diff --git a/packages/flutter/lib/src/material/scaffold.dart b/packages/flutter/lib/src/material/scaffold.dart
index 7cfd99ce579..bb958422327 100644
--- a/packages/flutter/lib/src/material/scaffold.dart
+++ b/packages/flutter/lib/src/material/scaffold.dart
@@ -39,7 +39,7 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     // in this case the toolbar appears -after- the body in the stacking order,
     // so the toolbar's shadow is drawn on top of the body.
 
-    final BoxConstraints fullWidthConstraints = looseConstraints.tightenWidth(size.width);
+    final BoxConstraints fullWidthConstraints = looseConstraints.tighten(width: size.width);
     Size toolBarSize = Size.zero;
 
     if (isChild(_Child.toolBar)) {
@@ -49,7 +49,7 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
 
     if (isChild(_Child.body)) {
       final double bodyHeight = size.height - toolBarSize.height;
-      final BoxConstraints bodyConstraints = fullWidthConstraints.tightenHeight(bodyHeight);
+      final BoxConstraints bodyConstraints = fullWidthConstraints.tighten(height: bodyHeight);
       layoutChild(_Child.body, bodyConstraints);
       positionChild(_Child.body, new Offset(0.0, toolBarSize.height));
     }
diff --git a/packages/flutter/lib/src/rendering/box.dart b/packages/flutter/lib/src/rendering/box.dart
index e19fe07d19d..b15120617fc 100644
--- a/packages/flutter/lib/src/rendering/box.dart
+++ b/packages/flutter/lib/src/rendering/box.dart
@@ -124,22 +124,14 @@ class BoxConstraints extends Constraints {
     );
   }
 
-  /// Returns new box constraints with a tight width as close to the given width
-  /// as possible while still respecting the original box constraints.
-  BoxConstraints tightenWidth(double width) {
-    return new BoxConstraints(minWidth: math.max(math.min(maxWidth, width), minWidth),
-                              maxWidth: math.max(math.min(maxWidth, width), minWidth),
-                              minHeight: minHeight,
-                              maxHeight: maxHeight);
-  }
-
-  /// Returns new box constraints with a tight height as close to the given
-  /// height as possible while still respecting the original box constraints.
-  BoxConstraints tightenHeight(double height) {
-    return new BoxConstraints(minWidth: minWidth,
-                              maxWidth: maxWidth,
-                              minHeight: math.max(math.min(maxHeight, height), minHeight),
-                              maxHeight: math.max(math.min(maxHeight, height), minHeight));
+  /// Returns new box constraints with a tight width and/or height as close to
+  /// the given width and height as possible while still respecting the original
+  /// box constraints.
+  BoxConstraints tighten({ double width, double height }) {
+    return new BoxConstraints(minWidth: width == null ? minWidth : math.max(math.min(maxWidth, width), minWidth),
+                              maxWidth: width == null ? maxWidth : math.max(math.min(maxWidth, width), minWidth),
+                              minHeight: height == null ? minHeight : math.max(math.min(maxHeight, height), minHeight),
+                              maxHeight: height == null ? maxHeight : math.max(math.min(maxHeight, height), minHeight));
   }
 
   /// Returns box constraints with the same width constraints but with
diff --git a/packages/flutter/lib/src/rendering/proxy_box.dart b/packages/flutter/lib/src/rendering/proxy_box.dart
index 675e016c86c..7a27c6faf69 100644
--- a/packages/flutter/lib/src/rendering/proxy_box.dart
+++ b/packages/flutter/lib/src/rendering/proxy_box.dart
@@ -415,7 +415,7 @@ class RenderIntrinsicWidth extends RenderProxyBox {
       return constraints;
     double width = child.getMaxIntrinsicWidth(constraints);
     assert(width == constraints.constrainWidth(width));
-    return constraints.tightenWidth(_applyStep(width, _stepWidth));
+    return constraints.tighten(width: _applyStep(width, _stepWidth));
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
@@ -451,7 +451,7 @@ class RenderIntrinsicWidth extends RenderProxyBox {
     if (child != null) {
       BoxConstraints childConstraints = _getInnerConstraints(constraints);
       if (_stepHeight != null)
-        childConstraints.tightenHeight(getMaxIntrinsicHeight(childConstraints));
+        childConstraints.tighten(height: getMaxIntrinsicHeight(childConstraints));
       child.layout(childConstraints, parentUsesSize: true);
       size = child.size;
     } else {
@@ -485,7 +485,7 @@ class RenderIntrinsicHeight extends RenderProxyBox {
       return constraints;
     double height = child.getMaxIntrinsicHeight(constraints);
     assert(height == constraints.constrainHeight(height));
-    return constraints.tightenHeight(height);
+    return constraints.tighten(height: height);
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
diff --git a/packages/flutter/lib/src/rendering/stack.dart b/packages/flutter/lib/src/rendering/stack.dart
index d28645582f4..ff691ff72ee 100644
--- a/packages/flutter/lib/src/rendering/stack.dart
+++ b/packages/flutter/lib/src/rendering/stack.dart
@@ -333,14 +333,14 @@ abstract class RenderStackBase extends RenderBox
         BoxConstraints childConstraints = const BoxConstraints();
 
         if (childParentData.left != null && childParentData.right != null)
-          childConstraints = childConstraints.tightenWidth(size.width - childParentData.right - childParentData.left);
+          childConstraints = childConstraints.tighten(width: size.width - childParentData.right - childParentData.left);
         else if (childParentData.width != null)
-          childConstraints = childConstraints.tightenWidth(childParentData.width);
+          childConstraints = childConstraints.tighten(width: childParentData.width);
 
         if (childParentData.top != null && childParentData.bottom != null)
-          childConstraints = childConstraints.tightenHeight(size.height - childParentData.bottom - childParentData.top);
+          childConstraints = childConstraints.tighten(height: size.height - childParentData.bottom - childParentData.top);
         else if (childParentData.height != null)
-          childConstraints = childConstraints.tightenHeight(childParentData.height);
+          childConstraints = childConstraints.tighten(height: childParentData.height);
 
         child.layout(childConstraints, parentUsesSize: true);
 
diff --git a/packages/flutter/lib/src/widgets/basic.dart b/packages/flutter/lib/src/widgets/basic.dart
index ad6637c678e..a1b14c91eef 100644
--- a/packages/flutter/lib/src/widgets/basic.dart
+++ b/packages/flutter/lib/src/widgets/basic.dart
@@ -509,12 +509,7 @@ class SizedBox extends OneChildRenderObjectWidget {
   );
 
   BoxConstraints get _additionalConstraints {
-    BoxConstraints result = const BoxConstraints();
-    if (width != null)
-      result = result.tightenWidth(width);
-    if (height != null)
-      result = result.tightenHeight(height);
-    return result;
+    return new BoxConstraints.tightFor(width: width, height: height);
   }
 
   void updateRenderObject(RenderConstrainedBox renderObject, SizedBox oldWidget) {
diff --git a/packages/flutter/test/rendering/box_test.dart b/packages/flutter/test/rendering/box_test.dart
index 6908efc073e..0fcda753057 100644
--- a/packages/flutter/test/rendering/box_test.dart
+++ b/packages/flutter/test/rendering/box_test.dart
@@ -25,7 +25,7 @@ void main() {
 
   test('Flex and padding', () {
     RenderBox size = new RenderConstrainedBox(
-      additionalConstraints: new BoxConstraints().tightenHeight(100.0)
+      additionalConstraints: new BoxConstraints().tighten(height: 100.0)
     );
     RenderBox inner = new RenderDecoratedBox(
       decoration: new BoxDecoration(