diff --git a/stylesheets/zen/_grids.scss b/stylesheets/zen/_grids.scss
index 59a411b..82757ee 100644
--- a/stylesheets/zen/_grids.scss
+++ b/stylesheets/zen/_grids.scss
@@ -92,10 +92,6 @@ $zen-reverse-all-floats: false !default;
   $reverse-all-floats: $zen-reverse-all-floats,
   $auto-include-item-base: $zen-auto-include-item-base
 ) {
-
-  // Calculate the unit width.
-  $unit-width: zen-unit-width($column-count, $grid-width);
-
   // Determine the float direction and its reverse.
   $dir: $float-direction;
   @if $reverse-all-floats {
@@ -104,19 +100,10 @@ $zen-reverse-all-floats: false !default;
   $rev: zen-direction-flip($dir);
 
   float: $dir;
-  $width: $column-span * $unit-width;
-  @if $box-sizing == content-box {
-    @if not comparable($width, $gutter-width) {
-      $units-gutter: unit($gutter-width);
-      $units-grid: unit($grid-width);
-      @warn "The layout cannot be calculated correctly; when using box-sizing: content-box, the units of the gutter (#{$units-gutter} did not match the units of the grid width (#{$units-grid}).";
-    }
-    $width: $width - $gutter-width;
-  }
-  width: $width;
+  width: zen-grid-item-width($column-span, $column-position, $column-count, $gutter-width, $grid-width, $box-sizing);
   margin: {
-    #{$dir}: ($column-position - 1) * $unit-width;
-    #{$rev}: (1 - $column-position - $column-span) * $unit-width;
+    #{$dir}: zen-grid-item-width(($column-position - 1), 1, $column-count, $gutter-width, $grid-width, border-box);
+    #{$rev}: - zen-grid-item-width(($column-position + $column-span - 1), 1, $column-count, $gutter-width, $grid-width, border-box);
   }
 
   // Auto-apply the unit base mixin.
@@ -218,6 +205,7 @@ $zen-reverse-all-floats: false !default;
 //
 @mixin zen-grid-flow-item (
   $column-span,
+  $column-position,
   $parent-column-count: false,
   $alpha-gutter: false,
   $omega-gutter: true,
@@ -261,16 +249,7 @@ $zen-reverse-all-floats: false !default;
   }
 
   // Calculate the item's width.
-  $width: zen-grid-item-width($column-span, $columns, $grid-width);
-  @if $box-sizing == content-box {
-    @if not comparable($width, $gutter-width) {
-      $units-gutter: unit($gutter-width);
-      $units-grid: unit($grid-width);
-      @warn "The layout cannot be calculated correctly; when using box-sizing: content-box, the units of the gutter (#{$units-gutter} did not match the units of the grid width (#{$units-grid}).";
-    }
-    $width: $width - $gutter-width;
-  }
-  width: $width;
+  $width: zen-grid-item-width($column-span, $column-position, $columns, $gutter-width, $grid-width, $box-sizing);
 
   // Auto-apply the unit base mixin.
   @if $auto-include-flow-item-base {
@@ -316,11 +295,23 @@ $zen-reverse-all-floats: false !default;
 
 // Calculates the width of a grid item that spans the specified number of columns.
 @function zen-grid-item-width (
-  $column-span,
+  $column-span: $zen-column-count,
+  $column-position: 1,
   $column-count: $zen-column-count,
-  $grid-width: $zen-grid-width
+  $gutter-width: $zen-gutter-width,
+  $grid-width: $zen-grid-width,
+  $box-sizing: $zen-box-sizing
 ) {
-  @return $column-span * zen-unit-width($column-count, $grid-width);
+  $width: $column-span * zen-unit-width($column-count, $grid-width);
+  @if $box-sizing == content-box {
+    @if not comparable($width, $gutter-width) {
+      $units-gutter: unit($gutter-width);
+      $units-grid: unit($grid-width);
+      @warn "The layout cannot be calculated correctly; when using box-sizing: content-box, the units of the gutter (#{$units-gutter} did not match the units of the grid width (#{$units-grid}).";
+    }
+    $width: $width - $gutter-width;
+  }
+  @return $width;
 }
 
 // Returns the opposite direction, given "left" or "right".