diff --git a/substrate/core/executor/src/allocator.rs b/substrate/core/executor/src/allocator.rs
index 9e61a2c6ef..504b96987f 100644
--- a/substrate/core/executor/src/allocator.rs
+++ b/substrate/core/executor/src/allocator.rs
@@ -67,7 +67,6 @@ impl FreeingBumpHeapAllocator {
 		let current_size: Bytes = mem.current_size().into();
 		let current_size = current_size.0 as u32;
 		let used_size = mem.used_size().0 as u32;
-		let heap_size = current_size - used_size;
 
 		let mut ptr_offset = used_size;
 		let padding = ptr_offset % ALIGNMENT;
@@ -75,6 +74,8 @@ impl FreeingBumpHeapAllocator {
 			ptr_offset += ALIGNMENT - padding;
 		}
 
+		let heap_size = current_size - ptr_offset;
+
 		FreeingBumpHeapAllocator {
 			bumper: 0,
 			heads: [0; N],