The code below is taken from an Arduino website. Maybe that will help.
// Read in 16 bits,
// 15 = 0 always
// 14..2 = 0.25 degree counts MSB First
// 2 = 1 if thermocouple is open circuit
// 1..0 = uninteresting status
v = shiftIn(MAX6675_SO, MAX6675_SCK, MSBFIRST);
v <<= 8;
v |= shiftIn(MAX6675_SO, MAX6675_SCK, MSBFIRST);
digitalWrite(MAX6675_CS, HIGH);
if (v & 0x4)
{
// Bit 2 indicates if the thermocouple is disconnected
return NAN;
}
// The lower three bits (0,1,2) are discarded status bits
v >>= 3;
// The remaining bits are the number of 0.25 degree (C) counts
return v*0.25;
}
So it looks like this might be the answer:
Temperature (celsius)= max6675 output (12bit)* 0.25