diff --git a/esphome/components/web_server/web_server.cpp b/esphome/components/web_server/web_server.cpp
index ee0d713334..3a3ac63a20 100644
--- a/esphome/components/web_server/web_server.cpp
+++ b/esphome/components/web_server/web_server.cpp
@@ -1272,8 +1272,9 @@ std::string WebServer::select_json(select::Select *obj, const char *value, JsonD
 }
 #endif
 
-// Longest: HORIZONTAL
-#define PSTR_LOCAL(mode_s) ESPHOME_strncpy_P(buf, (ESPHOME_PGM_P) ((mode_s)), 15)
+// Longest: HORIZONTAL (10 chars + null terminator, rounded up)
+static constexpr size_t PSTR_LOCAL_SIZE = 16;
+#define PSTR_LOCAL(mode_s) ESPHOME_strncpy_P(buf, (ESPHOME_PGM_P) ((mode_s)), PSTR_LOCAL_SIZE - 1)
 
 #ifdef USE_CLIMATE
 void WebServer::on_climate_update(climate::Climate *obj) {
@@ -1332,7 +1333,7 @@ std::string WebServer::climate_json(climate::Climate *obj, JsonDetail start_conf
   const auto traits = obj->get_traits();
   int8_t target_accuracy = traits.get_target_temperature_accuracy_decimals();
   int8_t current_accuracy = traits.get_current_temperature_accuracy_decimals();
-  char buf[16];
+  char buf[PSTR_LOCAL_SIZE];
 
   if (start_config == DETAIL_ALL) {
     JsonArray opt = root["modes"].to<JsonArray>();
@@ -1482,7 +1483,7 @@ std::string WebServer::lock_json(lock::Lock *obj, lock::LockState value, JsonDet
   json::JsonBuilder builder;
   JsonObject root = builder.root();
 
-  char buf[lock::LOCK_STATE_STR_SIZE];
+  char buf[PSTR_LOCAL_SIZE];
   set_json_icon_state_value(root, obj, "lock", PSTR_LOCAL(lock::lock_state_to_string(value)), value, start_config);
   if (start_config == DETAIL_ALL) {
     this->add_sorting_info_(root, obj);
@@ -1644,7 +1645,7 @@ std::string WebServer::alarm_control_panel_json(alarm_control_panel::AlarmContro
   json::JsonBuilder builder;
   JsonObject root = builder.root();
 
-  char buf[16];
+  char buf[PSTR_LOCAL_SIZE];
   set_json_icon_state_value(root, obj, "alarm-control-panel", PSTR_LOCAL(alarm_control_panel_state_to_string(value)),
                             value, start_config);
   if (start_config == DETAIL_ALL) {