diff --git a/runtime/observatory/lib/src/elements/class_view.dart b/runtime/observatory/lib/src/elements/class_view.dart
index 998f82d6a91..8c67ebcfd67 100644
--- a/runtime/observatory/lib/src/elements/class_view.dart
+++ b/runtime/observatory/lib/src/elements/class_view.dart
@@ -15,6 +15,7 @@ import 'package:polymer/polymer.dart';
 class ClassViewElement extends ObservatoryElement {
   @published Class cls;
   @observable ServiceMap instances;
+  @observable int reachableBytes;
   @observable int retainedBytes;
   @observable ObservableList mostRetained;
   SampleBufferControlElement sampleBufferControlElement;
@@ -44,6 +45,12 @@ class ClassViewElement extends ObservatoryElement {
   }
 
   // TODO(koda): Add no-arg "calculate-link" instead of reusing "eval-link".
+  Future<ServiceObject> reachableSize(var dummy) {
+    return cls.isolate.getReachableSize(cls).then((Instance obj) {
+      reachableBytes = int.parse(obj.valueAsString);
+    });
+  }
+
   Future<ServiceObject> retainedSize(var dummy) {
     return cls.isolate.getRetainedSize(cls).then((Instance obj) {
       retainedBytes = int.parse(obj.valueAsString);
diff --git a/runtime/observatory/lib/src/elements/class_view.html b/runtime/observatory/lib/src/elements/class_view.html
index 5dadba6884e..6e5988794b7 100644
--- a/runtime/observatory/lib/src/elements/class_view.html
+++ b/runtime/observatory/lib/src/elements/class_view.html
@@ -180,6 +180,19 @@
               </template>
             </div>
           </div>
+          <div class="memberItem">
+            <div class="memberName">total reachable memory size</div>
+            <div class="memberValue">
+              <template if="{{ reachableBytes == null }}">
+                <eval-link callback="{{ reachableSize }}"
+                           label="[calculate]">
+                </eval-link>
+              </template>
+              <template if="{{ reachableBytes != null }}">
+                {{ reachableBytes | formatSize }}
+              </template>
+            </div>
+          </div>
           <div class="memberItem">
             <div class="memberName">total retained memory size</div>
             <div class="memberValue">
diff --git a/runtime/observatory/lib/src/elements/object_common.dart b/runtime/observatory/lib/src/elements/object_common.dart
index eea85ffcaf0..1deb5d60231 100644
--- a/runtime/observatory/lib/src/elements/object_common.dart
+++ b/runtime/observatory/lib/src/elements/object_common.dart
@@ -15,10 +15,18 @@ class ObjectCommonElement extends ObservatoryElement {
   @published ServiceMap path;
   @published ServiceMap inboundReferences;
   @observable int retainedBytes = null;
+  @observable int reachableBytes = null;
 
   ObjectCommonElement.created() : super.created();
 
   // TODO(koda): Add no-arg "calculate-link" instead of reusing "eval-link".
+  Future<ServiceObject> reachableSize(var dummy) {
+    return object.isolate.getReachableSize(object).then((Instance obj) {
+      // TODO(turnidge): Handle collected/expired objects gracefully.
+      reachableBytes = int.parse(obj.valueAsString);
+    });
+  }
+
   Future<ServiceObject> retainedSize(var dummy) {
     return object.isolate.getRetainedSize(object).then((Instance obj) {
       // TODO(turnidge): Handle collected/expired objects gracefully.
diff --git a/runtime/observatory/lib/src/elements/object_common.html b/runtime/observatory/lib/src/elements/object_common.html
index 7cad87a8fb2..14311e072e2 100644
--- a/runtime/observatory/lib/src/elements/object_common.html
+++ b/runtime/observatory/lib/src/elements/object_common.html
@@ -26,6 +26,20 @@
         <div class="memberValue">{{ object.size | formatSize }}</div>
       </div>
 
+      <div class="memberItem" title="Space reachable from this object, excluding class references">
+        <div class="memberName">reachable size</div>
+        <div class="memberValue">
+          <template if="{{ reachableBytes == null }}">
+            <eval-link callback="{{ reachableSize }}"
+                       label="[calculate]">
+            </eval-link>
+          </template>
+          <template if="{{ reachableBytes != null }}">
+            {{ reachableBytes | formatSize }}
+          </template>
+        </div>
+      </div>
+
       <div class="memberItem" title="Space that would be reclaimed if references to this object were replaced with null">
         <div class="memberName">retained size</div>
         <div class="memberValue">
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index f7775c16a62..8f5f7a40fd9 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -1688,6 +1688,13 @@ class Isolate extends ServiceObjectOwner with Coverage {
     return invokeRpc('evaluateInFrame', params);
   }
 
+  Future<ServiceObject> getReachableSize(ServiceObject target) {
+    Map params = {
+      'targetId': target.id,
+    };
+    return invokeRpc('_getReachableSize', params);
+  }
+
   Future<ServiceObject> getRetainedSize(ServiceObject target) {
     Map params = {
       'targetId': target.id,
diff --git a/runtime/observatory/tests/service/reachable_size_test.dart b/runtime/observatory/tests/service/reachable_size_test.dart
new file mode 100644
index 00000000000..5c346e351ef
--- /dev/null
+++ b/runtime/observatory/tests/service/reachable_size_test.dart
@@ -0,0 +1,71 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// VMOptions=--error_on_bad_type --error_on_bad_override
+
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+import 'test_helper.dart';
+
+class Pair {
+  var x, y;
+}
+
+var p1;
+var p2;
+
+buildGraph() {
+  p1 = new Pair();
+  p2 = new Pair();
+
+  // Adds to both reachable and retained size.
+  p1.x = new List();
+  p2.x = new List();
+
+  // Adds to reachable size only.
+  p1.y = p2.y = new List();
+}
+
+getReachableSize(ServiceObject obj) {
+  return obj.isolate.getReachableSize(obj).then((Instance obj) {
+    return int.parse(obj.valueAsString);
+  });
+}
+
+getRetainedSize(ServiceObject obj) {
+  return obj.isolate.getRetainedSize(obj).then((Instance obj) {
+    return int.parse(obj.valueAsString);
+  });
+}
+
+var tests = [
+(Isolate isolate) async {
+  Instance p1 = await rootLibraryFieldValue(isolate, "p1");
+  Instance p2 = await rootLibraryFieldValue(isolate, "p2");
+
+  // In general, shallow <= retained <= reachable. In this program,
+  // 0 < shallow < retained < reachable.
+
+  int p1_shallow = p1.size;
+  int p1_retained = await getRetainedSize(p1);
+  int p1_reachable = await getReachableSize(p1);
+
+  expect(0, lessThan(p1_shallow));
+  expect(p1_shallow, lessThan(p1_retained));
+  expect(p1_retained, lessThan(p1_reachable));
+
+  int p2_shallow = p2.size;
+  int p2_retained = await getRetainedSize(p2);
+  int p2_reachable = await getReachableSize(p2);
+
+  expect(0, lessThan(p2_shallow));
+  expect(p2_shallow, lessThan(p2_retained));
+  expect(p2_retained, lessThan(p2_reachable));
+
+  expect(p1_shallow, equals(p2_shallow));
+  expect(p1_retained, equals(p2_retained));
+  expect(p1_reachable, equals(p2_reachable));
+},
+];
+
+main(args) => runIsolateTests(args, tests, testeeBefore: buildGraph);
diff --git a/runtime/observatory/tests/service/test_helper.dart b/runtime/observatory/tests/service/test_helper.dart
index 6248b43ffaf..58de4e660f9 100644
--- a/runtime/observatory/tests/service/test_helper.dart
+++ b/runtime/observatory/tests/service/test_helper.dart
@@ -319,6 +319,17 @@ Future<Class> getClassFromRootLib(Isolate isolate, String className) async {
 }
 
 
+Future<Instance> rootLibraryFieldValue(Isolate isolate,
+                                       String fieldName) async {
+  Library rootLib = await isolate.rootLibrary.load();
+  Field field = rootLib.variables.singleWhere((v) => v.name == fieldName);
+  await field.load();
+  Instance value = field.staticValue;
+  await value.load();
+  return value;
+}
+
+
 /// Runs [tests] in sequence, each of which should take an [Isolate] and
 /// return a [Future]. Code for setting up state can run before and/or
 /// concurrently with the tests. Uses [mainArgs] to determine whether
diff --git a/runtime/vm/object_graph.cc b/runtime/vm/object_graph.cc
index 2bc0d664374..ae1b1194b0d 100644
--- a/runtime/vm/object_graph.cc
+++ b/runtime/vm/object_graph.cc
@@ -186,7 +186,41 @@ void ObjectGraph::IterateObjectsFrom(const Object& root,
   RawObject* root_raw = root.raw();
   stack.VisitPointer(&root_raw);
   stack.TraverseGraph(visitor);
-  // TODO(koda): Optimize if we only visited a small subgraph.
+  Unmarker::UnmarkAll(isolate());
+}
+
+
+class InstanceAccumulator : public ObjectVisitor {
+ public:
+  explicit InstanceAccumulator(ObjectGraph::Stack* stack,
+                               intptr_t class_id,
+                               Isolate* isolate)
+    : ObjectVisitor(isolate), stack_(stack), class_id_(class_id) { }
+
+  void VisitObject(RawObject* obj) {
+    if (obj->GetClassId() == class_id_) {
+      RawObject* rawobj = obj;
+      stack_->VisitPointer(&rawobj);
+    }
+  }
+
+ private:
+  ObjectGraph::Stack* stack_;
+  const intptr_t class_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(InstanceAccumulator);
+};
+
+
+void ObjectGraph::IterateObjectsFrom(intptr_t class_id,
+                                     ObjectGraph::Visitor* visitor) {
+  NoSafepointScope no_safepoint_scope_;
+  Stack stack(isolate());
+
+  InstanceAccumulator accumulator(&stack, class_id, isolate());
+  isolate()->heap()->IterateObjects(&accumulator);
+
+  stack.TraverseGraph(visitor);
   Unmarker::UnmarkAll(isolate());
 }
 
@@ -240,6 +274,13 @@ intptr_t ObjectGraph::SizeRetainedByInstance(const Object& obj) {
 }
 
 
+intptr_t ObjectGraph::SizeReachableByInstance(const Object& obj) {
+  SizeVisitor total;
+  IterateObjectsFrom(obj, &total);
+  return total.size();
+}
+
+
 intptr_t ObjectGraph::SizeRetainedByClass(intptr_t class_id) {
   SizeVisitor total;
   IterateObjects(&total);
@@ -251,6 +292,13 @@ intptr_t ObjectGraph::SizeRetainedByClass(intptr_t class_id) {
 }
 
 
+intptr_t ObjectGraph::SizeReachableByClass(intptr_t class_id) {
+  SizeVisitor total;
+  IterateObjectsFrom(class_id, &total);
+  return total.size();
+}
+
+
 class RetainingPathVisitor : public ObjectGraph::Visitor {
  public:
   // We cannot use a GrowableObjectArray, since we must not trigger GC.
diff --git a/runtime/vm/object_graph.h b/runtime/vm/object_graph.h
index 0a65bae8dd3..5187d5e2d8b 100644
--- a/runtime/vm/object_graph.h
+++ b/runtime/vm/object_graph.h
@@ -63,12 +63,15 @@ class ObjectGraph : public StackResource {
   // Like 'IterateObjects', but restricted to objects reachable from 'root'
   // (including 'root' itself).
   void IterateObjectsFrom(const Object& root, Visitor* visitor);
+  void IterateObjectsFrom(intptr_t class_id, Visitor* visitor);
 
   // The number of bytes retained by 'obj'.
   intptr_t SizeRetainedByInstance(const Object& obj);
+  intptr_t SizeReachableByInstance(const Object& obj);
 
   // The number of bytes retained by the set of all objects of the given class.
   intptr_t SizeRetainedByClass(intptr_t class_id);
+  intptr_t SizeReachableByClass(intptr_t class_id);
 
   // Finds some retaining path from the isolate roots to 'obj'. Populates the
   // provided array with pairs of (object, offset from parent in words),
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 9b335ebfff5..2c2da5835ec 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -614,6 +614,7 @@ class RawObject {
   friend class Scavenger;
   friend class ScavengerVisitor;
   friend class SizeExcludingClassVisitor;  // GetClassId
+  friend class InstanceAccumulator;  // GetClassId
   friend class RetainingPathVisitor;  // GetClassId
   friend class SkippedCodeFunctions;  // StorePointer
   friend class InstructionsReader;  // tags_ check
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 454726c3fd9..39f4fec6246 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -1764,16 +1764,14 @@ static bool GetRetainingPath(Thread* thread, JSONStream* js) {
 
 static const MethodParameter* get_retained_size_params[] = {
   ISOLATE_PARAMETER,
+  new IdParameter("targetId", true),
   NULL,
 };
 
 
 static bool GetRetainedSize(Thread* thread, JSONStream* js) {
   const char* target_id = js->LookupParam("targetId");
-  if (target_id == NULL) {
-    PrintMissingParamError(js, "targetId");
-    return true;
-  }
+  ASSERT(target_id != NULL);
   ObjectIdRing::LookupResult lookup_result;
   Object& obj = Object::Handle(LookupHeapObject(thread, target_id,
                                                 &lookup_result));
@@ -1806,6 +1804,48 @@ static bool GetRetainedSize(Thread* thread, JSONStream* js) {
 }
 
 
+static const MethodParameter* get_reachable_size_params[] = {
+  ISOLATE_PARAMETER,
+  new IdParameter("targetId", true),
+  NULL,
+};
+
+
+static bool GetReachableSize(Thread* thread, JSONStream* js) {
+  const char* target_id = js->LookupParam("targetId");
+  ASSERT(target_id != NULL);
+  ObjectIdRing::LookupResult lookup_result;
+  Object& obj = Object::Handle(LookupHeapObject(thread, target_id,
+                                                &lookup_result));
+  if (obj.raw() == Object::sentinel().raw()) {
+    if (lookup_result == ObjectIdRing::kCollected) {
+      PrintSentinel(js, kCollectedSentinel);
+    } else if (lookup_result == ObjectIdRing::kExpired) {
+      PrintSentinel(js, kExpiredSentinel);
+    } else {
+      PrintInvalidParamError(js, "targetId");
+    }
+    return true;
+  }
+  // TODO(rmacnak): There is no way to get the size retained by a class object.
+  // SizeRetainedByClass should be a separate RPC.
+  if (obj.IsClass()) {
+    const Class& cls = Class::Cast(obj);
+    ObjectGraph graph(thread);
+    intptr_t retained_size = graph.SizeReachableByClass(cls.id());
+    const Object& result = Object::Handle(Integer::New(retained_size));
+    result.PrintJSON(js, true);
+    return true;
+  }
+
+  ObjectGraph graph(thread);
+  intptr_t retained_size = graph.SizeReachableByInstance(obj);
+  const Object& result = Object::Handle(Integer::New(retained_size));
+  result.PrintJSON(js, true);
+  return true;
+}
+
+
 static const MethodParameter* evaluate_params[] = {
   ISOLATE_PARAMETER,
   NULL,
@@ -3455,6 +3495,8 @@ static const ServiceMethodDescriptor service_methods_[] = {
     get_object_by_address_params },
   { "_getPorts", GetPorts,
     get_ports_params },
+  { "_getReachableSize", GetReachableSize,
+    get_reachable_size_params },
   { "_getRetainedSize", GetRetainedSize,
     get_retained_size_params },
   { "_getRetainingPath", GetRetainingPath,