diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index fb675fdc..41101884 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -4,6 +4,7 @@
 
 __lazy_modules__ = {"humanize.i18n", "math"}
 
+import re
 from math import log
 
 from humanize.i18n import _gettext as _
@@ -36,6 +37,10 @@
     "gnu": "KMGTPEZYRQ",
 }
 
+# Matches the numeric part of a rendered mantissa, ignoring any surrounding
+# text a custom `format` may add (e.g. "%.1f~" renders "976.6~").
+_MANTISSA_RE = re.compile(r"[-+]?(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][-+]?\d+)?")
+
 
 def naturalsize(
     value: float | str,
@@ -103,7 +108,13 @@ 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 extract the numeric
+    # mantissa from the rendered string before comparing it with `base`: parsing
+    # the whole string raises ValueError, and comparing rendered strings
+    # misjudges formats that do not round-trip through str -> float (e.g.
+    # "%.0e" % 1024 is "1e+03").
+    mantissa = _MANTISSA_RE.search(format % (abs_bytes / (base**exp)))
+    if exp < len(suffix) and mantissa and abs(float(mantissa.group())) >= 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 e6956399..ef212b42 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -91,6 +91,14 @@
         ([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"),
+        # The carry-over check must compare the numeric mantissa, not rendered
+        # strings: "%.0e" renders 1024 as "1e+03", which a string comparison
+        # misreads as already at the base and steps up one unit too early.
+        ([1048575, True, False, "%.0e"], "1e+03 KiB"),
     ],
 )
 def test_naturalsize(test_args: list[int] | list[int | bool], expected: str) -> None: