diff --git a/geodepy/convert.py b/geodepy/convert.py
index 30ffd63..251f5a4 100644
--- a/geodepy/convert.py
+++ b/geodepy/convert.py
@@ -399,8 +399,14 @@ def geo2grid(lat, lon, zone=0, ellipsoid=grs80, prj=utm):
             amgzone = int(amgzone)
             zone = int(f"{amgzone}{subzone}")
         else:
-            zone = int(
-                (float(lon) - (prj.initialcm - (1.5 * prj.zonewidth))) / prj.zonewidth
+            # Floating-point rounding at +180 (or immediately below it)
+            # can otherwise select zone 61, outside the UTM 1-60 range.
+            zone = min(
+                60,
+                int(
+                    (float(lon) - (prj.initialcm - (1.5 * prj.zonewidth)))
+                    / prj.zonewidth
+                ),
             )
     if prj == isg:
         amgzone = int(str(zone)[:2])
diff --git a/geodepy/tests/test_utm_antimeridian_zone.py b/geodepy/tests/test_utm_antimeridian_zone.py
new file mode 100644
index 0000000..04cb101
--- /dev/null
+++ b/geodepy/tests/test_utm_antimeridian_zone.py
@@ -0,0 +1,52 @@
+"""Regression tests for the UTM zone limit at positive 180 degrees."""
+
+import math
+
+import pytest
+
+from geodepy.convert import geo2grid, grid2geo
+
+
+@pytest.mark.parametrize("latitude", [-75.0, -35.0, 0.0, 35.0, 75.0])
+@pytest.mark.parametrize("longitude", [math.nextafter(180.0, -math.inf), 180.0])
+def test_positive_antimeridian_selects_zone_60(latitude, longitude):
+    # The same location using an explicit valid zone already works.
+    actual = geo2grid(latitude, longitude)
+    assert actual == geo2grid(latitude, longitude, zone=60)
+    assert actual[1] == 60
+
+
+@pytest.mark.parametrize("latitude", [-75.0, -35.0, 0.0, 35.0, 75.0])
+@pytest.mark.parametrize("longitude", [-180.0, math.nextafter(-180.0, math.inf)])
+def test_negative_antimeridian_remains_zone_1(latitude, longitude):
+    actual = geo2grid(latitude, longitude)
+    assert actual == geo2grid(latitude, longitude, zone=1)
+    assert actual[1] == 1
+
+
+@pytest.mark.parametrize("latitude", [-35.0, 0.0, 35.0])
+@pytest.mark.parametrize("longitude", [179.9, -179.9, 174.5, -174.5, 0.0])
+def test_unaffected_locations_keep_their_zone(latitude, longitude):
+    zone = int((longitude + 180.0) // 6.0) + 1
+    assert geo2grid(latitude, longitude) == geo2grid(
+        latitude, longitude, zone=zone
+    )
+
+
+@pytest.mark.parametrize("latitude", [-35.0, 0.0, 35.0])
+@pytest.mark.parametrize(
+    "longitude", [180.0, math.nextafter(180.0, -math.inf), -180.0]
+)
+def test_antimeridian_grid_round_trip(latitude, longitude):
+    hemisphere, zone, east, north, *_ = geo2grid(latitude, longitude)
+    lat2, lon2, *_ = grid2geo(zone, east, north, hemisphere=hemisphere)
+    assert abs(lat2 - latitude) < 1e-8
+    longitude_error = ((lon2 - longitude + 180.0) % 360.0) - 180.0
+    assert abs(longitude_error) < 1e-8
+
+
+def test_out_of_range_longitudes_remain_invalid():
+    with pytest.raises(ValueError, match="Invalid Longitude"):
+        geo2grid(0.0, math.nextafter(180.0, math.inf))
+    with pytest.raises(ValueError, match="Invalid Longitude"):
+        geo2grid(0.0, math.nextafter(-180.0, -math.inf))