diff --git a/esphome/components/modbus_controller/modbus_controller.h b/esphome/components/modbus_controller/modbus_controller.h
index 35aab81e90..7fbd0f17e1 100644
--- a/esphome/components/modbus_controller/modbus_controller.h
+++ b/esphome/components/modbus_controller/modbus_controller.h
@@ -271,29 +271,31 @@ class ServerRegister {
 
   // Formats a raw value into a string representation based on the value type for debugging
   std::string format_value(int64_t value) const {
+    // max 48: float with %.1f can be up to 42 chars (3.4e38 → 38 integer digits + decimal + 1 digit + sign + null)
+    // int64_t max is 20 chars + sign + null = 22, so 48 covers both
+    char buf[48];
     switch (this->value_type) {
       case SensorValueType::U_WORD:
       case SensorValueType::U_DWORD:
       case SensorValueType::U_DWORD_R:
       case SensorValueType::U_QWORD:
       case SensorValueType::U_QWORD_R:
-        return std::to_string(static_cast<uint64_t>(value));
+        buf_append_printf(buf, sizeof(buf), 0, "%" PRIu64, static_cast<uint64_t>(value));
+        return buf;
       case SensorValueType::S_WORD:
       case SensorValueType::S_DWORD:
       case SensorValueType::S_DWORD_R:
       case SensorValueType::S_QWORD:
       case SensorValueType::S_QWORD_R:
-        return std::to_string(value);
-      case SensorValueType::FP32_R:
-      case SensorValueType::FP32: {
-        // max 48: float with %.1f can be up to 42 chars incl. null (3.4e38 → 38 integer digits + decimal point + 1
-        // decimal digit + optional sign)
-        char buf[48];
-        snprintf(buf, sizeof(buf), "%.1f", bit_cast<float>(static_cast<uint32_t>(value)));
+        buf_append_printf(buf, sizeof(buf), 0, "%" PRId64, value);
+        return buf;
+      case SensorValueType::FP32_R:
+      case SensorValueType::FP32:
+        buf_append_printf(buf, sizeof(buf), 0, "%.1f", bit_cast<float>(static_cast<uint32_t>(value)));
         return buf;
-      }
       default:
-        return std::to_string(value);
+        buf_append_printf(buf, sizeof(buf), 0, "%" PRId64, value);
+        return buf;
     }
   }