diff --git a/include/spdlog/details/log_msg.h b/include/spdlog/details/log_msg.h
index a9fe9201..850b9a81 100644
--- a/include/spdlog/details/log_msg.h
+++ b/include/spdlog/details/log_msg.h
@@ -11,6 +11,7 @@
 
 #include <string>
 #include <utility>
+#include <unordered_map>
 
 namespace spdlog
 {
@@ -19,10 +20,11 @@ namespace details
 struct log_msg
 {
     log_msg() = default;
-    log_msg(const std::string *loggers_name, level::level_enum lvl) :
+    log_msg(const std::string *loggers_name, level::level_enum lvl, std::unordered_map<char, std::string>* flags) :
         logger_name(loggers_name),
         level(lvl),
-        msg_id(0)
+        msg_id(0),
+		custom_flags(flags)
     {
 #ifndef SPDLOG_NO_DATETIME
         time = os::now();
@@ -45,6 +47,7 @@ struct log_msg
     fmt::MemoryWriter raw;
     fmt::MemoryWriter formatted;
     size_t msg_id;
+	std::unordered_map<char, std::string>* custom_flags;
 };
 }
 }
diff --git a/include/spdlog/details/logger_impl.h b/include/spdlog/details/logger_impl.h
index 88044747..bfb39333 100644
--- a/include/spdlog/details/logger_impl.h
+++ b/include/spdlog/details/logger_impl.h
@@ -64,7 +64,7 @@ inline void spdlog::logger::log(level::level_enum lvl, const char* fmt, const Ar
 
     try
     {
-        details::log_msg log_msg(&_name, lvl);
+        details::log_msg log_msg(&_name, lvl, &_custom_flags);
 
 #if defined(SPDLOG_FMT_PRINTF)
         fmt::printf(log_msg.raw, fmt, args...);
@@ -90,7 +90,7 @@ inline void spdlog::logger::log(level::level_enum lvl, const char* msg)
     if (!should_log(lvl)) return;
     try
     {
-        details::log_msg log_msg(&_name, lvl);
+        details::log_msg log_msg(&_name, lvl, &_custom_flags);
         log_msg.raw << msg;
         _sink_it(log_msg);
     }
@@ -111,7 +111,7 @@ inline void spdlog::logger::log(level::level_enum lvl, const T& msg)
     if (!should_log(lvl)) return;
     try
     {
-        details::log_msg log_msg(&_name, lvl);
+        details::log_msg log_msg(&_name, lvl,&_custom_flags);
         log_msg.raw << msg;
         _sink_it(log_msg);
     }
@@ -279,6 +279,25 @@ inline void spdlog::logger::set_level(spdlog::level::level_enum log_level)
     _level.store(log_level);
 }
 
+inline void spdlog::logger::set_custom_flag(char flag,const std::string& value)
+{
+	_custom_flags[flag] = value;
+}
+
+inline const std::string& spdlog::logger::value_custom_flag(char flag)
+{
+	auto end = _custom_flags.end();
+	auto it = _custom_flags.find(flag);
+	if (it != end)
+	{
+		return it->second;
+	}
+	else
+	{
+		return "";
+	}
+}
+
 inline void spdlog::logger::set_error_handler(spdlog::log_err_handler err_handler)
 {
     _err_handler = err_handler;
diff --git a/include/spdlog/details/pattern_formatter_impl.h b/include/spdlog/details/pattern_formatter_impl.h
index a73f5dea..2dd67863 100644
--- a/include/spdlog/details/pattern_formatter_impl.h
+++ b/include/spdlog/details/pattern_formatter_impl.h
@@ -414,6 +414,27 @@ private:
     char _ch;
 };
 
+class custom_formatter SPDLOG_FINAL:public flag_formatter
+{
+public:
+	explicit custom_formatter(char flag) : _flag(_flag)
+	{}
+	void format(details::log_msg& msg, const std::tm&) override
+	{
+		auto end = msg.custom_flags->end();
+		auto it = msg.custom_flags->find(_flag);
+		if (it != end)
+		{
+			msg.formatted << it->second;
+		}
+		else {
+			msg.formatted << _flag;
+		}
+	}
+private:
+	char _flag;
+};
+
 
 //aggregate user chars to display as is
 class aggregate_formatter SPDLOG_FINAL:public flag_formatter
@@ -655,8 +676,7 @@ inline void spdlog::pattern_formatter::handle_flag(char flag)
         break;
 
     default: //Unknown flag appears as is
-        _formatters.push_back(std::unique_ptr<details::flag_formatter>(new details::ch_formatter('%')));
-        _formatters.push_back(std::unique_ptr<details::flag_formatter>(new details::ch_formatter(flag)));
+        _formatters.push_back(std::unique_ptr<details::flag_formatter>(new details::custom_formatter(flag)));
         break;
     }
 }
diff --git a/include/spdlog/details/registry.h b/include/spdlog/details/registry.h
index b68b9f5a..9ded577a 100644
--- a/include/spdlog/details/registry.h
+++ b/include/spdlog/details/registry.h
@@ -155,6 +155,28 @@ public:
         _level = log_level;
     }
 
+	void set_custom_flag(char flag, const std::string& value)
+	{
+		std::lock_guard<Mutex> lock(_mutex);
+		for (auto& l : _loggers)
+			l.second->set_custom_flag(flag, value);
+		_custom_flags[flag] = value;
+	}
+
+	const std::string& value_custom_flag(char flag)
+	{
+		auto end = _custom_flags.end();
+		auto it = _custom_flags.find(flag);
+		if (it != end)
+		{
+			return it->second;
+		}
+		else
+		{
+			return std::move(std::string(""));
+		}
+	}
+
     void flush_on(level::level_enum log_level)
     {
         std::lock_guard<Mutex> lock(_mutex);
@@ -215,6 +237,7 @@ private:
     std::function<void()> _worker_warmup_cb = nullptr;
     std::chrono::milliseconds _flush_interval_ms;
     std::function<void()> _worker_teardown_cb = nullptr;
+	std::unordered_map<char, std::string> _custom_flags;
 };
 #ifdef SPDLOG_NO_REGISTRY_MUTEX
 typedef registry_t<spdlog::details::null_mutex> registry;
diff --git a/include/spdlog/details/spdlog_impl.h b/include/spdlog/details/spdlog_impl.h
index 25c7b7e9..bcbf3014 100644
--- a/include/spdlog/details/spdlog_impl.h
+++ b/include/spdlog/details/spdlog_impl.h
@@ -236,6 +236,16 @@ inline void spdlog::set_level(level::level_enum log_level)
     return details::registry::instance().set_level(log_level);
 }
 
+inline void spdlog::set_custom_flag(char flag, const std::string& value)
+{
+	return details::registry::instance().set_custom_flag(flag, value);
+}
+
+inline const std::string& spdlog::value_custom_flag(char flag)
+{
+	return details::registry::instance().value_custom_flag(flag);
+}
+
 inline void spdlog::flush_on(level::level_enum log_level)
 {
     return details::registry::instance().flush_on(log_level);
diff --git a/include/spdlog/logger.h b/include/spdlog/logger.h
index 742f667f..6f0c50ad 100644
--- a/include/spdlog/logger.h
+++ b/include/spdlog/logger.h
@@ -18,6 +18,7 @@
 #include <vector>
 #include <memory>
 #include <string>
+#include <unordered_map>
 
 namespace spdlog
 {
@@ -66,6 +67,8 @@ public:
 
     bool should_log(level::level_enum) const;
     void set_level(level::level_enum);
+	void set_custom_flag(char flag, const std::string& value);
+	const std::string& value_custom_flag(char flag);
     level::level_enum level() const;
     const std::string& name() const;
     void set_pattern(const std::string&, pattern_time_type = pattern_time_type::local);
@@ -104,6 +107,7 @@ protected:
     log_err_handler _err_handler;
     std::atomic<time_t> _last_err_time;
     std::atomic<size_t> _msg_counter;
+	std::unordered_map<char, std::string> _custom_flags;
 };
 }
 
diff --git a/include/spdlog/spdlog.h b/include/spdlog/spdlog.h
index ff79ef6e..7cf14f2a 100644
--- a/include/spdlog/spdlog.h
+++ b/include/spdlog/spdlog.h
@@ -50,6 +50,15 @@ void flush_on(level::level_enum log_level);
 //
 void set_error_handler(log_err_handler);
 
+//
+// Set custom flag
+//
+void set_custom_flag(char flag, const std::string& value);
+
+//
+// value custom flag
+//
+const std::string& value_custom_flag(char flag);
 //
 // Turn on async mode (off by default) and set the queue size for each async_logger.
 // effective only for loggers created after this call.