diff --git a/layout/generic/nsGridContainerFrame.cpp b/layout/generic/nsGridContainerFrame.cpp
index 158871e8cca5..7fd6d73f9948 100644
--- a/layout/generic/nsGridContainerFrame.cpp
+++ b/layout/generic/nsGridContainerFrame.cpp
@@ -2282,6 +2282,21 @@ nsGridContainerFrame::LineRange::ToPositionAndLength(
   *aLength = length;
 }
 
+nscoord
+nsGridContainerFrame::LineRange::ToLength(
+  const nsTArray<TrackSize>& aTrackSizes) const
+{
+  MOZ_ASSERT(mStart != kAutoLine && mEnd != kAutoLine,
+             "expected a definite LineRange");
+  nscoord length = 0;
+  const uint32_t end = mEnd;
+  MOZ_ASSERT(end <= aTrackSizes.Length(), "aTrackSizes isn't large enough");
+  for (uint32_t i = mStart; i < end; ++i) {
+    length += aTrackSizes[i].mBase;
+  }
+  return length;
+}
+
 void
 nsGridContainerFrame::LineRange::ToPositionAndLengthForAbsPos(
   const nsTArray<TrackSize>& aTrackSizes, nscoord aGridOrigin,
@@ -2502,6 +2517,56 @@ nsGridContainerFrame::Reflow(nsPresContext*           aPresContext,
   NS_FRAME_SET_TRUNCATION(aStatus, aReflowState, aDesiredSize);
 }
 
+nscoord
+nsGridContainerFrame::IntrinsicISize(nsRenderingContext* aRenderingContext,
+                                     IntrinsicISizeType  aConstraint)
+{
+  // Calculate the sum of column sizes under aConstraint.
+  // http://dev.w3.org/csswg/css-grid/#intrinsic-sizes
+  GridReflowState state(this, *aRenderingContext);
+  InitImplicitNamedAreas(state.mGridStyle); // XXX optimize
+  PlaceGridItems(state);                    // XXX optimize
+  if (mGridColEnd == 0) {
+    return 0;
+  }
+  state.mCols.Initialize(state.mColFunctions, mGridColEnd,
+                         NS_UNCONSTRAINEDSIZE);
+  state.mIter.Reset();
+  state.mCols.CalculateSizes(state, mGridItems, state.mColFunctions,
+                             NS_UNCONSTRAINEDSIZE, &GridArea::mCols,
+                             aConstraint);
+  TranslatedLineRange allTracks(0, mGridColEnd);
+  return allTracks.ToLength(state.mCols.mSizes);
+}
+
+nscoord
+nsGridContainerFrame::GetMinISize(nsRenderingContext* aRC)
+{
+  DISPLAY_MIN_WIDTH(this, mCachedMinISize);
+  if (mCachedMinISize == NS_INTRINSIC_WIDTH_UNKNOWN) {
+    mCachedMinISize = IntrinsicISize(aRC, nsLayoutUtils::MIN_ISIZE);
+  }
+  return mCachedMinISize;
+}
+
+nscoord
+nsGridContainerFrame::GetPrefISize(nsRenderingContext* aRC)
+{
+  DISPLAY_PREF_WIDTH(this, mCachedPrefISize);
+  if (mCachedPrefISize == NS_INTRINSIC_WIDTH_UNKNOWN) {
+    mCachedPrefISize = IntrinsicISize(aRC, nsLayoutUtils::PREF_ISIZE);
+  }
+  return mCachedPrefISize;
+}
+
+void
+nsGridContainerFrame::MarkIntrinsicISizesDirty()
+{
+  mCachedMinISize = NS_INTRINSIC_WIDTH_UNKNOWN;
+  mCachedPrefISize = NS_INTRINSIC_WIDTH_UNKNOWN;
+  nsContainerFrame::MarkIntrinsicISizesDirty();
+}
+
 nsIAtom*
 nsGridContainerFrame::GetType() const
 {
diff --git a/layout/generic/nsGridContainerFrame.h b/layout/generic/nsGridContainerFrame.h
index b1c310e1458d..9c3c403fe2cf 100644
--- a/layout/generic/nsGridContainerFrame.h
+++ b/layout/generic/nsGridContainerFrame.h
@@ -33,6 +33,9 @@ public:
               nsHTMLReflowMetrics&     aDesiredSize,
               const nsHTMLReflowState& aReflowState,
               nsReflowStatus&          aStatus) override;
+  nscoord GetMinISize(nsRenderingContext* aRenderingContext) override;
+  nscoord GetPrefISize(nsRenderingContext* aRenderingContext) override;
+  void MarkIntrinsicISizesDirty() override;
   virtual nsIAtom* GetType() const override;
 
   void BuildDisplayList(nsDisplayListBuilder*   aBuilder,
@@ -107,7 +110,11 @@ protected:
   struct GridReflowState;
   friend nsContainerFrame* NS_NewGridContainerFrame(nsIPresShell* aPresShell,
                                                     nsStyleContext* aContext);
-  explicit nsGridContainerFrame(nsStyleContext* aContext) : nsContainerFrame(aContext) {}
+  explicit nsGridContainerFrame(nsStyleContext* aContext)
+    : nsContainerFrame(aContext)
+    , mCachedMinISize(NS_INTRINSIC_WIDTH_UNKNOWN)
+    , mCachedPrefISize(NS_INTRINSIC_WIDTH_UNKNOWN)
+  {}
 
   /**
    * A LineRange can be definite or auto - when it's definite it represents
@@ -182,6 +189,11 @@ protected:
      */
     void ToPositionAndLength(const nsTArray<TrackSize>& aTrackSizes,
                              nscoord* aPos, nscoord* aLength) const;
+    /**
+     * Given an array of track sizes, return the length of the tracks in this
+     * line range.
+     */
+    nscoord ToLength(const nsTArray<TrackSize>& aTrackSizes) const;
     /**
      * Given an array of track sizes and a grid origin coordinate, adjust the
      * abs.pos. containing block along an axis given by aPos and aLength.
@@ -523,6 +535,12 @@ protected:
                       nsHTMLReflowMetrics& aDesiredSize,
                       nsReflowStatus&      aStatus);
 
+  /**
+   * Helper for GetMinISize / GetPrefISize.
+   */
+  nscoord IntrinsicISize(nsRenderingContext* aRenderingContext,
+                         IntrinsicISizeType  aConstraint);
+
 #ifdef DEBUG
   void SanityCheckAnonymousGridItems() const;
 #endif // DEBUG
@@ -568,6 +586,12 @@ private:
   uint32_t mExplicitGridOffsetCol;
   uint32_t mExplicitGridOffsetRow;
 
+  /**
+   * Cached values to optimize GetMinISize/GetPrefISize.
+   */
+  nscoord mCachedMinISize;
+  nscoord mCachedPrefISize;
+
   /**
    * True iff the normal flow children are already in CSS 'order' in the
    * order they occur in the child frame list.