diff --git a/tests/test_metrics.py b/tests/test_metrics.py
new file mode 100644
index 0000000000..17897711a1
--- /dev/null
+++ b/tests/test_metrics.py
@@ -0,0 +1,81 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# 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.
+
+
+from synapse.metrics import InFlightGauge
+
+from tests import unittest
+
+
+class TestMauLimit(unittest.TestCase):
+    def test_basic(self):
+        gauge = InFlightGauge(
+            "test1", "",
+            labels=["test_label"],
+            sub_metrics=["foo", "bar"],
+        )
+
+        def handle1(metrics):
+            metrics.foo += 2
+            metrics.bar = max(metrics.bar, 5)
+
+        def handle2(metrics):
+            metrics.foo += 3
+            metrics.bar = max(metrics.bar, 7)
+
+        gauge.register(("key1",), handle1)
+
+        self.assert_dict({
+            "test1_total": {("key1",): 1},
+            "test1_foo": {("key1",): 2},
+            "test1_bar": {("key1",): 5},
+        }, self.get_metrics_from_gauge(gauge))
+
+        gauge.unregister(("key1",), handle1)
+
+        self.assert_dict({
+            "test1_total": {("key1",): 0},
+            "test1_foo": {("key1",): 0},
+            "test1_bar": {("key1",): 0},
+        }, self.get_metrics_from_gauge(gauge))
+
+        gauge.register(("key1",), handle1)
+        gauge.register(("key2",), handle2)
+
+        self.assert_dict({
+            "test1_total": {("key1",): 1, ("key2",): 1},
+            "test1_foo": {("key1",): 2, ("key2",): 3},
+            "test1_bar": {("key1",): 5, ("key2",): 7},
+        }, self.get_metrics_from_gauge(gauge))
+
+        gauge.unregister(("key2",), handle2)
+        gauge.register(("key1",), handle2)
+
+        self.assert_dict({
+            "test1_total": {("key1",): 2, ("key2",): 0},
+            "test1_foo": {("key1",): 5, ("key2",): 0},
+            "test1_bar": {("key1",): 7, ("key2",): 0},
+        }, self.get_metrics_from_gauge(gauge))
+
+    def get_metrics_from_gauge(self, gauge):
+        results = {}
+
+        for r in gauge.collect():
+            results[r.name] = {
+                tuple(labels[x] for x in gauge.labels): value
+                for _, labels, value in r.samples
+            }
+
+        return results