diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index fb675fd..35fb59c 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -103,7 +103,9 @@ def naturalsize(
     # mantissa afterward; rounding can push it up to `base` (e.g. 999999 is
     # 999.999 kB, which formats to "1000.0 kB"). When that happens and a larger
     # suffix is available, step up one suffix so the result reads "1.0 MB".
-    if exp < len(suffix) and abs(float(format % (abs_bytes / (base**exp)))) >= base:
+    # `format` may contain text around the conversion, so compare the rendered
+    # mantissa with the rendered base instead of parsing it back to a float.
+    if exp < len(suffix) and format % (abs_bytes / (base**exp)) == format % base:
         exp += 1
     space = "" if gnu else " "
     ret: str = format % (bytes_ / (base**exp)) + space + _(suffix[exp - 1])
diff --git a/tests/test_filesize.py b/tests/test_filesize.py
index e695639..fc1ceef 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -91,6 +91,10 @@
         ([1024**2 - 1, True], "1.0 MiB"),
         ([1024**3 - 1, True], "1.0 GiB"),
         ([1024**2 - 1, False, True], "1.0M"),
+        # A custom format may contain text around the numeric conversion, which
+        # must not break the rounding carry-over check above.
+        ([999999, False, True, "%.1f~"], "976.6~K"),
+        ([999999, False, False, "%.1f~"], "1.0~ MB"),
     ],
 )
 def test_naturalsize(test_args: list[int] | list[int | bool], expected: str) -> None: