Final answer:
The ratio for roux is 1 part flour to 1 part fat by weight and 2 parts flour to 2 parts fat to 1 part liquid by volume.
Step-by-step explanation:
The ratio for roux can be expressed both by weight and by volume. When expressed by weight, the ratio for roux is typically 1 part flour to 1 part fat. For example, if you use 100 grams of flour, you would also use 100 grams of fat. When expressed by volume, the ratio for roux is typically 2 parts flour to 2 parts fat to 1 part liquid. For example, if you use 2 cups of flour, you would also use 2 cups of fat and 1 cup of liquid.