diff --git a/app/models/quantity.rb b/app/models/quantity.rb
index eab2dfa..eb81c98 100644
--- a/app/models/quantity.rb
+++ b/app/models/quantity.rb
@@ -29,6 +29,24 @@ class Quantity < ActiveRecord::Base
     if new_record?
       self.domain ||= :diet
     end
+    @formula = Formula.new(self.project, self.formula)
+  end
+
+  def formula=(value)
+    @formula = Formula.new(self.project, value)
+    super(value)
+  end
+
+  def formula_valid?
+    @formula.valid?
+  end
+
+  def formula_quantities
+    @formula.get_quantities
+  end
+
+  def calculate(inputs)
+    @formula.calculate(inputs)
   end
 
   def movable?(direction)
@@ -47,14 +65,6 @@ class Quantity < ActiveRecord::Base
     end
   end
 
-  def formula_quantities
-    Formula.new(self.project, self.formula).get_quantities
-  end
-
-  def calculate(inputs)
-    Formula.new(self.project, self.formula).calculate(inputs)
-  end
-
   def self.filter(project, filters)
     quantities = all
 
diff --git a/app/views/quantities/_index.html.erb b/app/views/quantities/_index.html.erb
index d80b7c1..8cd2434 100644
--- a/app/views/quantities/_index.html.erb
+++ b/app/views/quantities/_index.html.erb
@@ -43,7 +43,7 @@
           </td>
           <td class="domain"><%= q.domain %></td>
           <td class="description"><%= q.description %></td>
-          <td class="formula"><%= checked_image q.formula.present? %></td>
+          <td class="formula"><%= checked_image q.formula? %></td>
           <td class="action unwrappable">
             <%= link_to l(:button_edit), edit_quantity_path(q), {
               remote: true,
diff --git a/lib/body_tracking/formula.rb b/lib/body_tracking/formula.rb
index 3f38e81..bfd63ef 100644
--- a/lib/body_tracking/formula.rb
+++ b/lib/body_tracking/formula.rb
@@ -4,6 +4,7 @@ module BodyTracking
     require 'set'
 
     class InvalidFormula < RuntimeError; end
+    class InvalidInputs < RuntimeError; end
 
     class Formula
       def initialize(project, formula)
@@ -33,28 +34,36 @@ module BodyTracking
       end
 
       def get_quantities
-        raise RuntimeError, 'Invalid formula' unless self.valid?
+        raise(InvalidFormula, 'Invalid formula') unless self.valid?
 
         @quantities.to_a
       end
 
-      #"params.values.first.each_with_index.map { |*, _index| #{@paramed_formula} }"
       def calculate(inputs)
-        raise RuntimeError, 'Invalid formula' unless self.valid?
+        byebug
+        params = inputs.map { |q, v| [q.name, v.transpose[0]] }.to_h
+        length = params.values.first.length
 
-        values = inputs.map { |q, v| [q.name, v.transpose[0]] }.to_h
-        puts values.inspect
-        begin
-          get_binding(values).eval(@paramed_formula).map { |x| [x, nil] }
-        rescue Exception => e
-          puts e.message
-          [[nil, nil]] * inputs.values.first.length
+        raise(InvalidFormula, 'Invalid formula') unless self.valid?
+        raise InvalidInputs unless params.values.all? { |v| v.length == length }
+
+        args = []
+        @parts.each do |p|
+          args << get_binding(params, length)
+            .eval(
+              p[:type] == :indexed ? "length.times.map { |_index| #{p[:content]} }" :
+                p[:content]
+            )
         end
+        args.last.map { |v| [v, nil] }
+      rescue Exception => e
+        puts e.message
+        [[nil, nil]] * length
       end
 
       private
 
-      def get_binding(params)
+      def get_binding(params, _length)
         binding
       end
     end
@@ -73,6 +82,8 @@ module BodyTracking
     end
 
 
+    # List of events with parameter count:
+    # https://github.com/racker/ruby-1.9.3-lucid/blob/master/ext/ripper/eventids1.c
     class FormulaBuilder < Ripper::SexpBuilder
       def initialize(*args)
         super(*args)
@@ -109,7 +120,7 @@ module BodyTracking
 
       def on_program(stmts)
         @parts << {type: :indexed, content: join_stmts(stmts)}
-        [@identifiers, @parts]
+        [@identifiers.to_a, @parts]
       end
 
       def on_string_content
diff --git a/lib/body_tracking/items_with_quantities.rb b/lib/body_tracking/items_with_quantities.rb
index ded289f..919c3db 100644
--- a/lib/body_tracking/items_with_quantities.rb
+++ b/lib/body_tracking/items_with_quantities.rb
@@ -67,7 +67,7 @@ module BodyTracking
         q, deps = unchecked_q.shift
 
         # quantity not computable (no formula) or not requiring calculation/computed
-        if q.formula.blank? || (subitems[q.name].length == items.count)
+        if !q.formula? || !q.formula_valid? || (subitems[q.name].length == items.count)
           completed_q[q.name] = subitems.delete(q.name) { {} }
           completed_q[q.name].default = [nil, nil]
           next
@@ -83,6 +83,7 @@ module BodyTracking
         # quantity with formula has all dependencies satisfied, requires calculation
         if deps.empty?
           output_ids = items.select { |i| subitems[q.name][i.id].nil? }.map(&:id)
+        byebug
           input_q = q.formula_quantities
           inputs = input_q.map { |i_q| [i_q, completed_q[i_q.name].values_at(*output_ids)] }
           q.calculate(inputs.to_h).each_with_index do |result, index|