Made several mistakes above, sorry - fixed code below Assuming the conversion is from string to string (e.g. "10" => "A"), we can take several steps to convert: 1. Convert number string to int or uint (assume decStr is a decimal number string): unsigned int num = 0; for (int i = 0; i > 4; if (hex < 10) hexStr = (hex+'0') + hexStr; else hexStr = (hex - 10 +'A') + hexStr; // Need to subtract 10 }