diff --git a/src/human_readable/files.py b/src/human_readable/files.py
index b28376b..13d1a0e 100644
--- a/src/human_readable/files.py
+++ b/src/human_readable/files.py
@@ -61,5 +61,10 @@ def file_size(
     for i, suffix in enumerate(suffixes):
         unit = base ** (i + 2)
         if byte_size < unit:
-            return f"{base * byte_size / unit:{formatting}}{suffix}"
+            formatted = f"{base * byte_size / unit:{formatting}}"
+            # Rounding can push the mantissa to base (e.g. 999,999 bytes is
+            # 999.999 KB, which rounds to "1000.0 KB"). Step up one suffix.
+            if float(formatted) >= base and i + 1 < len(suffixes):
+                return f"{byte_size / unit:{formatting}}{suffixes[i + 1]}"
+            return f"{formatted}{suffix}"
     return f"{base * byte_size / unit:{formatting}}{suffix}"
diff --git a/tests/unit/test_files.py b/tests/unit/test_files.py
index 4c6ee6e..b173448 100644
--- a/tests/unit/test_files.py
+++ b/tests/unit/test_files.py
@@ -15,6 +15,10 @@
         (2900000, "2.9 MB"),  # millions number
         (2000000000, "2.0 GB"),  # billions number
         (10**26 * 30, "3000.0 YB"),  # giant number
+        # Rollover: mantissa rounds up to base, must carry to the next suffix
+        (999999, "1.0 MB"),
+        (999999999, "1.0 GB"),
+        (999999999999, "1.0 TB"),
     ],
 )
 def test_file_size(params: int, expected: str) -> None: