diff --git a/synthpop/ipf/ipf.py b/synthpop/ipf/ipf.py
index 5b396d3..b3beddb 100644
--- a/synthpop/ipf/ipf.py
+++ b/synthpop/ipf/ipf.py
@@ -52,20 +52,16 @@ def calc_diff(x, y):
 
     iterations = 0
 
+    list_of_loc = [
+        ((flat_joint_dist[idx[0]] == idx[1]).values, marginals[idx])
+        for idx in marginals.index
+    ]
+
     while calc_diff(constraints, prev_constraints) > tolerance:
         prev_constraints[:] = constraints
 
-        for idx in marginals.index:
-            # get the locations of the things we're updating
-            cat = idx[0]  # top level category (col name in flat_joint_dist)
-            subcat = idx[1]  # subcategory (col values in flat_joint_dist)
-            loc = (flat_joint_dist[cat] == subcat).values
-
-            # figure out the proportions for this update
-            proportions = constraints[loc] / constraints[loc].sum()
-
-            # distribute new total for these classes
-            constraints[loc] = proportions * marginals[idx]
+        for loc, target in list_of_loc:
+            constraints[loc] *= target / constraints[loc].sum()
 
         iterations += 1