diff --git a/pkg/sql/rowenc/index_fetch.go b/pkg/sql/rowenc/index_fetch.go
index edec233e02a8..86a54aa01e20 100644
--- a/pkg/sql/rowenc/index_fetch.go
+++ b/pkg/sql/rowenc/index_fetch.go
@@ -15,6 +15,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog"
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb"
 	"github.com/cockroachdb/cockroach/pkg/util/buildutil"
+	"github.com/cockroachdb/cockroach/pkg/util/encoding"
 	"github.com/cockroachdb/errors"
 )
 
@@ -46,8 +47,9 @@ func InitIndexFetchSpec(
 
 	maxKeysPerRow := table.IndexKeysPerRow(index)
 	s.MaxKeysPerRow = uint32(maxKeysPerRow)
-	// TODO(radu): calculate the length without actually generating a throw-away key.
-	s.KeyPrefixLength = uint32(len(MakeIndexKeyPrefix(codec, table.GetID(), index.GetID())))
+	s.KeyPrefixLength = uint32(len(codec.TenantPrefix()) +
+		encoding.EncodedLengthUvarintAscending(uint64(s.TableID)) +
+		encoding.EncodedLengthUvarintAscending(uint64(index.GetID())))
 
 	s.FamilyDefaultColumns = table.FamilyDefaultColumns()
 
diff --git a/pkg/util/encoding/encoding.go b/pkg/util/encoding/encoding.go
index 634989f3478a..72bda65cef8e 100644
--- a/pkg/util/encoding/encoding.go
+++ b/pkg/util/encoding/encoding.go
@@ -374,6 +374,31 @@ func EncodeUvarintAscending(b []byte, v uint64) []byte {
 	}
 }
 
+// EncodedLengthUvarintAscending returns the length of the variable length
+// representation, i.e. the number of bytes appended by EncodeUvarintAscending.
+func EncodedLengthUvarintAscending(v uint64) int {
+	switch {
+	case v <= intSmall:
+		return 1
+	case v <= 0xff:
+		return 2
+	case v <= 0xffff:
+		return 3
+	case v <= 0xffffff:
+		return 4
+	case v <= 0xffffffff:
+		return 5
+	case v <= 0xffffffffff:
+		return 6
+	case v <= 0xffffffffffff:
+		return 7
+	case v <= 0xffffffffffffff:
+		return 8
+	default:
+		return 9
+	}
+}
+
 // EncodeUvarintDescending encodes the uint64 value so that it sorts in
 // reverse order, from largest to smallest.
 func EncodeUvarintDescending(b []byte, v uint64) []byte {
diff --git a/pkg/util/encoding/encoding_test.go b/pkg/util/encoding/encoding_test.go
index 926d4f2f3e93..723db2838a85 100644
--- a/pkg/util/encoding/encoding_test.go
+++ b/pkg/util/encoding/encoding_test.go
@@ -323,6 +323,17 @@ func TestEncodeDecodeUvarint(t *testing.T) {
 	testCustomEncodeUint64(testCases, EncodeUvarintAscending, t)
 }
 
+func TestEncodedLengthUvarintAscending(t *testing.T) {
+	for i := 0; i < 100; i++ {
+		v := rand.Uint64()
+		exp := len(EncodeUvarintAscending(nil, v))
+		actual := EncodedLengthUvarintAscending(v)
+		if actual != exp {
+			t.Fatalf("incorrect encoded length for %d: %d (expected %d)", v, actual, exp)
+		}
+	}
+}
+
 func TestEncodeDecodeUvarintDescending(t *testing.T) {
 	testBasicEncodeDecodeUint64(EncodeUvarintDescending, DecodeUvarintDescending, true, true, true, t)
 	testCases := []testCaseUint64{