Self-raising flour
Plain flour with raising agent already blended in — convenient, and the reason it goes wrong is that the leavening is on a clock you cannot see.
Also called self rising flour (US); self-rising flour (US); sr flour (UK); self raising flour substitute (UK)
Self-raising flour is soft plain flour with baking powder — and usually a little salt — already mixed through it. The convenience is real and so are two consequences most people are never told.
The first is that the leavening ages. Baking powder loses activity with exposure to moisture, and in a flour bag it has been exposed since it was packed. An old bag of self-raising flour is not merely old flour; it is flour whose raising agent has partly spent itself, and it produces flat, dense results that look like a technique failure.
The second is that the ratio is fixed. A formulation that wants more or less lift than the standard blend provides cannot be adjusted without adding a second leavener on top of an unknown quantity of the first.
Substituting is straightforward in one direction: plain flour plus baking powder gives you self-raising flour, at roughly two teaspoons of baking powder per 150 g of flour as a common convention. Going the other way is not — you cannot take the raising agent out again, so a recipe calling for plain flour made with self-raising will over-rise and often collapse.
British self-raising flour is also typically a softer, lower-protein flour than plain flour, which is part of why it suits cakes and scones and not bread.
What changes if it is left out
Plain flour with baking powder added replaces it directly. What that does not replicate is the lower protein content of a typical British self-raising flour, so the result may be slightly chewier than intended.
The two-teaspoons-per-150-g figure is a widely shared convention rather than a measured standard, and blends differ between manufacturers. BakeHQ reports it as a convention and not as a specification.
RecordedEstablished practice, not a measured result
Why an old bag behaves like a technique failure
A flat scone or a dense cake made with self-raising flour is usually blamed on overmixing, on the oven, or on the person.\n\nThe likelier explanation is the flour. Baking powder in an opened bag has been absorbing atmospheric moisture and reacting slowly for however long the bag has been in the cupboard, and nothing about the flour looks different when it has.\n\nThe test is the same one used for baking powder itself: stir a spoonful of the flour into hot water and watch for fizzing. No reaction means the raising agent is spent, and the flour is now plain flour that you are treating as though it were not.
Related
What it does
Structure building
Soft wheat flour, so it builds structure by a gluten network like any wheat flour — less of one, because the protein is lower.
Aeration
THE POINT OF THE PRODUCT. The raising agent is already in it, which is convenient and means the quantity is fixed and its age is unknown.
Covered in
The flours, and what actually separates them
Plain flour with a modifier, and the one whose behaviour changes with age.
Substituting one flour for another, honestly
The swap that changes the leavening as well as the flour, and the route by which cakes come to taste soapy.
Used in
Common in British recipes; flour and leavener together.
Compared in
Plain flour vs self-raising flour
One side of the comparison.