd..882f956552168 100644
--- a/util/chunk/chunk_test.go
+++ b/util/chunk/chunk_test.go
@@ -536,13 +536,14 @@ func (s *testChunkSuite) TestChunkMemoryUsage(c *check.C) {
 	initCap := 10
 	chk := NewChunkWithCapacity(fieldTypes, initCap)
 
+	sizeTime := 16
 	//cap(c.nullBitmap) + cap(c.offsets)*4 + cap(c.data) + cap(c.elemBuf)
 	colUsage := make([]int, len(fieldTypes))
-	colUsage[0] = initCap>>3 + 0 + initCap*4 + 4
-	colUsage[1] = initCap>>3 + (initCap+1)*4 + initCap*8 + 0
-	colUsage[2] = initCap>>3 + (initCap+1)*4 + initCap*8 + 0
-	colUsage[3] = initCap>>3 + 0 + initCap*16 + 16
-	colUsage[4] = initCap>>3 + 0 + initCap*8 + 8
+	colUsage[0] = (initCap+7)>>3 + 0 + initCap*4 + 4
+	colUsage[1] = (initCap+7)>>3 + (initCap+1)*4 + initCap*8 + 0
+	colUsage[2] = (initCap+7)>>3 + (initCap+1)*4 + initCap*8 + 0
+	colUsage[3] = (initCap+7)>>3 + 0 + initCap*sizeTime + sizeTime
+	colUsage[4] = (initCap+7)>>3 + 0 + initCap*8 + 8
 
 	expectedUsage := 0
 	for i := range colUsage {
@@ -572,7 +573,7 @@ func (s *testChunkSuite) TestChunkMemoryUsage(c *check.C) {
 	chk.AppendDuration(4, durationObj)
 
 	memUsage = chk.MemoryUsage()
-	colUsage[1] = initCap>>3 + (initCap+1)*4 + cap(chk.columns[1].data) + 0
+	colUsage[1] = (initCap+7)>>3 + (initCap+1)*4 + cap(chk.columns[1].data) + 0
 	expectedUsage = 0
 	for i := range colUsage {
 		expectedUsage += colUsage[i] + int(unsafe.Sizeof(*chk.columns[i]))