diff --git a/bucket.go b/bucket.go
index b0fd8bb..d70b4bb 100644
--- a/bucket.go
+++ b/bucket.go
@@ -22,13 +22,12 @@ func (b *bucket) set(key string, value interface{}, duration time.Duration) (*It
 	b.Lock()
 	defer b.Unlock()
 	if existing, exists := b.lookup[key]; exists {
-		s := existing.size
 		existing.value = value
 		existing.expires = expires
 		d := int64(0)
 		if sized, ok := value.(Sized); ok {
 			newSize := sized.Size()
-			d = newSize - s
+			d = newSize - existing.size
 			if d != 0 {
 				atomic.StoreInt64(&existing.size, newSize)
 			}
@@ -40,15 +39,23 @@ func (b *bucket) set(key string, value interface{}, duration time.Duration) (*It
 	return item, true, int64(item.size)
 }
 
-func (b *bucket) replace(key string, value interface{}) bool {
+func (b *bucket) replace(key string, value interface{}) (bool, int64) {
 	b.Lock()
 	defer b.Unlock()
 	existing, exists := b.lookup[key]
 	if exists == false {
-		return false
+		return false, 0
+	}
+	d := int64(0)
+	if sized, ok := value.(Sized); ok {
+		newSize := sized.Size()
+		d = newSize - existing.size
+		if d != 0 {
+			atomic.StoreInt64(&existing.size, newSize)
+		}
 	}
 	existing.value = value
-	return true
+	return true, d
 }
 
 func (b *bucket) delete(key string) *Item {
diff --git a/cache.go b/cache.go
index 5561af8..daf251b 100644
--- a/cache.go
+++ b/cache.go
@@ -82,7 +82,11 @@ func (c *Cache) Set(key string, value interface{}, duration time.Duration) {
 // Returns true if the item existed an was replaced, false otherwise.
 // Replace does not reset item's TTL nor does it alter its position in the LRU
 func (c *Cache) Replace(key string, value interface{}) bool {
-	return c.bucket(key).replace(key, value)
+	exists, d := c.bucket(key).replace(key, value)
+	if d != 0 {
+		atomic.AddInt64(&c.size, d)
+	}
+	return exists
 }
 
 // Attempts to get the value from the cache and calles fetch on a miss.
diff --git a/cache_test.go b/cache_test.go
index 088bff2..1bbc5d3 100644
--- a/cache_test.go
+++ b/cache_test.go
@@ -102,6 +102,30 @@ func (_ CacheTests) SetUpdatesSizeOnDelta() {
 	Expect(cache.size).To.Equal(int64(2))
 }
 
+func (_ CacheTests) ReplaceDoesNotchangeSizeIfNotSet() {
+	cache := New(Configure())
+	cache.Set("1", &SizedItem{1, 2}, time.Minute)
+	cache.Set("2", &SizedItem{1, 2}, time.Minute)
+	cache.Set("3", &SizedItem{1, 2}, time.Minute)
+	cache.Replace("4", &SizedItem{1, 2})
+	Expect(cache.size).To.Equal(int64(6))
+}
+
+func (_ CacheTests) ReplaceChangesSize() {
+	cache := New(Configure())
+	cache.Set("1", &SizedItem{1, 2}, time.Minute)
+	cache.Set("2", &SizedItem{1, 2}, time.Minute)
+
+	cache.Replace("2", &SizedItem{1, 2})
+	Expect(cache.size).To.Equal(int64(4))
+
+	cache.Replace("2", &SizedItem{1, 1})
+	Expect(cache.size).To.Equal(int64(3))
+
+	cache.Replace("2", &SizedItem{1, 3})
+	Expect(cache.size).To.Equal(int64(5))
+}
+
 type SizedItem struct {
 	id int
 	s  int64
diff --git a/layeredbucket.go b/layeredbucket.go
index 73a0ddc..b541c76 100644
--- a/layeredbucket.go
+++ b/layeredbucket.go
@@ -35,12 +35,12 @@ func (b *layeredBucket) set(primary, secondary string, value interface{}, durati
 	return item, new, d
 }
 
-func (b *layeredBucket) replace(primary, secondary string, value interface{}) bool {
+func (b *layeredBucket) replace(primary, secondary string, value interface{}) (bool, int64) {
 	b.Lock()
 	bucket, exists := b.buckets[primary]
 	b.Unlock()
 	if exists == false {
-		return false
+		return false, 0
 	}
 	return bucket.replace(secondary, value)
 }
diff --git a/layeredcache.go b/layeredcache.go
index 77d817c..79f6d20 100644
--- a/layeredcache.go
+++ b/layeredcache.go
@@ -93,7 +93,11 @@ func (c *LayeredCache) Set(primary, secondary string, value interface{}, duratio
 // Returns true if the item existed an was replaced, false otherwise.
 // Replace does not reset item's TTL nor does it alter its position in the LRU
 func (c *LayeredCache) Replace(primary, secondary string, value interface{}) bool {
-	return c.bucket(primary).replace(primary, secondary, value)
+	exists, d := c.bucket(primary).replace(primary, secondary, value)
+	if d != 0 {
+		atomic.AddInt64(&c.size, d)
+	}
+	return exists
 }
 
 // Attempts to get the value from the cache and calles fetch on a miss.
diff --git a/layeredcache_test.go b/layeredcache_test.go
index fde1439..67dfa5c 100644
--- a/layeredcache_test.go
+++ b/layeredcache_test.go
@@ -171,3 +171,27 @@ func (_ LayeredCacheTests) SetUpdatesSizeOnDelta() {
 	time.Sleep(time.Millisecond * 10)
 	Expect(cache.size).To.Equal(int64(5))
 }
+
+func (_ LayeredCacheTests) ReplaceDoesNotchangeSizeIfNotSet() {
+	cache := Layered(Configure())
+	cache.Set("pri", "1", &SizedItem{1, 2}, time.Minute)
+	cache.Set("pri", "2", &SizedItem{1, 2}, time.Minute)
+	cache.Set("pri", "3", &SizedItem{1, 2}, time.Minute)
+	cache.Replace("sec", "3", &SizedItem{1, 2})
+	Expect(cache.size).To.Equal(int64(6))
+}
+
+func (_ LayeredCacheTests) ReplaceChangesSize() {
+	cache := Layered(Configure())
+	cache.Set("pri", "1", &SizedItem{1, 2}, time.Minute)
+	cache.Set("pri", "2", &SizedItem{1, 2}, time.Minute)
+
+	cache.Replace("pri", "2", &SizedItem{1, 2})
+	Expect(cache.size).To.Equal(int64(4))
+
+	cache.Replace("pri", "2", &SizedItem{1, 1})
+	Expect(cache.size).To.Equal(int64(3))
+
+	cache.Replace("pri", "2", &SizedItem{1, 3})
+	Expect(cache.size).To.Equal(int64(5))
+}