diff --git a/kim_tools/kimunits.py b/kim_tools/kimunits.py
index 841768e..aaf6a96 100644
--- a/kim_tools/kimunits.py
+++ b/kim_tools/kimunits.py
@@ -161,30 +161,41 @@ def convert_inner(x, fit=None):
 
 def add_si_units(doc, convert=convert):
     """Given a document, add all of the appropriate si-units fields"""
+
+    # dict specifying which SI keys correspond with which source keys
+    corresponding_keys = {
+        "source-value": "si-value",
+        "source-std-uncert-value": "si-std-uncert-value",
+        "source-expand-uncert-value": "si-expand-uncert-value",
+        "source-asym-std-uncert-neg": "si-asym-std-uncert-neg",
+        "source-asym-std-uncert-pos": "si-asym-std-uncert-pos",
+        "source-asym-expand-uncert-neg": "si-asym-expand-uncert-neg",
+        "source-asym-expand-uncert-pos": "si-asym-expand-uncert-pos",
+    }
+
     if isinstance(doc, dict):
         # check for a source-unit to defined a value with units
         if "source-unit" in doc:
             # we've found a place to add
             assert "source-value" in doc, "Badly formed doc"
-            o_value = doc.get("source-value", None)
-            o_unit = doc.get("source-unit", None)
-
-            if o_value is None:
-                raise UnitConversion("No source-value provided")
-            if o_unit is None:
-                raise UnitConversion("No source-unit provided")
-
-            # convert the units and insert
-            value, unit = convert_list(o_value, o_unit, convert=convert)
-            si_dict = {"si-unit": unit, "si-value": value}
-            doc = doc.copy()
-            doc.update(si_dict)
-            return doc
+            # search for any convertable source values or uncertanties
+            output = doc.copy()
+            for source_key in corresponding_keys.keys():
+                o_value = doc.get(source_key, None)
+                o_unit = doc.get("source-unit", None)
+
+                # convertable value or uncertianty found
+                if o_value is not None:
+                    # convert the units and insert
+                    value, unit = convert_list(o_value, o_unit, convert=convert)
+                    # look up the corresponding SI key for the converted value
+                    converted_key = corresponding_keys[source_key]
+                    si_dict = {"si-unit": unit, converted_key: value}
+                    output.update(si_dict)
+            return output
         else:
             # recurse
-            return type(doc)(
-                (key, add_si_units(value)) for key, value in list(doc.items())
-            )
+            return type(doc)((key, add_si_units(value)) for key, value in doc.items())
 
     elif isinstance(doc, (list, tuple)):
         return type(doc)(add_si_units(x) for x in doc)
diff --git a/tests/test_units.py b/tests/test_units.py
index f3ae5b2..336675e 100644
--- a/tests/test_units.py
+++ b/tests/test_units.py
@@ -1,6 +1,6 @@
 import numpy as np
 
-from kim_tools.kimunits import convert_list, convert_units
+from kim_tools.kimunits import add_si_units, convert_list, convert_units
 
 
 def test_units() -> None:
@@ -9,3 +9,15 @@ def test_units() -> None:
     assert one_kilogram[1] == "kg"
     assert np.isclose(convert_units(0, "tempC", "tempF", suppress_unit=True), 32)
     assert np.allclose(convert_list([1000, 1000], "g")[0], [1, 1])
+    assert np.isclose(
+        add_si_units(
+            {"source-value": 1000, "source-unit": "mm", "source-std-uncert-value": 1}
+        )["si-value"],
+        1.0,
+    )
+    assert np.isclose(
+        add_si_units(
+            {"source-value": 1000, "source-unit": "mm", "source-std-uncert-value": 1}
+        )["si-std-uncert-value"],
+        0.001,
+    )