diff --git a/ast.c b/ast.c
index adb7287ed3..d60e5d3fcf 100644
--- a/ast.c
+++ b/ast.c
@@ -377,7 +377,7 @@ rest_arg(rb_ast_t *ast, const NODE *rest_arg)
 static VALUE
 node_children(rb_ast_t *ast, const NODE *node)
 {
-    char name[DECIMAL_SIZE_OF_BITS(sizeof(long) * CHAR_BIT) + 2]; /* including '$' */
+    char name[sizeof("$") + DECIMAL_SIZE_OF(long)];
 
     enum node_type type = nd_type(node);
     switch (type) {
diff --git a/compile.c b/compile.c
index 66dafe0169..41e908ecd0 100644
--- a/compile.c
+++ b/compile.c
@@ -8275,7 +8275,7 @@ compile_builtin_function_call(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NOD
     }
     else {
 # define BUILTIN_INLINE_PREFIX "_bi"
-        char inline_func[DECIMAL_SIZE_OF_BITS(sizeof(int) * CHAR_BIT) + sizeof(BUILTIN_INLINE_PREFIX)];
+        char inline_func[sizeof(BUILTIN_INLINE_PREFIX) + DECIMAL_SIZE_OF(int)];
         bool cconst = false;
       retry:;
         const struct rb_builtin_function *bf = iseq_builtin_function_lookup(iseq, builtin_func);
diff --git a/include/ruby/util.h b/include/ruby/util.h
index e8727a3200..ee11bc940a 100644
--- a/include/ruby/util.h
+++ b/include/ruby/util.h
@@ -36,6 +36,15 @@ RBIMPL_SYMBOL_EXPORT_BEGIN()
 /** an approximation of ceil(n * log10(2)), up to 65536 at least */
 #define DECIMAL_SIZE_OF_BITS(n) (((n) * 3010 + 9998) / 9999)
 
+/** an approximation of decimal representation size for n-bytes */
+#define DECIMAL_SIZE_OF_BYTES(n) DECIMAL_SIZE_OF_BITS((n) * CHAR_BIT)
+
+/**
+ * An approximation of decimal representation size. `expr` may be a
+ * type name
+ */
+#define DECIMAL_SIZE_OF(expr) DECIMAL_SIZE_OF_BYTES(sizeof(expr))
+
 /**
  * Character to  number mapping  like `'a'`  -> `10`, `'b'`  -> `11`  etc.  For
  * punctuation etc.,  the value is  -1.  "36"  terminology comes from  the fact