diff --git a/storage/local/chunk/chunk.go b/storage/local/chunk/chunk.go
index 11c42b39d..733028d16 100644
--- a/storage/local/chunk/chunk.go
+++ b/storage/local/chunk/chunk.go
@@ -260,7 +260,7 @@ func (d *Desc) MaybeEvict() bool {
 // Chunk is the interface for all chunks. Chunks are generally not
 // goroutine-safe.
 type Chunk interface {
-	// add adds a SamplePair to the chunks, performs any necessary
+	// Add adds a SamplePair to the chunks, performs any necessary
 	// re-encoding, and adds any necessary overflow chunks. It returns the
 	// new version of the original chunk, followed by overflow chunks, if
 	// any. The first chunk returned might be the same as the original one
@@ -276,6 +276,10 @@ type Chunk interface {
 	UnmarshalFromBuf([]byte) error
 	Encoding() Encoding
 	Utilization() float64
+
+	// Len returns the number of samples in the chunk.  Implementations may be
+	// expensive.
+	Len() int
 }
 
 // Iterator enables efficient access to the content of a chunk. It is
diff --git a/storage/local/chunk/chunk_test.go b/storage/local/chunk/chunk_test.go
new file mode 100644
index 000000000..f5f21f67d
--- /dev/null
+++ b/storage/local/chunk/chunk_test.go
@@ -0,0 +1,46 @@
+// Copyright 2016 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Note: this file has tests for code in both delta.go and doubledelta.go --
+// it may make sense to split those out later, but given that the tests are
+// near-identical and share a helper, this feels simpler for now.
+
+package chunk
+
+import (
+	"testing"
+
+	"github.com/prometheus/common/model"
+)
+
+func TestLen(t *testing.T) {
+	chunks := []Chunk{}
+	for _, encoding := range []Encoding{Delta, DoubleDelta, Varbit} {
+		c, err := NewForEncoding(encoding)
+		if err != nil {
+			t.Fatal(err)
+		}
+		chunks = append(chunks, c)
+	}
+
+	for _, c := range chunks {
+		for i := 0; i <= 10; i++ {
+			if c.Len() != i {
+				t.Errorf("chunk type %s should have %d samples, had %d", c.Encoding(), i, c.Len())
+			}
+
+			cs, _ := c.Add(model.SamplePair{model.Time(i), model.SampleValue(i)})
+			c = cs[0]
+		}
+	}
+}
diff --git a/storage/local/chunk/delta.go b/storage/local/chunk/delta.go
index 4e04bca47..2be00cce9 100644
--- a/storage/local/chunk/delta.go
+++ b/storage/local/chunk/delta.go
@@ -75,7 +75,7 @@ func newDeltaEncodedChunk(tb, vb deltaBytes, isInt bool, length int) *deltaEncod
 // Add implements chunk.
 func (c deltaEncodedChunk) Add(s model.SamplePair) ([]Chunk, error) {
 	// TODO(beorn7): Since we return &c, this method might cause an unnecessary allocation.
-	if c.len() == 0 {
+	if c.Len() == 0 {
 		c = c[:deltaHeaderBytes]
 		binary.LittleEndian.PutUint64(c[deltaHeaderBaseTimeOffset:], uint64(s.Timestamp))
 		binary.LittleEndian.PutUint64(c[deltaHeaderBaseValueOffset:], math.Float64bits(float64(s.Value)))
@@ -191,7 +191,7 @@ func (c deltaEncodedChunk) FirstTime() model.Time {
 
 // NewIterator implements chunk.
 func (c *deltaEncodedChunk) NewIterator() Iterator {
-	return newIndexAccessingChunkIterator(c.len(), &deltaEncodedIndexAccessor{
+	return newIndexAccessingChunkIterator(c.Len(), &deltaEncodedIndexAccessor{
 		c:      *c,
 		baseT:  c.baseTime(),
 		baseV:  c.baseValue(),
@@ -296,7 +296,8 @@ func (c deltaEncodedChunk) sampleSize() int {
 	return int(c.timeBytes() + c.valueBytes())
 }
 
-func (c deltaEncodedChunk) len() int {
+// Len implements Chunk. Runs in constant time.
+func (c deltaEncodedChunk) Len() int {
 	if len(c) < deltaHeaderBytes {
 		return 0
 	}
diff --git a/storage/local/chunk/doubledelta.go b/storage/local/chunk/doubledelta.go
index 9a4744130..2a1221461 100644
--- a/storage/local/chunk/doubledelta.go
+++ b/storage/local/chunk/doubledelta.go
@@ -83,14 +83,14 @@ func newDoubleDeltaEncodedChunk(tb, vb deltaBytes, isInt bool, length int) *doub
 // Add implements chunk.
 func (c doubleDeltaEncodedChunk) Add(s model.SamplePair) ([]Chunk, error) {
 	// TODO(beorn7): Since we return &c, this method might cause an unnecessary allocation.
-	if c.len() == 0 {
+	if c.Len() == 0 {
 		return c.addFirstSample(s), nil
 	}
 
 	tb := c.timeBytes()
 	vb := c.valueBytes()
 
-	if c.len() == 1 {
+	if c.Len() == 1 {
 		return c.addSecondSample(s, tb, vb)
 	}
 
@@ -103,10 +103,10 @@ func (c doubleDeltaEncodedChunk) Add(s model.SamplePair) ([]Chunk, error) {
 		return addToOverflowChunk(&c, s)
 	}
 
-	projectedTime := c.baseTime() + model.Time(c.len())*c.baseTimeDelta()
+	projectedTime := c.baseTime() + model.Time(c.Len())*c.baseTimeDelta()
 	ddt := s.Timestamp - projectedTime
 
-	projectedValue := c.baseValue() + model.SampleValue(c.len())*c.baseValueDelta()
+	projectedValue := c.baseValue() + model.SampleValue(c.Len())*c.baseValueDelta()
 	ddv := s.Value - projectedValue
 
 	ntb, nvb, nInt := tb, vb, c.isInt()
@@ -198,7 +198,7 @@ func (c doubleDeltaEncodedChunk) FirstTime() model.Time {
 
 // NewIterator( implements chunk.
 func (c *doubleDeltaEncodedChunk) NewIterator() Iterator {
-	return newIndexAccessingChunkIterator(c.len(), &doubleDeltaEncodedIndexAccessor{
+	return newIndexAccessingChunkIterator(c.Len(), &doubleDeltaEncodedIndexAccessor{
 		c:      *c,
 		baseT:  c.baseTime(),
 		baseΔT: c.baseTimeDelta(),
@@ -279,7 +279,7 @@ func (c doubleDeltaEncodedChunk) Encoding() Encoding { return DoubleDelta }
 
 // Utilization implements chunk.
 func (c doubleDeltaEncodedChunk) Utilization() float64 {
-	return float64(len(c)) / float64(cap(c))
+	return float64(len(c)-doubleDeltaHeaderIsIntOffset-1) / float64(cap(c))
 }
 
 func (c doubleDeltaEncodedChunk) baseTime() model.Time {
@@ -336,7 +336,8 @@ func (c doubleDeltaEncodedChunk) sampleSize() int {
 	return int(c.timeBytes() + c.valueBytes())
 }
 
-func (c doubleDeltaEncodedChunk) len() int {
+// Len implements Chunk. Runs in constant time.
+func (c doubleDeltaEncodedChunk) Len() int {
 	if len(c) <= doubleDeltaHeaderIsIntOffset+1 {
 		return 0
 	}
diff --git a/storage/local/chunk/varbit.go b/storage/local/chunk/varbit.go
index 42de4adb2..2ec59efd3 100644
--- a/storage/local/chunk/varbit.go
+++ b/storage/local/chunk/varbit.go
@@ -328,6 +328,15 @@ func (c varbitChunk) Utilization() float64 {
 	return math.Min(float64(c.nextSampleOffset()/8+15)/float64(cap(c)), 1)
 }
 
+// Len implements chunk.  Runs in O(n).
+func (c varbitChunk) Len() int {
+	it := c.NewIterator()
+	i := 0
+	for ; it.Scan(); i++ {
+	}
+	return i
+}
+
 // FirstTime implements chunk.
 func (c varbitChunk) FirstTime() model.Time {
 	return model.Time(