diff --git a/include/ruby/util.h b/include/ruby/util.h
index ee11bc940a..12e69c4b80 100644
--- a/include/ruby/util.h
+++ b/include/ruby/util.h
@@ -33,7 +33,9 @@
 
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 
-/** an approximation of ceil(n * log10(2)), up to 65536 at least */
+/** an approximation of ceil(n * log10(2)), up to 1,048,576 (1<<20)
+ * without overflow within 32-bit calculation
+ */
 #define DECIMAL_SIZE_OF_BITS(n) (((n) * 3010 + 9998) / 9999)
 
 /** an approximation of decimal representation size for n-bytes */