diff --git a/moto/cognitoidp/models.py b/moto/cognitoidp/models.py
index 3fe810b2b..dd1c52055 100644
--- a/moto/cognitoidp/models.py
+++ b/moto/cognitoidp/models.py
@@ -76,6 +76,7 @@ class CognitoIdpUserPool(BaseModel):
             "CreationDate": time.mktime(self.creation_date.timetuple()),
             "LastModifiedDate": time.mktime(self.last_modified_date.timetuple()),
             "MfaConfiguration": self.mfa_config,
+            "EstimatedNumberOfUsers": len(self.users),
         }
 
     def to_json(self, extended=False):
diff --git a/tests/test_cognitoidp/test_cognitoidp.py b/tests/test_cognitoidp/test_cognitoidp.py
index dbe59f656..ea4870b8a 100644
--- a/tests/test_cognitoidp/test_cognitoidp.py
+++ b/tests/test_cognitoidp/test_cognitoidp.py
@@ -215,6 +215,22 @@ def test_describe_user_pool():
     result["UserPool"]["LambdaConfig"]["PreSignUp"].should.equal(value)
 
 
+@mock_cognitoidp
+def test_describe_user_pool_estimated_number_of_users():
+    conn = boto3.client("cognito-idp", "us-west-2")
+    user_pool_id = conn.create_user_pool(PoolName=str(uuid.uuid4()))["UserPool"]["Id"]
+
+    result = conn.describe_user_pool(UserPoolId=user_pool_id)
+    result["UserPool"]["EstimatedNumberOfUsers"].should.equal(0)
+
+    users_count = random.randint(2, 6)
+    for _ in range(users_count):
+        conn.admin_create_user(UserPoolId=user_pool_id, Username=str(uuid.uuid4()))
+
+    result = conn.describe_user_pool(UserPoolId=user_pool_id)
+    result["UserPool"]["EstimatedNumberOfUsers"].should.equal(users_count)
+
+
 @mock_cognitoidp
 def test_describe_user_pool_resource_not_found():
     conn = boto3.client("cognito-idp", "us-east-1")