Yaw calculation from RAW data of MPU 6050

Question

I am trying to calculate yaw pitch and roll for my Quadcopter project. I have taken the code from Here. I have been succeeded in calculating roll and pitch in degrees and applying complimentary filter to these values and these values are very near to the values calculated by Jeff Rowberg's code. I want to calculate the yaw value from this RAW data instead of using Jeff's code. I know yaw calculated by RAW data will not be that accurate as Jeff's but can anyone please tell me the formula or maths behind calculating the yaw value from accelerometer and gyro RAW data? here is the code I am using

#include<Wire.h>
const int MPU=0x68;  // I2C address of the MPU-6050
int16_t AcX,AcY,AcZ,Tmp,GyX,GyY,GyZ;
double compAngleX,compAngleY,/*gzangle,*/compAnglez,timer;
  double accXangle ,accYangle,acczangle ,gyroXrate ,gyroYrate,gyroZrate;
  double gyroXAngle, gyroYAngle, gyroZAngle;
  // float rgyro,w;
  int ap=0.955;

  void setup()
{
   Serial.begin(115200);
/////////////////////// SENSOR READING//////////
 Wire.begin();
  Wire.beginTransmission(MPU);
  Wire.write(0x6B);  // PWR_MGMT_1 register
  Wire.write(0);     // set to zero (wakes up the MPU-6050)
  Wire.endTransmission(true);
  ///////////////////////////////////////
}
void loop()
{

  Wire.beginTransmission(MPU);
  Wire.write(0x3B);  // starting with register 0x3B (ACCEL_XOUT_H)
  Wire.endTransmission(false);
  Wire.requestFrom(MPU,14,true);  // request a total of 14 registers
  AcX=Wire.read()<<8|Wire.read();  // 0x3B (ACCEL_XOUT_H) & 0x3C (ACCEL_XOUT_L)    
  AcY=Wire.read()<<8|Wire.read();  // 0x3D (ACCEL_YOUT_H) & 0x3E (ACCEL_YOUT_L)
  AcZ=Wire.read()<<8|Wire.read();  // 0x3F (ACCEL_ZOUT_H) & 0x40 (ACCEL_ZOUT_L)
  Tmp=Wire.read()<<8|Wire.read();  // 0x41 (TEMP_OUT_H) & 0x42 (TEMP_OUT_L)
  GyX=Wire.read()<<8|Wire.read();  // 0x43 (GYRO_XOUT_H) & 0x44 (GYRO_XOUT_L)
  GyY=Wire.read()<<8|Wire.read();  // 0x45 (GYRO_YOUT_H) & 0x46 (GYRO_YOUT_L)
  GyZ=Wire.read()<<8|Wire.read();  // 0x47 (GYRO_ZOUT_H) & 0x48 (GYRO_ZOUT_L)



 accXangle = (atan2(AcY, AcZ) * RAD_TO_DEG);
 accYangle = (atan2(AcX, AcZ) * RAD_TO_DEG);
acczangle = (atan2(AcX,AcY) * RAD_TO_DEG);/* my attempt to calculate yaw but not correct*/
 gyroXrate = GyX / 16.5;
 gyroYrate = GyY / 16.5;
 gyroZrate = GyZ/ 16.5;
 timer = millis();
 //angular position
 gyroXAngle += gyroXrate * (millis()-timer)/1000;
 gyroYAngle += gyroYrate * (millis()-timer)/1000;
 gyroZAngle += gyroZrate * (millis()-timer)/1000;/* my attempt to calculate yaw but not correct*/
//---------------------------\\
 //COMPLIMENTRY FILTER STARTED\\
 //---------------------------\\

 compAngleX = ap * (compAngleX + gyroXAngle) + (1-ap) * accXangle;
 compAngleY = ap * (compAngleY + gyroYAngle) + (1-ap) * accYangle;
 compAnglez = ap * (compAnglez + gyroZAngle) + (1-ap) * acczangle; /*yaw but not correct*/
 //---------------------------\\
 //COMPLIMENTRY FILTER ENDED  \\
 //---------------------------\\

   Serial.print("ypr\t");
            Serial.print(compAnglez);
            Serial.print("\t");
            Serial.print(compAngleY);
            Serial.print("\t");
            Serial.println(compAngleX);
}