In British recipes, "a mug" of flour is a common informal measurement used by some home cooks. It is not a standard measurement like grams or ounces, so its exact quantity can vary depending on the size of the mug being used.
When a recipe calls for "a mug" of flour, it typically means filling a standard-sized mug (approximately 250-300 milliliters or 8-10 fluid ounces) with flour. However, it's important to note that this measurement is not as precise as using a kitchen scale or specific measuring cups, so the actual amount of flour used may vary slightly.
If you encounter a recipe that calls for "a mug" of flour and you prefer to use a more precise measurement, it is advisable to convert it to grams or ounces using standard conversion charts for flour. This will ensure consistent results in your baking or cooking.