diff --git a/voussoirkit/ratemeter.py b/voussoirkit/ratemeter.py
index 4b4b58d..5abbd3c 100644
--- a/voussoirkit/ratemeter.py
+++ b/voussoirkit/ratemeter.py
@@ -1,5 +1,5 @@
 import collections
-import math
+import threading
 import time
 
 class RateMeter:
@@ -9,17 +9,18 @@ class RateMeter:
         units per second over `span` seconds.
 
         Set `span` to None to calculate unit/s over the lifetime of the object
-        after the first digest, rather than over a span.
-        This saves the effort of tracking timestamps. Don't just use a large number!
+        after the first digest, rather than over a span. This saves the effort
+        of tracking timestamps; so don't just use a large number!
         '''
         self.sum = 0
         self.span = span
 
+        self.lock = threading.Lock()
         self.tracking = collections.deque()
         self.first_digest = None
 
-    def digest(self, value):
-        now = time.time()
+    def _digest(self, value):
+        now = time.monotonic()
         self.sum += value
 
         if self.span is None:
@@ -27,8 +28,8 @@ class RateMeter:
                 self.first_digest = now
             return
 
-        earlier = now - self.span
-        while len(self.tracking) > 0 and self.tracking[0][0] < earlier:
+        expire_cutoff = now - self.span
+        while len(self.tracking) > 0 and self.tracking[0][0] < expire_cutoff:
             (timestamp, pop_value) = self.tracking.popleft()
             self.sum -= pop_value
 
@@ -37,19 +38,16 @@ class RateMeter:
         else:
             self.tracking[-1][1] += value
 
-    def report(self):
-        '''
-        Return a tuple containing the running sum, the time span
-        over which the rate is being calculated, and the rate in
-        units per second.
+    def digest(self, value):
+        with self.lock:
+            return self._digest(value)
 
-        (sum, time_interval, rate)
-        '''
+    def _report(self):
         # Flush the old values, ensure self.first_digest exists.
-        self.digest(0)
+        self._digest(0)
 
         if self.span is None:
-            now = math.ceil(time.time())
+            now = time.monotonic()
             time_interval = now - self.first_digest
         else:
             # No risk of IndexError because the digest(0) ensures we have
@@ -58,7 +56,16 @@ class RateMeter:
 
         if time_interval == 0:
             return (self.sum, 0, self.sum)
+
         rate = self.sum / time_interval
-        time_interval = round(time_interval, 3)
-        rate = round(rate, 3)
         return (self.sum, time_interval, rate)
+
+    def report(self):
+        '''
+        Return a tuple containing the running sum, the time span over which the
+        rate has been calculated, and the rate in units per second.
+
+        (sum, time_interval, rate)
+        '''
+        with self.lock:
+            return self._report()