diff --git a/esphome/components/modbus_controller/modbus_controller.h b/esphome/components/modbus_controller/modbus_controller.h
index 466598e9cd..111b0d574e 100644
--- a/esphome/components/modbus_controller/modbus_controller.h
+++ b/esphome/components/modbus_controller/modbus_controller.h
@@ -286,7 +286,7 @@ class ServerRegister {
         return std::to_string(value);
       case SensorValueType::FP32_R:
       case SensorValueType::FP32: {
-        // max 48: float with %.1f can be up to 41 chars (3.4e38 → 39 digits + sign + decimal + 1 digit) + null
+        // 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)));
         return buf;