diff --git a/subiquity/models/filesystem.py b/subiquity/models/filesystem.py
index b31655c3..a0119949 100644
--- a/subiquity/models/filesystem.py
+++ b/subiquity/models/filesystem.py
@@ -34,11 +34,14 @@ class FS:
 
 
 def humanize_size(size):
-    size = abs(size)
     if size == 0:
         return "0B"
-    p = math.floor(math.log(size, 2) / 10)
-    return "%.3f%s" % (size / math.pow(1024, p), HUMAN_UNITS[int(p)])
+    p = int(math.floor(math.log(size, 2) / 10))
+    # We want to truncate the non-integral part, not round to nearest.
+    s = "{:.17f}".format(size / 2**(10*p))
+    i = s.index('.')
+    s = s[:i+4]
+    return s + HUMAN_UNITS[int(p)]
 
 
 def dehumanize_size(size):
diff --git a/subiquity/models/tests/__init__.py b/subiquity/models/tests/__init__.py
new file mode 100644
index 00000000..792d6005
--- /dev/null
+++ b/subiquity/models/tests/__init__.py
@@ -0,0 +1 @@
+#
diff --git a/subiquity/models/tests/test_filesystem.py b/subiquity/models/tests/test_filesystem.py
index 43181520..b2a9be89 100644
--- a/subiquity/models/tests/test_filesystem.py
+++ b/subiquity/models/tests/test_filesystem.py
@@ -1,7 +1,23 @@
 
 import unittest
 
-from subiquity.models.filesystem import dehumanize_size
+from subiquity.models.filesystem import dehumanize_size, humanize_size
+
+class TestHumanizeSize(unittest.TestCase):
+
+
+    basics = [
+        ('1.000M', 2**20),
+        ('1.500M', 2**20+2**19),
+        ('1.500M', 2**20+2**19),
+        ('1023.000M', 1023*2**20),
+        ('1.000G', 1024*2**20),
+        ]
+
+    def test_basics(self):
+        for string, integer in self.basics:
+            with self.subTest(input=string):
+                self.assertEqual(string, humanize_size(integer))
 
 class TestDehumanizeSize(unittest.TestCase):