diff --git a/gee-cache/day1-lru/geecache/lru/lru.go b/gee-cache/day1-lru/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day1-lru/geecache/lru/lru.go
+++ b/gee-cache/day1-lru/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day1-lru/geecache/lru/lru_test.go b/gee-cache/day1-lru/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day1-lru/geecache/lru/lru_test.go
+++ b/gee-cache/day1-lru/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/day2-single-node/geecache/lru/lru.go b/gee-cache/day2-single-node/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day2-single-node/geecache/lru/lru.go
+++ b/gee-cache/day2-single-node/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day2-single-node/geecache/lru/lru_test.go b/gee-cache/day2-single-node/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day2-single-node/geecache/lru/lru_test.go
+++ b/gee-cache/day2-single-node/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/day3-http-server/geecache/lru/lru.go b/gee-cache/day3-http-server/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day3-http-server/geecache/lru/lru.go
+++ b/gee-cache/day3-http-server/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day3-http-server/geecache/lru/lru_test.go b/gee-cache/day3-http-server/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day3-http-server/geecache/lru/lru_test.go
+++ b/gee-cache/day3-http-server/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/day4-consistent-hash/geecache/lru/lru.go b/gee-cache/day4-consistent-hash/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day4-consistent-hash/geecache/lru/lru.go
+++ b/gee-cache/day4-consistent-hash/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day4-consistent-hash/geecache/lru/lru_test.go b/gee-cache/day4-consistent-hash/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day4-consistent-hash/geecache/lru/lru_test.go
+++ b/gee-cache/day4-consistent-hash/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/day5-multi-nodes/geecache/lru/lru.go b/gee-cache/day5-multi-nodes/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day5-multi-nodes/geecache/lru/lru.go
+++ b/gee-cache/day5-multi-nodes/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day5-multi-nodes/geecache/lru/lru_test.go b/gee-cache/day5-multi-nodes/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day5-multi-nodes/geecache/lru/lru_test.go
+++ b/gee-cache/day5-multi-nodes/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/day6-single-flight/geecache/lru/lru.go b/gee-cache/day6-single-flight/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day6-single-flight/geecache/lru/lru.go
+++ b/gee-cache/day6-single-flight/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day6-single-flight/geecache/lru/lru_test.go b/gee-cache/day6-single-flight/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day6-single-flight/geecache/lru/lru_test.go
+++ b/gee-cache/day6-single-flight/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/day7-proto-buf/geecache/lru/lru.go b/gee-cache/day7-proto-buf/geecache/lru/lru.go
index 81eee43..6fa617a 100644
--- a/gee-cache/day7-proto-buf/geecache/lru/lru.go
+++ b/gee-cache/day7-proto-buf/geecache/lru/lru.go
@@ -37,6 +37,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}
diff --git a/gee-cache/day7-proto-buf/geecache/lru/lru_test.go b/gee-cache/day7-proto-buf/geecache/lru/lru_test.go
index 7308322..f2d3470 100644
--- a/gee-cache/day7-proto-buf/geecache/lru/lru_test.go
+++ b/gee-cache/day7-proto-buf/geecache/lru/lru_test.go
@@ -53,3 +53,13 @@ func TestOnEvicted(t *testing.T) {
 		t.Fatalf("Call OnEvicted failed, expect keys equals to %s", expect)
 	}
 }
+
+func TestAdd(t *testing.T) {
+	lru := New(int64(0), nil)
+	lru.Add("key", String("1"))
+	lru.Add("key", String("111"))
+
+	if lru.nbytes != int64(len("key")+len("111")) {
+		t.Fatal("expected 6 but got", lru.nbytes)
+	}
+}
diff --git a/gee-cache/doc/geecache-day1.md b/gee-cache/doc/geecache-day1.md
index 6848421..064d90a 100644
--- a/gee-cache/doc/geecache-day1.md
+++ b/gee-cache/doc/geecache-day1.md
@@ -153,6 +153,7 @@ func (c *Cache) Add(key string, value Value) {
 	if ele, ok := c.cache[key]; ok {
 		c.ll.MoveToFront(ele)
 		kv := ele.Value.(*entry)
+		c.nbytes += int64(value.Len()) - int64(kv.value.Len())
 		kv.value = value
 		return
 	}