diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index a730abd59b..ab26c2376a 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -2,35 +2,64 @@ import collections
 
 
 class Ratelimiter(object):
+    """
+    Ratelimit message sending by user.
+    """
 
     def __init__(self):
         self.message_counts = collections.OrderedDict()
 
-    def prune_message_counts(self, time_now):
-        for user_id in self.message_counts.keys():
-            message_count, time_start, msg_rate_hz = (
-                self.message_counts[user_id]
-            )
-            time_delta = time_now - time_start
-            if message_count - time_delta * msg_rate_hz > 0:
-                break
-            else:
-                del self.message_counts[user_id]
-
-    def send_message(self, user_id, time_now, msg_rate_hz, burst_count):
-        self.prune_message_counts(time_now)
+    def send_message(self, user_id, time_now_s, msg_rate_hz, burst_count):
+        """Can the user send a message?
+        Args:
+            user_id: The user sending a message.
+            time_now_s: The time now.
+            msg_rate_hz: The long term number of messages a user can send in a
+                second.
+            burst_count: How many messages the user can send before being
+                limited.
+        Returns:
+            A pair of a bool indicating if they can send a message now and a
+                time in seconds of when they can next send a message.
+        """
+        self.prune_message_counts(time_now_s)
         message_count, time_start, _ignored = self.message_counts.pop(
-            user_id, (0., time_now, None),
+            user_id, (0., time_now_s, None),
         )
-        time_delta = time_now - time_start
-        if message_count - time_delta * msg_rate_hz < 0:
-            a
-        if message_count - (time_now - time_start) * msg_rate_hz > burst_count:
+        time_delta = time_now_s - time_start
+        sent_count = message_count - time_delta * msg_rate_hz
+        if sent_count < 0:
+            allowed = True
+            time_start = time_now_s
+            messagecount = 1.
+        elif sent_count > burst_count - 1.:
             allowed = False
         else:
             allowed = True
             message_count += 1
+
         self.message_counts[user_id] = (
             message_count, time_start, msg_rate_hz
         )
-        return allowed
+
+        if msg_rate_hz > 0:
+            time_allowed = (
+                time_start + (message_count - burst_count + 1) / msg_rate_hz
+            )
+            if time_allowed < time_now_s:
+                time_allowed = time_now_s
+        else:
+            time_allowed = -1
+
+        return allowed, time_allowed
+
+    def prune_message_counts(self, time_now_s):
+        for user_id in self.message_counts.keys():
+            message_count, time_start, msg_rate_hz = (
+                self.message_counts[user_id]
+            )
+            time_delta = time_now_s - time_start
+            if message_count - time_delta * msg_rate_hz > 0:
+                break
+            else:
+                del self.message_counts[user_id]
diff --git a/tests/api/__init__.py b/tests/api/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/tests/api/test_ratelimiting.py b/tests/api/test_ratelimiting.py
new file mode 100644
index 0000000000..dc2f83c7eb
--- /dev/null
+++ b/tests/api/test_ratelimiting.py
@@ -0,0 +1,39 @@
+from synapse.api.ratelimiting import Ratelimiter
+
+import unittest
+
+class TestRatelimiter(unittest.TestCase):
+
+    def test_allowed(self):
+        limiter = Ratelimiter()
+        allowed, time_allowed = limiter.send_message(
+            user_id="test_id", time_now_s=0, msg_rate_hz=0.1, burst_count=1,
+        )
+        self.assertTrue(allowed)
+        self.assertEquals(10., time_allowed)
+
+        allowed, time_allowed = limiter.send_message(
+            user_id="test_id", time_now_s=5, msg_rate_hz=0.1, burst_count=1,
+        )
+        self.assertFalse(allowed)
+        self.assertEquals(10., time_allowed)
+
+        allowed, time_allowed = limiter.send_message(
+            user_id="test_id", time_now_s=10, msg_rate_hz=0.1, burst_count=1
+        )
+        self.assertTrue(allowed)
+        self.assertEquals(20., time_allowed)
+
+    def test_pruning(self):
+        limiter = Ratelimiter()
+        allowed, time_allowed = limiter.send_message(
+            user_id="test_id_1", time_now_s=0, msg_rate_hz=0.1, burst_count=1,
+        )
+
+        self.assertIn("test_id_1", limiter.message_counts)
+
+        allowed, time_allowed = limiter.send_message(
+            user_id="test_id_2", time_now_s=10, msg_rate_hz=0.1, burst_count=1
+        )
+
+        self.assertNotIn("test_id_1", limiter.message_counts)