diff --git a/src/core/instance/instance.cpp b/src/core/instance/instance.cpp
index d00129c70d8..94b93028953 100644
--- a/src/core/instance/instance.cpp
+++ b/src/core/instance/instance.cpp
@@ -351,9 +351,7 @@ Instance &Instance::Get(uint8_t aIdx)
 
 uint8_t Instance::GetIdx(Instance *aInstance)
 {
-    return static_cast<uint8_t>(
-        (reinterpret_cast<uint8_t *>(aInstance) - reinterpret_cast<uint8_t *>(gMultiInstanceRaw)) /
-        INSTANCE_SIZE_ALIGNED);
+    return static_cast<uint8_t>((reinterpret_cast<uint64_t *>(aInstance) - gMultiInstanceRaw) / INSTANCE_SIZE_ALIGNED);
 }
 
 #endif // #if OPENTHREAD_CONFIG_MULTIPLE_STATIC_INSTANCE_ENABLE