diff --git a/common/lib/calc/calc/calc.py b/common/lib/calc/calc/calc.py
index fc45f107bc..b55055dcd1 100644
--- a/common/lib/calc/calc/calc.py
+++ b/common/lib/calc/calc/calc.py
@@ -381,10 +381,19 @@ class ParseAugmenter(object):
         # Predefine recursive variables.
         expr = Forward()
 
-        # Handle variables passed in. They must start with letters/underscores
-        # and may contain numbers afterward.
-        inner_varname = Word(alphas + "_", alphanums + "_")
-        varname = Group(inner_varname)("variable")
+        # Handle variables passed in. They must start with a letter
+        # and may contain numbers and underscores afterward.
+        inner_varname = Combine(Word(alphas, alphanums + "_") + ZeroOrMore("'"))
+        # Alternative variable name in tensor format
+        # Tensor name must start with a letter, continue with alphanums
+        # Indices may be alphanumeric
+        # e.g., U_{ijk}^{123}
+        upper_indices = Literal("^{") + Word(alphanums) + Literal("}")
+        lower_indices = Literal("_{") + Word(alphanums) + Literal("}")
+        tensor_lower = Combine(Word(alphas, alphanums) + lower_indices + ZeroOrMore("'"))
+        tensor_mixed = Combine(Word(alphas, alphanums) + Optional(lower_indices) + upper_indices + ZeroOrMore("'"))
+        # Test for mixed tensor first, then lower tensor alone, then generic variable name
+        varname = Group(tensor_mixed | tensor_lower | inner_varname)("variable")
         varname.setParseAction(self.variable_parse_action)
 
         # Same thing for functions.
diff --git a/common/lib/calc/calc/tests/test_calc.py b/common/lib/calc/calc/tests/test_calc.py
index d8da18c93f..77acca9f48 100644
--- a/common/lib/calc/calc/tests/test_calc.py
+++ b/common/lib/calc/calc/tests/test_calc.py
@@ -440,7 +440,7 @@ class EvaluatorTest(unittest.TestCase):
         """
         Substitution of variables into simple equations
         """
-        variables = {'x': 9.72, 'y': 7.91, 'loooooong': 6.4}
+        variables = {'x': 9.72, 'y': 7.91, 'loooooong': 6.4, "f_0'": 2.0, "T_{ijk}^{123}''": 5.2}
 
         # Should not change value of constant
         # even with different numbers of variables...
@@ -452,6 +452,8 @@ class EvaluatorTest(unittest.TestCase):
         self.assertEqual(calc.evaluator(variables, {}, 'x'), 9.72)
         self.assertEqual(calc.evaluator(variables, {}, 'y'), 7.91)
         self.assertEqual(calc.evaluator(variables, {}, 'loooooong'), 6.4)
+        self.assertEqual(calc.evaluator(variables, {}, "f_0'"), 2.0)
+        self.assertEqual(calc.evaluator(variables, {}, "T_{ijk}^{123}''"), 5.2)
 
         # Test a simple equation
         self.assertAlmostEqual(