diff --git a/fs/filesize.py b/fs/filesize.py
index ed113e88..ce2d2523 100644
--- a/fs/filesize.py
+++ b/fs/filesize.py
@@ -33,12 +33,20 @@ def _to_str(size, suffixes, base):
     elif size < base:
         return "{:,} bytes".format(size)
 
-    # TODO (dargueta): Don't rely on unit or suffix being defined in the loop.
+    suffixes = list(suffixes)
     for i, suffix in enumerate(suffixes, 2):  # noqa: B007
         unit = base**i
         if size < unit:
             break
-    return "{:,.1f} {}".format((base * size / unit), suffix)
+
+    mantissa = base * size / unit
+    # Rounding can push the mantissa to `base` (e.g. 999,999 B is 999.999 kB,
+    # which formats as "1,000.0 kB"). Carry into the next suffix instead.
+    if round(mantissa, 1) >= base and i - 1 < len(suffixes):
+        suffix = suffixes[i - 1]
+        mantissa = size / unit
+
+    return "{:,.1f} {}".format(mantissa, suffix)
 
 
 def traditional(size):
diff --git a/tests/test_filesize.py b/tests/test_filesize.py
index 8900f671..9c983425 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -45,6 +45,21 @@ def test_decimal(self):
 
         self.assertEqual(filesize.decimal(1200 * 1000), "1.2 MB")
 
+    def test_rollover_decimal(self):
+        # Mantissa rounds up to base — must carry to the next suffix
+        self.assertEqual(filesize.decimal(999_999), "1.0 MB")
+        self.assertEqual(filesize.decimal(999_999_999), "1.0 GB")
+        self.assertEqual(filesize.decimal(999_999_999_999), "1.0 TB")
+
+    def test_rollover_traditional(self):
+        # 1024**2 - 1 bytes is 1023.999 KB, which rounds to 1,024.0 KB
+        self.assertEqual(filesize.traditional(1024**2 - 1), "1.0 MB")
+        self.assertEqual(filesize.traditional(1024**3 - 1), "1.0 GB")
+
+    def test_rollover_binary(self):
+        self.assertEqual(filesize.binary(1024**2 - 1), "1.0 MiB")
+        self.assertEqual(filesize.binary(1024**3 - 1), "1.0 GiB")
+
     def test_errors(self):
 
         with self.assertRaises(TypeError):