diff --git a/compile.c b/compile.c
index 879d332e89..f4ae64bacc 100644
--- a/compile.c
+++ b/compile.c
@@ -868,7 +868,7 @@ compile_data_alloc(rb_iseq_t *iseq, size_t size)
 	    alloc_size *= 2;
 	}
 	storage->next = (void *)ALLOC_N(char, alloc_size +
-					sizeof(struct iseq_compile_data_storage));
+					offsetof(struct iseq_compile_data_storage, buff));
 	storage = ISEQ_COMPILE_DATA(iseq)->storage_current = storage->next;
 	storage->next = 0;
 	storage->pos = 0;
diff --git a/file.c b/file.c
index 5947b3f68b..7fdca684cc 100644
--- a/file.c
+++ b/file.c
@@ -388,7 +388,7 @@ apply2files(int (*func)(const char *, void *), int argc, VALUE *argv, void *arg)
 {
     VALUE v;
     const size_t size = sizeof(struct apply_filename);
-    const long len = (long)(sizeof(struct apply_arg) + (size * argc));
+    const long len = (long)(offsetof(struct apply_arg, fn) + (size * argc));
     struct apply_arg *aa = ALLOCV(v, len);
 
     aa->errnum = 0;
diff --git a/iseq.c b/iseq.c
index 57ff506d0a..20ad68e65e 100644
--- a/iseq.c
+++ b/iseq.c
@@ -208,7 +208,7 @@ iseq_memsize(const rb_iseq_t *iseq)
 
 	cur = compile_data->storage_head;
 	while (cur) {
-	    size += cur->size + sizeof(struct iseq_compile_data_storage);
+	    size += cur->size + offsetof(struct iseq_compile_data_storage, buff);
 	    cur = cur->next;
 	}
     }
@@ -333,7 +333,7 @@ prepare_iseq_build(rb_iseq_t *iseq,
     ISEQ_COMPILE_DATA(iseq)->storage_head = ISEQ_COMPILE_DATA(iseq)->storage_current =
       (struct iseq_compile_data_storage *)
 	ALLOC_N(char, INITIAL_ISEQ_COMPILE_DATA_STORAGE_BUFF_SIZE +
-		sizeof(struct iseq_compile_data_storage));
+		offsetof(struct iseq_compile_data_storage, buff));
 
     RB_OBJ_WRITE(iseq, &ISEQ_COMPILE_DATA(iseq)->catch_table_ary, rb_ary_tmp_new(3));
     ISEQ_COMPILE_DATA(iseq)->storage_head->pos = 0;
diff --git a/iseq.h b/iseq.h
index daeac0c889..283f5dcdf4 100644
--- a/iseq.h
+++ b/iseq.h
@@ -256,11 +256,12 @@ static inline int
 iseq_catch_table_bytes(int n)
 {
     enum {
-	catch_table_entries_max = (INT_MAX - sizeof(struct iseq_catch_table)) / sizeof(struct iseq_catch_table_entry)
+	catch_table_entry_size = sizeof(struct iseq_catch_table_entry),
+	catch_table_entries_max = (INT_MAX - offsetof(struct iseq_catch_table, entries)) / catch_table_entry_size
     };
     if (n > catch_table_entries_max) rb_fatal("too large iseq_catch_table - %d", n);
-    return (int)(sizeof(struct iseq_catch_table) +
-		 n * sizeof(struct iseq_catch_table_entry));
+    return (int)(offsetof(struct iseq_catch_table, entries) +
+		 n * catch_table_entry_size);
 }
 
 #define INITIAL_ISEQ_COMPILE_DATA_STORAGE_BUFF_SIZE (512)
diff --git a/string.c b/string.c
index 22734e0e1b..63b4d57f82 100644
--- a/string.c
+++ b/string.c
@@ -6410,7 +6410,7 @@ rb_str_casemap(VALUE source, OnigCaseFoldType *flags, rb_encoding *enc)
 	if (CASEMAP_DEBUG) {
 	    fprintf(stderr, "Buffer allocation, capa is %"PRIuSIZE"\n", capa); /* for tuning */
 	}
-	current_buffer->next = xmalloc(sizeof(mapping_buffer) + capa);
+	current_buffer->next = xmalloc(offsetof(mapping_buffer, space) + capa);
 	current_buffer = current_buffer->next;
 	current_buffer->next = NULL;
 	current_buffer->capa = capa;
diff --git a/variable.c b/variable.c
index 836686316f..1923f28f52 100644
--- a/variable.c
+++ b/variable.c
@@ -1001,7 +1001,7 @@ generic_ivar_get(VALUE obj, ID id, VALUE undef)
 static size_t
 gen_ivtbl_bytes(size_t n)
 {
-    return sizeof(struct gen_ivtbl) + n * sizeof(VALUE);
+    return offsetof(struct gen_ivtbl, ivptr) + n * sizeof(VALUE);
 }
 
 static struct gen_ivtbl *