diff --git a/src/pygui/backend/wafer/__init__.py b/src/pygui/backend/wafer/__init__.py
index d662db0..0c098d5 100644
--- a/src/pygui/backend/wafer/__init__.py
+++ b/src/pygui/backend/wafer/__init__.py
@@ -1,4 +1,5 @@
 # ===== Wafer Sub-package =====
+from pygui.backend.wafer.family_spec import sensor_count_for
 from pygui.backend.wafer.wafer_layouts import available_families, load_layout
 from pygui.backend.wafer.zwafer_models import DataRecord, Sensor, ZWaferData
 from pygui.backend.wafer.zwafer_parser import ZWaferParser
@@ -7,4 +8,5 @@ __all__ = [
     "ZWaferData", "Sensor", "DataRecord",
     "ZWaferParser",
     "load_layout", "available_families",
+    "sensor_count_for",
 ]
diff --git a/src/pygui/backend/wafer/family_spec.py b/src/pygui/backend/wafer/family_spec.py
new file mode 100644
index 0000000..18b9a17
--- /dev/null
+++ b/src/pygui/backend/wafer/family_spec.py
@@ -0,0 +1,9 @@
+"""Single source of truth for the wafer sensor-count partition."""
+
+
+def sensor_count_for(family_code: str) -> int:
+    """Return the number of valid sensor readings for a wafer family code.
+
+    Returns 80 for the "X" family. Returns 244 for every other family
+    code."""
+    return 80 if family_code == "X" else 244
diff --git a/tests/test_family_spec.py b/tests/test_family_spec.py
new file mode 100644
index 0000000..083bb82
--- /dev/null
+++ b/tests/test_family_spec.py
@@ -0,0 +1,23 @@
+"""Tests for the shared wafer family sensor-count lookup."""
+from pygui.backend.wafer.family_spec import sensor_count_for
+
+
+def test_x_family_returns_80():
+    assert sensor_count_for("X") == 80
+
+
+def test_p_family_returns_244():
+    assert sensor_count_for("P") == 244
+
+
+def test_other_known_families_return_244():
+    for code in ("A", "B", "C", "D", "E", "F"):
+        assert sensor_count_for(code) == 244
+
+
+def test_unknown_family_returns_244():
+    assert sensor_count_for("Z") == 244
+
+
+def test_empty_string_returns_244():
+    assert sensor_count_for("") == 244