diff --git a/src/fsize/__init__.py b/src/fsize/__init__.py
index be419c8..21df579 100644
--- a/src/fsize/__init__.py
+++ b/src/fsize/__init__.py
@@ -200,7 +200,7 @@ def __format__(self, format_spec: str) -> str:
             raise AssertionError(f"unhandled unit: {unit!r}")
         n = self.real / self._convert ** _UNIT_POWERS[unit]
 
-        log_digits = math.ceil(math.log10(n)) if n > 0 else 0
+        log_digits = math.floor(math.log10(n)) + 1 if n > 0 else 0
         out_format_spec = (
             f"{fill}{align}{width}{grouping}"
             + "."
diff --git a/tests/test_fsize_init.py b/tests/test_fsize_init.py
index c2646d1..6379031 100755
--- a/tests/test_fsize_init.py
+++ b/tests/test_fsize_init.py
@@ -442,6 +442,13 @@ def test_format_empty_spec():
     assert format(z, "") == str(z)
 
 
+def test_format_no_scientific_notation():
+    """Test that exact powers of 10 do not produce scientific notation."""
+    assert format(FSize(10 * 1024), "K") == "10"
+    assert format(FSize(100 * 1024), "K") == "100"
+    assert format(FSize(1000 * 1024), "K") == "1000"
+
+
 def test_format_all_units():
     """Test that every unit in _UNIT_POWERS works via format."""
     # Binary FSize (1 EiB)