Achieving perfect espresso is easy if you follow the numbers.   Here are the definitive requirements to achieving perfect espresso.

Coffee Dose

8 +/- 1 grams of freshly ground coffee per single shot of espresso
7 - 9 grams of freshly ground coffee evenly distributed in a single portafilter
14 - 18 grams of freshly ground coffee evenly distributed in a double portafilter

Water temperature

90 +/- 2 degrees centigrade
88 - 92 degrees centigrade at the point of extraction

Extraction pressure

9 bar (130 psi)

Extraction time

5 seconds pre-infusion + 25 seconds extraction
30 seconds maximum

Volume in the cup

25 +/- 5 ml per per single shot of espresso
20 ml for a ristretto (restricted) espresso
30ml for a completely extracted espresso
2x 20ml for two ristrettos
2x 30ml for two espressos