diff --git a/changelog.d/3871.misc b/changelog.d/3871.misc
new file mode 100644
index 000000000000..dd9510ceb6ac
--- /dev/null
+++ b/changelog.d/3871.misc
@@ -0,0 +1 @@
+Add in flight real time metrics for Measure blocks
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index c49dbacd9371..c3542b935320 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -43,6 +43,7 @@
 from synapse.http.endpoint import matrix_federation_endpoint
 from synapse.util import logcontext
 from synapse.util.logcontext import make_deferred_yieldable
+from synapse.util.metrics import Measure
 
 logger = logging.getLogger(__name__)
 outbound_logger = logging.getLogger("synapse.http.outbound")
@@ -224,9 +225,11 @@ def _request(self, destination, method, path,
                         reactor=self.hs.get_reactor()
                     )
                     request_deferred.addTimeout(_sec_timeout, self.hs.get_reactor())
-                    response = yield make_deferred_yieldable(
-                        request_deferred,
-                    )
+
+                    with Measure(self.clock, "outbound_request"):
+                        response = yield make_deferred_yieldable(
+                            request_deferred,
+                        )
 
                     log_result = "%d %s" % (response.code, response.phrase,)
                     break
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 550f8443f78a..59900aa5d1b5 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -18,8 +18,11 @@
 import logging
 import os
 import platform
+import threading
 import time
 
+import six
+
 import attr
 from prometheus_client import Counter, Gauge, Histogram
 from prometheus_client.core import REGISTRY, GaugeMetricFamily
@@ -68,7 +71,7 @@ def collect(self):
             return
 
         if isinstance(calls, dict):
-            for k, v in calls.items():
+            for k, v in six.iteritems(calls):
                 g.add_metric(k, v)
         else:
             g.add_metric([], calls)
@@ -87,6 +90,109 @@ def _register(self):
         all_gauges[self.name] = self
 
 
+class InFlightGauge(object):
+    """Tracks number of things (e.g. requests, Measure blocks, etc) in flight
+    at any given time.
+
+    Each InFlightGauge will create a metric called `<name>_total` that counts
+    the number of in flight blocks, as well as a metrics for each item in the
+    given `sub_metrics` as `<name>_<sub_metric>` which will get updated by the
+    callbacks.
+
+    Args:
+        name (str)
+        desc (str)
+        labels (list[str])
+        sub_metrics (list[str]): A list of sub metrics that the callbacks
+            will update.
+    """
+
+    def __init__(self, name, desc, labels, sub_metrics):
+        self.name = name
+        self.desc = desc
+        self.labels = labels
+        self.sub_metrics = sub_metrics
+
+        # Create a class which have the sub_metrics values as attributes, which
+        # default to 0 on initialization. Used to pass to registered callbacks.
+        self._metrics_class = attr.make_class(
+            "_MetricsEntry",
+            attrs={x: attr.ib(0) for x in sub_metrics},
+            slots=True,
+        )
+
+        # Counts number of in flight blocks for a given set of label values
+        self._registrations = {}
+
+        # Protects access to _registrations
+        self._lock = threading.Lock()
+
+        self._register_with_collector()
+
+    def register(self, key, callback):
+        """Registers that we've entered a new block with labels `key`.
+
+        `callback` gets called each time the metrics are collected. The same
+        value must also be given to `unregister`.
+
+        `callback` gets called with an object that has an attribute per
+        sub_metric, which should be updated with the necessary values. Note that
+        the metrics object is shared between all callbacks registered with the
+        same key.
+
+        Note that `callback` may be called on a separate thread.
+        """
+        with self._lock:
+            self._registrations.setdefault(key, set()).add(callback)
+
+    def unregister(self, key, callback):
+        """Registers that we've exited a block with labels `key`.
+        """
+
+        with self._lock:
+            self._registrations.setdefault(key, set()).discard(callback)
+
+    def collect(self):
+        """Called by prometheus client when it reads metrics.
+
+        Note: may be called by a separate thread.
+        """
+        in_flight = GaugeMetricFamily(self.name + "_total", self.desc, labels=self.labels)
+
+        metrics_by_key = {}
+
+        # We copy so that we don't mutate the list while iterating
+        with self._lock:
+            keys = list(self._registrations)
+
+        for key in keys:
+            with self._lock:
+                callbacks = set(self._registrations[key])
+
+            in_flight.add_metric(key, len(callbacks))
+
+            metrics = self._metrics_class()
+            metrics_by_key[key] = metrics
+            for callback in callbacks:
+                callback(metrics)
+
+        yield in_flight
+
+        for name in self.sub_metrics:
+            gauge = GaugeMetricFamily("_".join([self.name, name]), "", labels=self.labels)
+            for key, metrics in six.iteritems(metrics_by_key):
+                gauge.add_metric(key, getattr(metrics, name))
+            yield gauge
+
+    def _register_with_collector(self):
+        if self.name in all_gauges.keys():
+            logger.warning("%s already registered, reregistering" % (self.name,))
+            REGISTRY.unregister(all_gauges.pop(self.name))
+
+        REGISTRY.register(self)
+        all_gauges[self.name] = self
+
+
 #
 # Detailed CPU metrics
 #
diff --git a/synapse/util/metrics.py b/synapse/util/metrics.py
index 97f1267380bf..4b4ac5f6c7d8 100644
--- a/synapse/util/metrics.py
+++ b/synapse/util/metrics.py
@@ -20,6 +20,7 @@
 
 from twisted.internet import defer
 
+from synapse.metrics import InFlightGauge
 from synapse.util.logcontext import LoggingContext
 
 logger = logging.getLogger(__name__)
@@ -45,6 +46,13 @@
 block_db_sched_duration = Counter(
     "synapse_util_metrics_block_db_sched_duration_seconds", "", ["block_name"])
 
+# Tracks the number of blocks currently active
+in_flight = InFlightGauge(
+    "synapse_util_metrics_block_in_flight", "",
+    labels=["block_name"],
+    sub_metrics=["real_time_max", "real_time_sum"],
+)
+
 
 def measure_func(name):
     def wrapper(func):
@@ -82,10 +90,14 @@ def __enter__(self):
 
         self.start_usage = self.start_context.get_resource_usage()
 
+        in_flight.register((self.name,), self._update_in_flight)
+
     def __exit__(self, exc_type, exc_val, exc_tb):
         if isinstance(exc_type, Exception) or not self.start_context:
             return
 
+        in_flight.unregister((self.name,), self._update_in_flight)
+
         duration = self.clock.time() - self.start
 
         block_counter.labels(self.name).inc()
@@ -120,3 +132,13 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
         if self.created_context:
             self.start_context.__exit__(exc_type, exc_val, exc_tb)
+
+    def _update_in_flight(self, metrics):
+        """Gets called when processing in flight metrics
+        """
+        duration = self.clock.time() - self.start
+
+        metrics.real_time_max = max(metrics.real_time_max, duration)
+        metrics.real_time_sum += duration
+
+        # TODO: Add other in flight metrics.
diff --git a/tests/test_metrics.py b/tests/test_metrics.py
new file mode 100644
index 000000000000..17897711a14b
--- /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