Short answer: self-raising flour already contains baking powder, and the typical ratio is about 1.5 teaspoons of baking powder per 150 grams of plain flour (or roughly 5–5.5% baking powder by weight). If you’re converting plain/all-purpose flour to self-raising for a recipe, add this amount of baking powder to the flour and sift well. Details and practical guidance

• Typical ratio (by weight)
  • About 5% baking powder relative to the weight of the flour. For 150 g of plain flour, this is roughly 7.5 g of baking powder, which corresponds to about 1.5 teaspoons. For 250 g of flour, that scales to about 12–13 g (roughly 2 to 2.5 teaspoons, depending on how packed the teaspoon is).
• Common conversion methods
  • If a recipe calls for self-raising flour and you only have plain flour:
    • Add 1.5 teaspoons of baking powder per 150 g of plain flour (or about 1 teaspoon per 100 g). Then mix thoroughly.
  • If you’re making a batch from scratch:
    • A practical rule used by many bakers is 1.5 teaspoons baking powder per 150 g plain flour, or 1 teaspoon per 100 g plain flour, with a pinch of salt sometimes optional depending on taste and recipe (salt is typically omitted in some sugar-heavy cakes).
• Quick references you’ll see in recipes
  • 2 teaspoons of baking powder per 150 g/1 cup plain flour is a common shorthand in many home-baking guides.
  • Some sources specify 12.5 g of baking powder per 237.5 g (roughly 1 cup) plain flour as a precise conversion.
• How to mix
  • Sift the plain flour and baking powder together before using to ensure even distribution and consistent rise.
• Storage and usage
  • Self-raising flour is best used soon after mixing. If you make a large batch, store it in an airtight container away from moisture to preserve leavening effectiveness.

If you’d like, share the exact amount of flour your recipe uses (grams or cups), and I’ll give you the precise baking powder amount to add.