From af93d314ef7825d316ab54c3f6bda0f93977258e Mon Sep 17 00:00:00 2001
From: Mayur Dhavan <118877752+mayur-dhavan@users.noreply.github.com>
Date: Mon, 26 Oct 2026 00:11:33 +0530
Subject: [PATCH 1/2] Handle metric rounding carry at the quetta boundary

Remove unnecessary condition for rounding carry in number formatting.
---
 src/humanize/number.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

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))

From b32f1f6a0ba247b5ca9e083cd5d21a2e78b8d9b1 Mon Sep 17 00:00:00 2001
From: Mayur Dhavan <118877752+mayur-dhavan@users.noreply.github.com>
Date: Mon, 26 Oct 2026 00:11:58 +0530
Subject: [PATCH 2/2] Add metric regression tests for the quetta rounding
 boundary

Add test cases for rounding carry at quetta boundary.
---
 tests/test_number.py | 6 ++++++
 1 file changed, 6 insertions(+)

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"),