diff --git a/src/humanize/number.py b/src/humanize/number.py
index 52a5356a..b1e62f18 100644
--- a/src/humanize/number.py
+++ b/src/humanize/number.py
@@ -578,12 +578,16 @@ def metric(value: float, unit: str = "", precision: int = 3) -> str:
     if exponent >= 33 or exponent < -30:
         return scientific(value, precision - 1) + unit
 
+    original = value
     old_bucket = exponent // 3 * 3
     value /= 10**old_bucket
     digits = int(max(0, precision - exponent % 3 - 1))
     # Absorb a rounding carry (9.999 -> 10.0) by bumping the exponent.
-    if exponent < 30 and round(abs(value), digits) >= 10 ** (exponent % 3 + 1):
+    if round(abs(value), digits) >= 10 ** (exponent % 3 + 1):
         exponent += 1
+        if exponent >= 33:
+            # Rounded past quetta: no larger prefix exists.
+            return scientific(original, precision - 1) + unit
         new_bucket = exponent // 3 * 3
         value /= 10 ** (new_bucket - old_bucket)
         digits = int(max(0, precision - exponent % 3 - 1))
diff --git a/tests/test_number.py b/tests/test_number.py
index 5fb12fa6..d940f8f1 100644
--- a/tests/test_number.py
+++ b/tests/test_number.py
@@ -332,6 +332,12 @@ def test_clamp(test_args: list[typing.Any], expected: str) -> None:
         ([1.235e29, "A"], "124 RA"),
         ([2.56e-30, "V"], "2.56 qV"),
         ([2.596e32, "F"], "260 QF"),
+        # Rounding carry at the quetta boundary (issue #423)
+        ([9.996e30], "10.0 Q"),
+        ([9.996e31], "100 Q"),
+        ([9.996e32], "1.00 x 10\u00b3\u00b3"),
+        ([9.999e-30], "10.0 q"),
+        ([-9.996e31], "-100 Q"),
         ([1e50], "1.00 x 10⁵⁰"),
         ([1e-50], "1.00 x 10⁻⁵⁰"),
         ([-1500, "V"], "-1.50 kV"),