The best coffee-to-water ratio for an espresso is a critical factor in achieving a well-extracted shot that is neither too bitter nor too sour. The golden standard in the specialty coffee industry is often referred to as the "gold cup" standards, which are guidelines set by the Specialty Coffee Association (SCA).

According to the SCA, the ideal espresso extraction ratio is:

• 1 gram of coffee to 2 to 2.5 grams of water (or 1:2 to 1:2.5 ratio)
• This should be achieved in approximately 25 to 30 seconds.

For a typical double shot of espresso, which uses about 18 to 20 grams of coffee, you would expect to extract approximately 36 to 50 grams of espresso liquid.

However, personal preferences can vary, and some may prefer a longer or shorter extraction time, which would result in a different ratio. Here are some common variations:

• Ristretto: A shorter extraction, around 15-20 seconds, with less water (e.g., 1 gram coffee to 1 gram water or 1:1 ratio).
• Lungo: A longer extraction, often more than 30 seconds, with more water (e.g., 1 gram coffee to 3-4 grams water or 1:3 to 1:4 ratio).

It's important to note that the grind size also plays a significant role in the extraction process. For espresso, the coffee should be ground very finely, similar to the texture of caster sugar. If the grind is too coarse, the water will pass through too quickly, and you'll end up with a weak and sour shot. If the grind is too fine, the water may not pass through fast enough, leading to an overly bitter and salty extraction.

To achieve the perfect espresso, you'll need to experiment with your specific espresso machine, grinder, and coffee beans. Factors such as the freshness of the beans, the grind size, the distribution of the grounds in the filter basket, the pressure of the machine, and the water temperature all contribute to the final result. It's an art that takes practice and fine-tuning to perfect.