diff --git a/setup.py b/setup.py
index 0a23c6d..d945d6c 100644
--- a/setup.py
+++ b/setup.py
@@ -33,6 +33,7 @@ setup(name="magic-wormhole",
           "autobahn[twisted] >= 0.14.1",
           "hkdf", "tqdm",
           "click",
+          "humanize",
       ],
       extras_require={
           ':sys_platform=="win32"': ["pypiwin32"],
diff --git a/src/wormhole/cli/cmd_receive.py b/src/wormhole/cli/cmd_receive.py
index 89caba2..c759ed0 100644
--- a/src/wormhole/cli/cmd_receive.py
+++ b/src/wormhole/cli/cmd_receive.py
@@ -1,6 +1,7 @@
 from __future__ import print_function
 import os, sys, six, tempfile, zipfile, hashlib
 from tqdm import tqdm
+from humanize import naturalsize
 from twisted.internet import reactor
 from twisted.internet.defer import inlineCallbacks, returnValue
 from twisted.python import log
@@ -194,8 +195,8 @@ class TwistedReceiver:
                                                   file_data["filename"])
         self.xfersize = file_data["filesize"]
 
-        self._msg(u"Receiving file (%d bytes) into: %s" %
-                  (self.xfersize, os.path.basename(self.abs_destname)))
+        self._msg(u"Receiving file (%s) into: %s" %
+                  (naturalsize(self.xfersize), os.path.basename(self.abs_destname)))
         self._ask_permission()
         tmp_destname = self.abs_destname + ".tmp"
         return open(tmp_destname, "wb")
@@ -210,10 +211,10 @@ class TwistedReceiver:
                                                   file_data["dirname"])
         self.xfersize = file_data["zipsize"]
 
-        self._msg(u"Receiving directory (%d bytes) into: %s/" %
-                  (self.xfersize, os.path.basename(self.abs_destname)))
-        self._msg(u"%d files, %d bytes (uncompressed)" %
-                  (file_data["numfiles"], file_data["numbytes"]))
+        self._msg(u"Receiving directory (%s) into: %s/" %
+                  (naturalsize(self.xfersize), os.path.basename(self.abs_destname)))
+        self._msg(u"%d files, %s (uncompressed)" %
+                  (file_data["numfiles"], naturalsize(file_data["numbytes"])))
         self._ask_permission()
         return tempfile.SpooledTemporaryFile()
 
diff --git a/src/wormhole/cli/cmd_send.py b/src/wormhole/cli/cmd_send.py
index a610b36..6fa7446 100644
--- a/src/wormhole/cli/cmd_send.py
+++ b/src/wormhole/cli/cmd_send.py
@@ -1,6 +1,7 @@
 from __future__ import print_function
 import os, sys, six, tempfile, zipfile, hashlib
 from tqdm import tqdm
+from humanize import naturalsize
 from twisted.python import log
 from twisted.protocols import basic
 from twisted.internet import reactor
@@ -167,7 +168,7 @@ class Sender:
             text = six.moves.input("Text to send: ")
 
         if text is not None:
-            print(u"Sending text message (%d bytes)" % len(text),
+            print(u"Sending text message (%s)" % naturalsize(len(text)),
                   file=args.stdout)
             offer = { "message": text }
             fd_to_send = None
@@ -187,7 +188,8 @@ class Sender:
                 "filename": basename,
                 "filesize": filesize,
                 }
-            print(u"Sending %d byte file named '%s'" % (filesize, basename),
+            print(u"Sending %s file named '%s'"
+                  % (naturalsize(filesize), basename),
                   file=args.stdout)
             fd_to_send = open(what, "rb")
             return offer, fd_to_send
@@ -222,8 +224,8 @@ class Sender:
                 "numbytes": num_bytes,
                 "numfiles": num_files,
                 }
-            print(u"Sending directory (%d bytes compressed) named '%s'"
-                  % (filesize, basename), file=args.stdout)
+            print(u"Sending directory (%s compressed) named '%s'"
+                  % (naturalsize(filesize), basename), file=args.stdout)
             return offer, fd_to_send
 
         raise TypeError("'%s' is neither file nor directory" % args.what)
diff --git a/src/wormhole/server/cmd_usage.py b/src/wormhole/server/cmd_usage.py
index c314313..227a220 100644
--- a/src/wormhole/server/cmd_usage.py
+++ b/src/wormhole/server/cmd_usage.py
@@ -2,6 +2,7 @@ from __future__ import print_function, unicode_literals
 import os, time, json
 from collections import defaultdict
 import click
+from humanize import naturalsize
 from .database import get_db
 
 def abbrev(t):
@@ -13,31 +14,6 @@ def abbrev(t):
         return "%.1fms" % (t*1e3)
     return "%.1fus" % (t*1e6)
 
-def abbreviate_space(s, SI=True):
-    if s is None:
-        return "-"
-    if SI:
-        U = 1000.0
-        isuffix = "B"
-    else:
-        U = 1024.0
-        isuffix = "iB"
-    def r(count, suffix):
-        return "%.2f %s%s" % (count, suffix, isuffix)
-
-    if s < 1024: # 1000-1023 get emitted as bytes, even in SI mode
-        return "%d B" % s
-    if s < U*U:
-        return r(s/U, "k")
-    if s < U*U*U:
-        return r(s/(U*U), "M")
-    if s < U*U*U*U:
-        return r(s/(U*U*U), "G")
-    if s < U*U*U*U*U:
-        return r(s/(U*U*U*U), "T")
-    if s < U*U*U*U*U*U:
-        return r(s/(U*U*U*U*U), "P")
-    return r(s/(U*U*U*U*U*U), "E")
 
 def print_event(event):
     event_type, started, result, total_bytes, waiting_time, total_time = event
@@ -49,7 +25,7 @@ def print_event(event):
            abbrev(total_time),
            abbrev(waiting_time),
            abbrev(followthrough),
-           abbreviate_space(total_bytes),
+           naturalsize(total_bytes),
            time.ctime(started),
           ))
 
@@ -108,8 +84,8 @@ def show_usage(args):
         print(" %d events in %s (%.2f per hour)" % (total, abbrev(elapsed),
                                                     (3600 * total / elapsed)))
         rate = total_transit_bytes / elapsed
-        print(" %s total bytes, %sps" % (abbreviate_space(total_transit_bytes),
-                                         abbreviate_space(rate)))
+        print(" %s total bytes, %sps" % (naturalsize(total_transit_bytes),
+                                         naturalsize(rate)))
         print("", ", ".join(["%s=%d (%d%%)" %
                              (k, counters[k], (100.0 * counters[k] / total))
                              for k in sorted(counters)
diff --git a/src/wormhole/test/test_scripts.py b/src/wormhole/test/test_scripts.py
index 3a45b0d..ea6c4fc 100644
--- a/src/wormhole/test/test_scripts.py
+++ b/src/wormhole/test/test_scripts.py
@@ -1,5 +1,6 @@
 from __future__ import print_function, unicode_literals
 import os, sys, re, io, zipfile, six, stat
+from humanize import naturalsize
 import mock
 from twisted.trial import unittest
 from twisted.python import procutils, log
@@ -367,7 +368,7 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
 
         # check sender
         if mode == "text":
-            expected = ("Sending text message ({bytes:d} bytes){NL}"
+            expected = ("Sending text message ({bytes:d} Bytes){NL}"
                         "On the other computer, please run: "
                         "wormhole receive{NL}"
                         "Wormhole code is: {code}{NL}{NL}"
@@ -376,8 +377,9 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
                                                         NL=NL)
             self.failUnlessEqual(send_stdout, expected)
         elif mode == "file":
-            self.failUnlessIn("Sending {bytes:d} byte file named '{name}'{NL}"
-                              .format(bytes=len(message), name=send_filename,
+            self.failUnlessIn("Sending {size:s} file named '{name}'{NL}"
+                              .format(size=naturalsize(len(message)),
+                                      name=send_filename,
                                       NL=NL), send_stdout)
             self.failUnlessIn("On the other computer, please run: "
                               "wormhole receive{NL}"
@@ -402,8 +404,8 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
         if mode == "text":
             self.failUnlessEqual(receive_stdout, message+NL)
         elif mode == "file":
-            self.failUnlessIn("Receiving file ({bytes:d} bytes) into: {name}"
-                              .format(bytes=len(message),
+            self.failUnlessIn("Receiving file ({size:s}) into: {name}"
+                              .format(size=naturalsize(len(message)),
                                       name=receive_filename), receive_stdout)
             self.failUnlessIn("Received file written to ", receive_stdout)
             fn = os.path.join(receive_dir, receive_filename)
@@ -411,7 +413,7 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
             with open(fn, "r") as f:
                 self.failUnlessEqual(f.read(), message)
         elif mode == "directory":
-            want = (r"Receiving directory \(\d+ bytes\) into: {name}/"
+            want = (r"Receiving directory \(\d+ \w+\) into: {name}/"
                     .format(name=receive_dirname))
             self.failUnless(re.search(want, receive_stdout),
                             (want, receive_stdout))
@@ -511,8 +513,9 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
                              (receive_stdout, receive_stderr))
 
         # check sender
-        self.failUnlessIn("Sending {bytes:d} byte file named '{name}'{NL}"
-                          .format(bytes=len(message), name=send_filename,
+        self.failUnlessIn("Sending {size:s} file named '{name}'{NL}"
+                          .format(size=naturalsize(len(message)),
+                                  name=send_filename,
                                   NL=NL), send_stdout)
         self.failUnlessIn("On the other computer, please run: "
                           "wormhole receive{NL}"