diff --git a/asherah/asherah.py b/asherah/asherah.py
index b66d2bb..3f70e1c 100644
--- a/asherah/asherah.py
+++ b/asherah/asherah.py
@@ -1,4 +1,5 @@
 """Main Asherah class, for encrypting and decrypting of data"""
+
 # pylint: disable=line-too-long
 
 from __future__ import annotations
@@ -6,17 +7,16 @@
 import json
 import os
 from typing import ByteString, Union
-
 from cobhan import Cobhan
-
 from . import exceptions, types
 
 
 class Asherah:
     """The main class for providing encryption and decryption functionality"""
 
-    JSON_OVERHEAD = 256
-    KEY_SIZE = 64
+    ENCRYPTION_OVERHEAD = 48
+    ENVELOPE_OVERHEAD = 185
+    BASE64_OVERHEAD = 1.34
 
     def __init__(self):
         self.__cobhan = Cobhan()
@@ -35,6 +35,7 @@ def __init__(self):
 
     def setup(self, config: types.AsherahConfig) -> None:
         """Set up/initialize the underlying encryption library."""
+        self.ik_overhead = len(config.service_name) + len(config.product_id)
         config_json = json.dumps(config.to_json())
         config_buf = self.__cobhan.str_to_buf(config_json)
         result = self.__libasherah.SetupJson(config_buf)
@@ -55,7 +56,13 @@ def encrypt(self, partition_id: str, data: Union[ByteString, str]):
         partition_id_buf = self.__cobhan.str_to_buf(partition_id)
         data_buf = self.__cobhan.bytearray_to_buf(data)
         # Outputs
-        json_buf = self.__cobhan.allocate_buf(len(data_buf) + self.JSON_OVERHEAD)
+        buffer_estimate = int(
+            self.ENVELOPE_OVERHEAD
+            + self.ik_overhead
+            + len(partition_id_buf)
+            + ((len(data_buf) + self.ENCRYPTION_OVERHEAD) * self.BASE64_OVERHEAD)
+        )
+        json_buf = self.__cobhan.allocate_buf(buffer_estimate)
 
         result = self.__libasherah.EncryptToJson(partition_id_buf, data_buf, json_buf)
         if result < 0:
diff --git a/tests/test_asherah.py b/tests/test_asherah.py
index 78321c2..6aab69c 100644
--- a/tests/test_asherah.py
+++ b/tests/test_asherah.py
@@ -35,3 +35,9 @@ def test_decrypted_data_equals_original_data(self):
         encrypted = self.asherah.encrypt("partition", data)
         decrypted = self.asherah.decrypt("partition", encrypted)
         self.assertEqual(decrypted, data)
+
+    def test_encrypt_decrypt_large_data(self):
+        data = b"a" * 1024 * 1024
+        encrypted = self.asherah.encrypt("partition", data)
+        decrypted = self.asherah.decrypt("partition", encrypted)
+        self.assertEqual(decrypted, data)