diff --git a/model/labels/labels_dedupelabels_test.go b/model/labels/labels_dedupelabels_test.go
index 5ef9255c21..baf0423db2 100644
--- a/model/labels/labels_dedupelabels_test.go
+++ b/model/labels/labels_dedupelabels_test.go
@@ -21,6 +21,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+var expectedSizeOfLabels = []uint64{ // Values must line up with testCaseLabels.
+	16,
+	0,
+	41,
+	270,
+	271,
+	325,
+}
+
 func TestVarint(t *testing.T) {
 	cases := []struct {
 		v        int
diff --git a/model/labels/labels_slicelabels_test.go b/model/labels/labels_slicelabels_test.go
new file mode 100644
index 0000000000..e9edc9152d
--- /dev/null
+++ b/model/labels/labels_slicelabels_test.go
@@ -0,0 +1,25 @@
+// Copyright 2025 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build slicelabels
+
+package labels
+
+var expectedSizeOfLabels = []uint64{ // Values must line up with testCaseLabels.
+	72,
+	0,
+	97,
+	326,
+	327,
+	549,
+}
diff --git a/model/labels/labels_stringlabels_test.go b/model/labels/labels_stringlabels_test.go
new file mode 100644
index 0000000000..aaa8b52415
--- /dev/null
+++ b/model/labels/labels_stringlabels_test.go
@@ -0,0 +1,25 @@
+// Copyright 2025 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !slicelabels && !dedupelabels
+
+package labels
+
+var expectedSizeOfLabels = []uint64{ // Values must line up with testCaseLabels.
+	12,
+	0,
+	37,
+	266,
+	270,
+	309,
+}
diff --git a/model/labels/labels_test.go b/model/labels/labels_test.go
index d9368ccfc3..3b4f802160 100644
--- a/model/labels/labels_test.go
+++ b/model/labels/labels_test.go
@@ -26,37 +26,33 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
+var (
+	s254 = strings.Repeat("x", 254) // Edge cases for stringlabels encoding.
+	s255 = strings.Repeat("x", 255)
+)
+
+var testCaseLabels = []Labels{
+	FromStrings("t1", "t1", "t2", "t2"),
+	{},
+	FromStrings("service.name", "t1", "whatever\\whatever", "t2"),
+	FromStrings("aaa", "111", "xx", s254),
+	FromStrings("aaa", "111", "xx", s255),
+	FromStrings("__name__", "kube_pod_container_status_last_terminated_exitcode", "cluster", "prod-af-north-0", " container", "prometheus", "instance", "kube-state-metrics-0:kube-state-metrics:ksm", "job", "kube-state-metrics/kube-state-metrics", " namespace", "observability-prometheus", "pod", "observability-prometheus-0", "uid", "d3ec90b2-4975-4607-b45d-b9ad64bb417e"),
+}
+
 func TestLabels_String(t *testing.T) {
-	s254 := strings.Repeat("x", 254) // Edge cases for stringlabels encoding.
-	s255 := strings.Repeat("x", 255)
-	cases := []struct {
-		labels   Labels
-		expected string
-	}{
-		{
-			labels:   FromStrings("t1", "t1", "t2", "t2"),
-			expected: "{t1=\"t1\", t2=\"t2\"}",
-		},
-		{
-			labels:   Labels{},
-			expected: "{}",
-		},
-		{
-			labels:   FromStrings("service.name", "t1", "whatever\\whatever", "t2"),
-			expected: `{"service.name"="t1", "whatever\\whatever"="t2"}`,
-		},
-		{
-			labels:   FromStrings("aaa", "111", "xx", s254),
-			expected: `{aaa="111", xx="` + s254 + `"}`,
-		},
-		{
-			labels:   FromStrings("aaa", "111", "xx", s255),
-			expected: `{aaa="111", xx="` + s255 + `"}`,
-		},
+	expected := []string{ // Values must line up with testCaseLabels.
+		"{t1=\"t1\", t2=\"t2\"}",
+		"{}",
+		`{"service.name"="t1", "whatever\\whatever"="t2"}`,
+		`{aaa="111", xx="` + s254 + `"}`,
+		`{aaa="111", xx="` + s255 + `"}`,
+		`{" container"="prometheus", " namespace"="observability-prometheus", __name__="kube_pod_container_status_last_terminated_exitcode", cluster="prod-af-north-0", instance="kube-state-metrics-0:kube-state-metrics:ksm", job="kube-state-metrics/kube-state-metrics", pod="observability-prometheus-0", uid="d3ec90b2-4975-4607-b45d-b9ad64bb417e"}`,
 	}
-	for _, c := range cases {
-		str := c.labels.String()
-		require.Equal(t, c.expected, str)
+	require.Len(t, expected, len(testCaseLabels))
+	for i, c := range expected {
+		str := testCaseLabels[i].String()
+		require.Equal(t, c, str)
 	}
 }
 
@@ -67,6 +63,17 @@ func BenchmarkString(b *testing.B) {
 	}
 }
 
+func TestSizeOfLabels(t *testing.T) {
+	require.Len(t, expectedSizeOfLabels, len(testCaseLabels))
+	for i, c := range expectedSizeOfLabels { // Declared in build-tag-specific files, e.g. labels_slicelabels_test.go.
+		var total uint64
+		testCaseLabels[i].Range(func(l Label) {
+			total += SizeOfLabels(l.Name, l.Value, 1)
+		})
+		require.Equal(t, c, total)
+	}
+}
+
 func TestLabels_MatchLabels(t *testing.T) {
 	labels := FromStrings(
 		"__name__", "ALERTS",