diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 75c033cc76..ced0f489be 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -291,7 +291,6 @@ uint16_t APIConnection::encode_message_to_buffer(ProtoMessage &msg, uint8_t mess
 
   // Get buffer size after allocation (which includes header padding)
   std::vector<uint8_t> &shared_buf = conn->parent_->get_shared_buffer_ref();
-  size_t size_before_encode = shared_buf.size();
 
   if (is_single || conn->flags_.batch_first_message) {
     // Single message or first batch message
@@ -302,11 +301,13 @@ uint16_t APIConnection::encode_message_to_buffer(ProtoMessage &msg, uint8_t mess
   } else {
     // Batch message second or later
     // Add padding for previous message footer + this message header
-    shared_buf.reserve(size_before_encode + total_calculated_size);
-    shared_buf.resize(size_before_encode + footer_size + header_padding);
+    size_t current_size = shared_buf.size();
+    shared_buf.reserve(current_size + total_calculated_size);
+    shared_buf.resize(current_size + footer_size + header_padding);
   }
 
   // Encode directly into buffer
+  size_t size_before_encode = shared_buf.size();
   msg.encode({&shared_buf});
 
   // Calculate actual encoded size (not including header that was already added)