diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index 8e0856aac587..61b35f990a12 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -316,6 +316,19 @@ public:
   nscoord GetCrossSize() const     { return mCrossSize;  }
   nscoord GetCrossPosition() const { return mCrossPosn; }
 
+  // Convenience methods to compute the main & cross size of our *margin-box*.
+  // The caller is responsible for telling us the right axis, so that we can
+  // pull out the appropriate components of our margin/border/padding structs.
+  nscoord GetOuterMainSize(AxisOrientationType aMainAxis) const
+  {
+    return mMainSize + GetMarginBorderPaddingSizeInAxis(aMainAxis);
+  }
+
+  nscoord GetOuterCrossSize(AxisOrientationType aCrossAxis) const
+  {
+    return mCrossSize + GetMarginBorderPaddingSizeInAxis(aCrossAxis);
+  }
+
   // Returns the distance between this FlexItem's baseline and the cross-start
   // edge of its margin-box. Used in baseline alignment.
   // (This function needs to be told what the cross axis is so that it can
@@ -1216,10 +1229,7 @@ FlexItem::GetBaselineOffsetFromOuterCrossStart(
   // margin-bottom edge) to the baseline is just the margin-box cross size
   // (i.e. outer cross size), minus the distance from margin-top to baseline
   // (already computed above).
-  nscoord outerCrossSize = mCrossSize +
-    GetMarginBorderPaddingSizeInAxis(aCrossAxis);
-
-  return outerCrossSize - marginTopToBaseline;
+  return GetOuterCrossSize(aCrossAxis) - marginTopToBaseline;
 }
 
 uint32_t
@@ -1787,10 +1797,7 @@ MainAxisPositionTracker::
   // with the *actual* amount of packing space.
   for (const FlexItem* item = aLine->GetFirstItem(); item;
        item = item->getNext()) {
-    nscoord itemMarginBoxMainSize =
-      item->GetMainSize() +
-      item->GetMarginBorderPaddingSizeInAxis(aAxisTracker.GetMainAxis());
-    mPackingSpaceRemaining -= itemMarginBoxMainSize;
+    mPackingSpaceRemaining -= item->GetOuterMainSize(mAxis);
     mNumAutoMarginsInMainAxis += item->GetNumAutoMarginsInAxis(mAxis);
   }
 
@@ -2074,8 +2081,8 @@ FlexLine::ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker)
   nscoord crossEndToFurthestBaseline = nscoord_MIN;
   nscoord largestOuterCrossSize = 0;
   for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
-    nscoord curOuterCrossSize = item->GetCrossSize() +
-      item->GetMarginBorderPaddingSizeInAxis(aAxisTracker.GetCrossAxis());
+    nscoord curOuterCrossSize =
+      item->GetOuterCrossSize(aAxisTracker.GetCrossAxis());
 
     if (item->GetAlignSelf() == NS_STYLE_ALIGN_ITEMS_BASELINE &&
         item->GetNumAutoMarginsInAxis(aAxisTracker.GetCrossAxis()) == 0) {
@@ -2180,7 +2187,7 @@ SingleLineCrossAxisPositionTracker::
   // Subtract the space that our item is already occupying, to see how much
   // space (if any) is available for its auto margins.
   nscoord spaceForAutoMargins = aLine.GetLineCrossSize() -
-    (aItem.GetCrossSize() + aItem.GetMarginBorderPaddingSizeInAxis(mAxis));
+    aItem.GetOuterCrossSize(mAxis);
 
   if (spaceForAutoMargins <= 0) {
     return; // No available space  --> nothing to do
@@ -2230,17 +2237,12 @@ SingleLineCrossAxisPositionTracker::
       // auto-sized items (which we've already done).
       break;
     case NS_STYLE_ALIGN_ITEMS_FLEX_END:
-      mPosition +=
-        aLine.GetLineCrossSize() -
-        (aItem.GetCrossSize() +
-         aItem.GetMarginBorderPaddingSizeInAxis(mAxis));
+      mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
       break;
     case NS_STYLE_ALIGN_ITEMS_CENTER:
       // Note: If cross-size is odd, the "after" space will get the extra unit.
       mPosition +=
-        (aLine.GetLineCrossSize() -
-         (aItem.GetCrossSize() +
-          aItem.GetMarginBorderPaddingSizeInAxis(mAxis))) / 2;
+        (aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis)) / 2;
       break;
     case NS_STYLE_ALIGN_ITEMS_BASELINE: {
       nscoord lineBaselineOffset =
@@ -2419,8 +2421,8 @@ nsFlexContainerFrame::GenerateFlexLines(
     }
 
     nscoord itemInnerHypotheticalMainSize = item->GetMainSize();
-    nscoord itemOuterHypotheticalMainSize = item->GetMainSize() +
-      item->GetMarginBorderPaddingSizeInAxis(aAxisTracker.GetMainAxis());
+    nscoord itemOuterHypotheticalMainSize =
+      item->GetOuterMainSize(aAxisTracker.GetMainAxis());
 
     // Check if we need to wrap |item| to a new line
     // (i.e. check if its outer hypothetical main size pushes our line over