diff --git a/examples/platforms/nrf52811/diag.c b/examples/platforms/nrf52811/diag.c
index 232f160a3..9401dd4e5 100644
--- a/examples/platforms/nrf52811/diag.c
+++ b/examples/platforms/nrf52811/diag.c
@@ -380,9 +380,9 @@ const struct PlatformDiagCommand sCommands[] = {
 
 void otPlatDiagProcess(otInstance *aInstance, int argc, char *argv[], char *aOutput, size_t aOutputMaxLen)
 {
-    uint32_t i;
+    size_t i;
 
-    for (i = 0; i < sizeof(sCommands) / sizeof(sCommands[0]); i++)
+    for (i = 0; i < otARRAY_LENGTH(sCommands); i++)
     {
         if (strcmp(argv[0], sCommands[i].mName) == 0)
         {
@@ -391,7 +391,7 @@ void otPlatDiagProcess(otInstance *aInstance, int argc, char *argv[], char *aOut
         }
     }
 
-    if (i == sizeof(sCommands) / sizeof(sCommands[0]))
+    if (i == otARRAY_LENGTH(sCommands))
     {
         snprintf(aOutput, aOutputMaxLen, "diag feature '%s' is not supported\r\n", argv[0]);
     }
diff --git a/examples/platforms/nrf52840/diag.c b/examples/platforms/nrf52840/diag.c
index 636363a62..e76aa1c6a 100644
--- a/examples/platforms/nrf52840/diag.c
+++ b/examples/platforms/nrf52840/diag.c
@@ -380,9 +380,9 @@ const struct PlatformDiagCommand sCommands[] = {
 
 void otPlatDiagProcess(otInstance *aInstance, int argc, char *argv[], char *aOutput, size_t aOutputMaxLen)
 {
-    uint32_t i;
+    size_t i;
 
-    for (i = 0; i < sizeof(sCommands) / sizeof(sCommands[0]); i++)
+    for (i = 0; i < otARRAY_LENGTH(sCommands); i++)
     {
         if (strcmp(argv[0], sCommands[i].mName) == 0)
         {
@@ -391,7 +391,7 @@ void otPlatDiagProcess(otInstance *aInstance, int argc, char *argv[], char *aOut
         }
     }
 
-    if (i == sizeof(sCommands) / sizeof(sCommands[0]))
+    if (i == otARRAY_LENGTH(sCommands))
     {
         snprintf(aOutput, aOutputMaxLen, "diag feature '%s' is not supported\r\n", argv[0]);
     }
diff --git a/examples/platforms/utils/code_utils.h b/examples/platforms/utils/code_utils.h
index 30a9e0168..90c4827e0 100644
--- a/examples/platforms/utils/code_utils.h
+++ b/examples/platforms/utils/code_utils.h
@@ -71,4 +71,14 @@
         }                                    \
     } while (0)
 
+/**
+ * This macro calculates the number of elements in an array.
+ *
+ * @param[in] aArray  Name of the array variable.
+ *
+ * @returns Number of elements in the array.
+ *
+ */
+#define otARRAY_LENGTH(aArray) (sizeof(aArray) / sizeof(aArray[0]))
+
 #endif // CODE_UTILS_H
diff --git a/src/diag/diag_process.cpp b/src/diag/diag_process.cpp
index d7e071b25..16cf2396b 100644
--- a/src/diag/diag_process.cpp
+++ b/src/diag/diag_process.cpp
@@ -46,9 +46,8 @@ namespace ot {
 namespace Diagnostics {
 
 const struct Diag::Command Diag::sCommands[] = {
-    {"start", &ProcessStart}, {"stop", &ProcessStop},   {"channel", &ProcessChannel},
-    {"power", &ProcessPower}, {"send", &ProcessSend},   {"repeat", &ProcessRepeat},
-    {"stats", &ProcessStats}, {"radio", &ProcessRadio}, {NULL, NULL},
+    {"start", &ProcessStart}, {"stop", &ProcessStop},     {"channel", &ProcessChannel}, {"power", &ProcessPower},
+    {"send", &ProcessSend},   {"repeat", &ProcessRepeat}, {"stats", &ProcessStats},     {"radio", &ProcessRadio},
 };
 
 struct Diag::DiagStats Diag::sStats;
@@ -86,11 +85,11 @@ void Diag::ProcessCmd(int aArgCount, char *aArgVector[], char *aOutput, size_t a
         ExitNow();
     }
 
-    for (const Command *command = &sCommands[0]; command->mName != NULL; command++)
+    for (size_t i = 0; i < OT_ARRAY_LENGTH(sCommands); i++)
     {
-        if (strcmp(aArgVector[0], command->mName) == 0)
+        if (strcmp(aArgVector[0], sCommands[i].mName) == 0)
         {
-            command->mHandler(aArgCount - 1, (aArgCount > 1) ? &aArgVector[1] : NULL, aOutput, aOutputMaxLen);
+            sCommands[i].mHandler(aArgCount - 1, (aArgCount > 1) ? &aArgVector[1] : NULL, aOutput, aOutputMaxLen);
             ExitNow();
         }
     }