diff --git a/courseware/capa/unit.py b/courseware/capa/unit.py
new file mode 100644
index 0000000000..c6a4d4591a
--- /dev/null
+++ b/courseware/capa/unit.py
@@ -0,0 +1,130 @@
+import math
+from numpy import eye, array
+import operator
+from pyparsing import Word, alphas, nums, oneOf, Literal
+from pyparsing import ZeroOrMore, OneOrMore, StringStart
+from pyparsing import StringEnd, Optional, Forward
+from pyparsing import CaselessLiteral, Group, StringEnd
+from pyparsing import NoMatch, stringEnd
+
+base_units = ['meter', 'gram', 'second', 'ampere', 'kelvin', 'mole', 'cd']
+unit_vectors = dict([(base_units[i], eye(len(base_units))[:,i]) for i in range(len(base_units))])
+
+
+def unit_evaluator(unit_string, units=unit_map):
+    ''' Evaluate an expression. Variables are passed as a dictionary
+    from string to value. Unary functions are passed as a dictionary
+    from string to function '''
+    if string.strip() == "":
+        return float('nan')
+    ops = { "^" : operator.pow,
+            "*" : operator.mul,
+            "/" : operator.truediv,
+            }
+    prefixes={'%':0.01,'k':1e3,'M':1e6,'G':1e9,
+              'T':1e12,#'P':1e15,'E':1e18,'Z':1e21,'Y':1e24,
+              'c':1e-2,'m':1e-3,'u':1e-6,
+              'n':1e-9,'p':1e-12}#,'f':1e-15,'a':1e-18,'z':1e-21,'y':1e-24}
+    
+    def super_float(text):
+        ''' Like float, but with si extensions. 1k goes to 1000'''
+        if text[-1] in suffixes:
+            return float(text[:-1])*suffixes[text[-1]]
+        else:
+            return float(text)
+
+    def number_parse_action(x): # [ '7' ] ->  [ 7 ]
+        return [super_float("".join(x))]
+    def exp_parse_action(x): # [ 2 ^ 3 ^ 2 ] -> 512
+        x = [e for e in x if type(e) == float] # Ignore ^
+        x.reverse()
+        x=reduce(lambda a,b:b**a, x)
+        return x
+    def parallel(x): # Parallel resistors [ 1 2 ] => 2/3
+        if len(x) == 1:
+            return x[0]
+        if 0 in x:
+            return float('nan')
+        x = [1./e for e in x if type(e) == float] # Ignore ^
+        return 1./sum(x)
+    def sum_parse_action(x): # [ 1 + 2 - 3 ] -> 0
+        total = 0.0
+        op = ops['+']
+        for e in x:
+            if e in set('+-'):
+                op = ops[e]
+            else:
+                total=op(total, e)
+        return total
+    def prod_parse_action(x): # [ 1 * 2 / 3 ] => 0.66
+        prod = 1.0
+        op = ops['*']
+        for e in x:
+            if e in set('*/'):
+                op = ops[e]
+            else:
+                prod=op(prod, e)
+        return prod
+    def func_parse_action(x):
+        return [functions[x[0]](x[1])]
+
+    number_suffix=reduce(lambda a,b:a|b, map(Literal,suffixes.keys()), NoMatch()) # SI suffixes and percent
+    (dot,minus,plus,times,div,lpar,rpar,exp)=map(Literal,".-+*/()^")
+    
+    number_part=Word(nums)
+    inner_number = ( number_part+Optional("."+number_part) ) | ("."+number_part) # 0.33 or 7 or .34
+    number=Optional(minus | plus)+ inner_number + \
+        Optional(CaselessLiteral("E")+Optional("-")+number_part)+ \
+        Optional(number_suffix) # 0.33k or -17
+    number=number.setParseAction( number_parse_action ) # Convert to number
+    
+    # Predefine recursive variables
+    expr = Forward() 
+    factor = Forward()
+    
+    def sreduce(f, l):
+        ''' Same as reduce, but handle len 1 and len 0 lists sensibly '''
+        if len(l)==0:
+            return NoMatch()
+        if len(l)==1:
+            return l[0]
+        return reduce(f, l)
+
+    # Handle variables passed in. E.g. if we have {'R':0.5}, we make the substitution. 
+    # Special case for no variables because of how we understand PyParsing is put together
+    if len(variables)>0:
+        varnames = sreduce(lambda x,y:x|y, map(lambda x: CaselessLiteral(x), variables.keys()))
+        varnames.setParseAction(lambda x:map(lambda y:variables[y], x))
+    else:
+        varnames=NoMatch()
+    # Same thing for functions. 
+    if len(functions)>0:
+        funcnames = sreduce(lambda x,y:x|y, map(lambda x: CaselessLiteral(x), functions.keys()))
+        function = funcnames+lpar.suppress()+expr+rpar.suppress()
+        function.setParseAction(func_parse_action)
+    else:
+        function = NoMatch()
+
+    atom = number | varnames | lpar+expr+rpar | function
+    factor << (atom + ZeroOrMore(exp+atom)).setParseAction(exp_parse_action) # 7^6
+    paritem = factor + ZeroOrMore(Literal('||')+factor) # 5k || 4k
+    paritem=paritem.setParseAction(parallel)
+    term = paritem + ZeroOrMore((times|div)+paritem) # 7 * 5 / 4 - 3
+    term = term.setParseAction(prod_parse_action)
+    expr << Optional((plus|minus)) + term + ZeroOrMore((plus|minus)+term) # -5 + 4 - 3
+    expr=expr.setParseAction(sum_parse_action)
+    return (expr+stringEnd).parseString(string)[0]
+
+if __name__=='__main__':
+    variables={'R1':2.0, 'R3':4.0}
+    functions={'sin':math.sin, 'cos':math.cos}
+    print "X",evaluator(variables, functions, "10000||sin(7+5)-6k")
+    print "X",evaluator(variables, functions, "13")
+    print evaluator({'R1': 2.0, 'R3':4.0}, {}, "13")
+    # 
+    print evaluator({'a': 2.2997471478310274, 'k': 9, 'm': 8, 'x': 0.66009498411213041}, {}, "5")
+    print evaluator({},{}, "-1")
+    print evaluator({},{}, "-(7+5)")
+    print evaluator({},{}, "-0.33")
+    print evaluator({},{}, "-.33")
+    print evaluator({},{}, "5+7 QWSEKO")