For a cup of flour, the typical recommended amount of baking powder is about 1 to 2 teaspoons. Most sources advise using around 1 ½ teaspoons of baking powder per cup of all-purpose flour to achieve the right leavening effect. Some guidance also suggests adding ¼ teaspoon of salt to mimic self-raising flour proportions when using plain flour.

More specifically:

• One common ratio is 1 teaspoon baking powder per 1 cup flour plus ¼ teaspoon salt.
• Another common suggestion is 1 ½ teaspoons baking powder per 1 cup flour, especially if not adding salt.
• A weight-based guideline is about 5% baking powder relative to the weight of the flour (e.g., 10 grams baking powder per 200 grams flour) which roughly translates to these volume measures.

Overall, about 1 ½ teaspoons baking powder per cup of flour is a safe and common rule of thumb.