diff --git a/tree.go b/tree.go
index 14a31b0..186de97 100644
--- a/tree.go
+++ b/tree.go
@@ -13,6 +13,7 @@ package arbo
 
 import (
 	"bytes"
+	"encoding/binary"
 	"encoding/hex"
 	"fmt"
 	"io"
@@ -41,8 +42,9 @@ const (
 )
 
 var (
-	dbKeyRoot  = []byte("root")
-	emptyValue = []byte{0}
+	dbKeyRoot   = []byte("root")
+	dbKeyNLeafs = []byte("nleafs")
+	emptyValue  = []byte{0}
 )
 
 // Tree defines the struct that implements the MerkleTree functionalities
@@ -55,6 +57,7 @@ type Tree struct {
 	root       []byte
 
 	hashFunction HashFunction
+	emptyHash    []byte
 }
 
 // NewTree returns a new Tree, if there is a Tree still in the given storage, it
@@ -63,18 +66,23 @@ func NewTree(storage db.Storage, maxLevels int, hash HashFunction) (*Tree, error
 	t := Tree{db: storage, maxLevels: maxLevels, hashFunction: hash}
 	t.updateAccessTime()
 
+	t.emptyHash = make([]byte, t.hashFunction.Len()) // empty
+
 	root, err := t.dbGet(dbKeyRoot)
 	if err == db.ErrNotFound {
 		// store new root 0
-		tx, err := t.db.NewTx()
+		t.tx, err = t.db.NewTx()
 		if err != nil {
 			return nil, err
 		}
-		t.root = make([]byte, t.hashFunction.Len()) // empty
-		if err = tx.Put(dbKeyRoot, t.root); err != nil {
+		t.root = t.emptyHash
+		if err = t.tx.Put(dbKeyRoot, t.root); err != nil {
+			return nil, err
+		}
+		if err = t.setNLeafs(0); err != nil {
 			return nil, err
 		}
-		if err = tx.Commit(); err != nil {
+		if err = t.tx.Commit(); err != nil {
 			return nil, err
 		}
 		return &t, err
@@ -129,6 +137,10 @@ func (t *Tree) AddBatch(keys, values [][]byte) ([]int, error) {
 	if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
 		return indexes, err
 	}
+	// update nLeafs
+	if err = t.incNLeafs(uint64(len(keys) - len(indexes))); err != nil {
+		return indexes, err
+	}
 
 	if err := t.tx.Commit(); err != nil {
 		return nil, err
@@ -159,6 +171,10 @@ func (t *Tree) Add(k, v []byte) error {
 	if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
 		return err
 	}
+	// update nLeafs
+	if err = t.incNLeafs(1); err != nil {
+		return err
+	}
 	return t.tx.Commit()
 }
 
@@ -208,8 +224,7 @@ func (t *Tree) down(newKey, currKey []byte, siblings [][]byte,
 	}
 	var err error
 	var currValue []byte
-	emptyKey := make([]byte, t.hashFunction.Len())
-	if bytes.Equal(currKey, emptyKey) {
+	if bytes.Equal(currKey, t.emptyHash) {
 		// empty value
 		return currKey, emptyValue, siblings, nil
 	}
@@ -277,8 +292,7 @@ func (t *Tree) downVirtually(siblings [][]byte, oldKey, newKey []byte, oldPath,
 	}
 
 	if oldPath[l] == newPath[l] {
-		emptyKey := make([]byte, t.hashFunction.Len())
-		siblings = append(siblings, emptyKey)
+		siblings = append(siblings, t.emptyHash)
 
 		siblings, err = t.downVirtually(siblings, oldKey, newKey, oldPath, newPath, l+1)
 		if err != nil {
@@ -599,9 +613,8 @@ func CheckProof(hashFunc HashFunction, k, v, root, packedSiblings []byte) (bool,
 
 func (t *Tree) dbGet(k []byte) ([]byte, error) {
 	// if key is empty, return empty as value
-	empty := make([]byte, t.hashFunction.Len())
-	if bytes.Equal(k, empty) {
-		return empty, nil
+	if bytes.Equal(k, t.emptyHash) {
+		return t.emptyHash, nil
 	}
 
 	v, err := t.db.Get(k)
@@ -614,6 +627,38 @@ func (t *Tree) dbGet(k []byte) ([]byte, error) {
 	return nil, db.ErrNotFound
 }
 
+// Warning: should be called with a Tree.tx created, and with a Tree.tx.Commit
+// after the setNLeafs call.
+func (t *Tree) incNLeafs(nLeafs uint64) error {
+	oldNLeafs, err := t.GetNLeafs()
+	if err != nil {
+		return err
+	}
+	newNLeafs := oldNLeafs + nLeafs
+	return t.setNLeafs(newNLeafs)
+}
+
+// Warning: should be called with a Tree.tx created, and with a Tree.tx.Commit
+// after the setNLeafs call.
+func (t *Tree) setNLeafs(nLeafs uint64) error {
+	b := make([]byte, 8)
+	binary.LittleEndian.PutUint64(b, nLeafs)
+	if err := t.tx.Put(dbKeyNLeafs, b); err != nil {
+		return err
+	}
+	return nil
+}
+
+// GetNLeafs returns the number of Leafs of the Tree.
+func (t *Tree) GetNLeafs() (uint64, error) {
+	b, err := t.dbGet(dbKeyNLeafs)
+	if err != nil {
+		return 0, err
+	}
+	nLeafs := binary.LittleEndian.Uint64(b)
+	return nLeafs, nil
+}
+
 // Iterate iterates through the full Tree, executing the given function on each
 // node of the Tree.
 func (t *Tree) Iterate(f func([]byte, []byte)) error {
@@ -677,9 +722,13 @@ func (t *Tree) Dump() ([]byte, error) {
 func (t *Tree) ImportDump(b []byte) error {
 	t.updateAccessTime()
 	r := bytes.NewReader(b)
+	count := 0
+	// TODO instead of adding one by one, use AddBatch (once AddBatch is
+	// optimized)
+	var err error
 	for {
 		l := make([]byte, 2)
-		_, err := io.ReadFull(r, l)
+		_, err = io.ReadFull(r, l)
 		if err == io.EOF {
 			break
 		} else if err != nil {
@@ -699,6 +748,19 @@ func (t *Tree) ImportDump(b []byte) error {
 		if err != nil {
 			return err
 		}
+		count++
+	}
+	// update nLeafs (once ImportDump uses AddBatch method, this will not be
+	// needed)
+	t.tx, err = t.db.NewTx()
+	if err != nil {
+		return err
+	}
+	if err := t.incNLeafs(uint64(count)); err != nil {
+		return err
+	}
+	if err = t.tx.Commit(); err != nil {
+		return err
 	}
 	return nil
 }
@@ -711,7 +773,6 @@ node [fontname=Monospace,fontsize=10,shape=box]
 `)
 	nChars := 4
 	nEmpties := 0
-	empty := make([]byte, t.hashFunction.Len())
 	err := t.Iterate(func(k, v []byte) {
 		switch v[0] {
 		case PrefixValueEmpty:
@@ -729,13 +790,13 @@ node [fontname=Monospace,fontsize=10,shape=box]
 			lStr := hex.EncodeToString(l[:nChars])
 			rStr := hex.EncodeToString(r[:nChars])
 			eStr := ""
-			if bytes.Equal(l, empty) {
+			if bytes.Equal(l, t.emptyHash) {
 				lStr = fmt.Sprintf("empty%v", nEmpties)
 				eStr += fmt.Sprintf("\"%v\" [style=dashed,label=0];\n",
 					lStr)
 				nEmpties++
 			}
-			if bytes.Equal(r, empty) {
+			if bytes.Equal(r, t.emptyHash) {
 				rStr = fmt.Sprintf("empty%v", nEmpties)
 				eStr += fmt.Sprintf("\"%v\" [style=dashed,label=0];\n",
 					rStr)
diff --git a/tree_test.go b/tree_test.go
index 53d1c15..963d2bc 100644
--- a/tree_test.go
+++ b/tree_test.go
@@ -325,6 +325,54 @@ func TestRWMutex(t *testing.T) {
 	}
 }
 
+func TestSetGetNLeafs(t *testing.T) {
+	c := qt.New(t)
+	tree, err := NewTree(memory.NewMemoryStorage(), 100, HashFunctionPoseidon)
+	c.Assert(err, qt.IsNil)
+
+	// 0
+	tree.tx, err = tree.db.NewTx()
+	c.Assert(err, qt.IsNil)
+
+	err = tree.setNLeafs(0)
+	c.Assert(err, qt.IsNil)
+
+	err = tree.tx.Commit()
+	c.Assert(err, qt.IsNil)
+
+	n, err := tree.GetNLeafs()
+	c.Assert(err, qt.IsNil)
+	c.Assert(n, qt.Equals, uint64(0))
+
+	// 1024
+	tree.tx, err = tree.db.NewTx()
+	c.Assert(err, qt.IsNil)
+
+	err = tree.setNLeafs(1024)
+	c.Assert(err, qt.IsNil)
+
+	err = tree.tx.Commit()
+	c.Assert(err, qt.IsNil)
+
+	n, err = tree.GetNLeafs()
+	c.Assert(err, qt.IsNil)
+	c.Assert(n, qt.Equals, uint64(1024))
+
+	// 2**64 -1
+	tree.tx, err = tree.db.NewTx()
+	c.Assert(err, qt.IsNil)
+
+	err = tree.setNLeafs(18446744073709551615)
+	c.Assert(err, qt.IsNil)
+
+	err = tree.tx.Commit()
+	c.Assert(err, qt.IsNil)
+
+	n, err = tree.GetNLeafs()
+	c.Assert(err, qt.IsNil)
+	c.Assert(n, qt.Equals, uint64(18446744073709551615))
+}
+
 func BenchmarkAdd(b *testing.B) {
 	// prepare inputs
 	var ks, vs [][]byte