diff --git a/news/getmud.rst b/news/getmud.rst
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index 0000000..d1de864
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+++ b/news/getmud.rst
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+**Added:**
+
+* New ``getmuD`` module with ``get_diameter`` function to calculate capillary diameter.
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>
diff --git a/src/diffpy/labpdfproc/getmud.py b/src/diffpy/labpdfproc/getmud.py
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+++ b/src/diffpy/labpdfproc/getmud.py
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+from diffpy.utils.tools import compute_mu_using_xraydb
+
+
+def get_diameter(
+    mud,
+    sample_composition,
+    xray_energy,
+    sample_mass_density=None,
+    packing_fraction=None,
+):
+    """
+    Compute capillary diameter (mm) from muD, sample composition, energy,
+    and either sample mass density or packing fraction.
+
+    Parameters
+    ----------
+    mud : float
+        The given muD of the sample.
+    sample_composition : str
+        The chemical formula of the material (e.g. "ZrO2").
+    xray_energy : float
+        The energy of the incident x-rays in keV.
+    sample_mass_density : float, optional
+        The mass density of the packed sample in g/cm^3.
+    packing_fraction : float, optional
+        The packing fraction of the sample (0–1).
+
+    Returns
+    -------
+    diameter : float
+        Computed capillary diameter in mm.
+    """
+    return mud / compute_mu_using_xraydb(
+        sample_composition,
+        xray_energy,
+        sample_mass_density,
+        packing_fraction,
+    )
diff --git a/tests/test_getmud.py b/tests/test_getmud.py
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index 0000000..3dd2edd
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+++ b/tests/test_getmud.py
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+import pytest
+
+from diffpy.labpdfproc.getmud import get_diameter
+
+
+@pytest.mark.parametrize(
+    "inputs, expected_diameter",
+    [
+        (  # C1: User specifies target mud,
+            # sample composition, energy, and mass density
+            # expect to return diameter
+            {
+                "mud": 2.0,
+                "sample_composition": "ZrO2",
+                "xray_energy": 17.45,
+                "sample_mass_density": 1.20,
+            },
+            1.3439,
+        ),
+    ],
+)
+def test_compute_diameter(inputs, expected_diameter):
+    actual_diameter = get_diameter(**inputs)
+    assert actual_diameter == pytest.approx(
+        expected_diameter, rel=0.01, abs=0.1
+    )