diff --git a/src/basic/macro.h b/src/basic/macro.h
index 072fed4378..6507728a51 100644
--- a/src/basic/macro.h
+++ b/src/basic/macro.h
@@ -213,7 +213,7 @@ static inline size_t GREEDY_ALLOC_ROUND_UP(size_t l) {
  *          Contrary to strlen(), this is a constant expression.
  * @x: a string literal.
  */
-#define STRLEN(x) (sizeof(""x"") - 1)
+#define STRLEN(x) ((unsigned) sizeof(""x"") - 1)
 
 /*
  * container_of - cast a member of a structure out to the containing structure
@@ -341,10 +341,10 @@ static inline int __coverity_check_and_return__(int condition) {
  * negative '-' prefix (hence works correctly on signed
  * types). Includes space for the trailing NUL. */
 #define DECIMAL_STR_MAX(type)                                           \
-        (2+(sizeof(type) <= 1 ? 3 :                                     \
-            sizeof(type) <= 2 ? 5 :                                     \
-            sizeof(type) <= 4 ? 10 :                                    \
-            sizeof(type) <= 8 ? 20 : sizeof(int[-2*(sizeof(type) > 8)])))
+        (2U+(sizeof(type) <= 1 ? 3U :                                   \
+             sizeof(type) <= 2 ? 5U :                                   \
+             sizeof(type) <= 4 ? 10U :                                  \
+             sizeof(type) <= 8 ? 20U : (unsigned) sizeof(int[-2*(sizeof(type) > 8)])))
 
 #define DECIMAL_STR_WIDTH(x)                            \
         ({                                              \
diff --git a/src/test/test-format-util.c b/src/test/test-format-util.c
index 5562ac8761..4e6c90adfa 100644
--- a/src/test/test-format-util.c
+++ b/src/test/test-format-util.c
@@ -4,6 +4,15 @@
 #include "macro.h"
 #include "string-util.h"
 
+/* Do some basic checks on STRLEN() and DECIMAL_STR_MAX() */
+assert_cc(STRLEN("xxx") == 3);
+assert_cc(STRLEN("") == 0);
+assert_cc(DECIMAL_STR_MAX(uint8_t) == 5);
+assert_cc(DECIMAL_STR_MAX(int8_t) == 5);
+assert_cc(DECIMAL_STR_MAX(uint64_t) == 22);
+assert_cc(DECIMAL_STR_MAX(char) == 5);
+assert_cc(CONST_MAX(DECIMAL_STR_MAX(int8_t), STRLEN("xxx")) == 5);
+
 static void test_format_bytes_one(uint64_t val, bool trailing_B, const char *iec_with_p, const char *iec_without_p,
                                   const char *si_with_p, const char *si_without_p) {
         char buf[FORMAT_BYTES_MAX];