A substitution is a trade
Every list online presents it as a swap. No buttermilk? Milk and lemon juice. That is half the answer, and the missing half is the one that decides whether it works: the acid is the point, it is there to react with the bicarbonate of soda, and swapping in plain milk leaves the raising agent with nothing to push against.
So every entry here gives you the ratio and what it costs you, because standing in a kitchen at eight o'clock with a bowl half made, what you need to know is not just what to put in but what you are giving up by putting it in.
Self-raising flour, and the reason the American ratio is wrong here
Two teaspoons of baking powder and a quarter teaspoon of salt per 150 g of plain flour. Whisk it right through, because a pocket of baking powder is a bitter spot somebody will find.
The salt is not seasoning and it is not optional: it is in the commercial blend, and leaving it out gives a noticeably flatter-tasting bake. The other thing nobody mentions is that your homemade version does not keep. Baking powder starts losing strength once it is out of the tin and mixed through flour, so make it as you need it.
And note that the quantity follows the WEIGHT of the flour. Every American list gives this per cup, which is a volume, and a cup of flour is anywhere from 120 to 150 g depending entirely on how it got into the cup. That is the whole problem this hub exists to solve, and it turns up here as a raising agent that is out by a fifth.
The two with no honest answer
An egg that is doing the lifting. Ask what the egg was for before reaching for a substitute. If it was binding a flapjack, ground flaxseed in water does the job. If it was whisked to eight times its volume and set around the air, as in a genoise, a meringue, a souffle or an angel cake, then nothing in a domestic cupboard replaces it, and a list offering you a flax egg for a sponge has not asked the question. Aquafaba, the liquid out of a tin of chickpeas, genuinely does whip and hold air and is a real substitute for egg white. It is not a substitute for a whole egg.
Vanilla extract. Leave it out. In almost every recipe it is a background note rather than structure, so going without costs a little roundness and nothing more. What it is not is interchangeable with vanilla essence, which is a synthetic imitation and tastes like one. A flavour that is present and wrong is worse than one that is absent.
Butter to oil changes the structure, not the flavour
About 80 g of oil replaces 100 g of butter, because butter is roughly 80 percent fat and the rest is water and milk solids. That part is arithmetic. The part that matters is that butter is a solid fat, so beating it with sugar whips air into it, and that trapped air is a real share of the lift in a creamed cake. Oil is a liquid and cannot hold any.
The result is denser, more evenly crumbed and stays soft for days longer, which is why carrot cake, most muffins and a lot of chocolate cake use oil on purpose rather than as a compromise. In any recipe where the butter was going to be melted anyway, which is most traybakes and brownies, it is a clean swap and nothing is lost.
Bicarbonate and baking powder are not two grades of the same thing
Baking powder is bicarbonate of soda already mixed with a dry acid, plus a little starch to keep them apart in the tin. So you can make one from the other in both directions, and the two directions are not equally good.
Making baking powder from scratch is free: half a teaspoon of cream of tartar to a quarter of bicarbonate. Two parts acid to one part alkali, which is the ratio that leaves neither in excess.
Going the other way costs you something. Replacing a teaspoon of bicarbonate takes three teaspoons of baking powder, which drags three times the acid and salt in with it. In a pinch, fine. In a recipe wanting a full teaspoon, you can taste it, and the browning is different too: bicarbonate makes a dough alkaline, and alkalinity is what gives biscuits their colour and their spread. A cookie made with baking powder instead is paler and puffier, and that is a real change rather than a subtlety.
The ones that are genuinely free
A few of these cost nothing at all and are worth doing even when you are not desperate. Caster sugar is granulated ground finer, so thirty seconds in a food processor makes it, and the reason it matters is that the smaller crystals dissolve into butter while you cream it. Granulated used straight leaves a slightly gritty crumb and a flatter cake.
Brown sugar is white sugar with treacle in it, so a tablespoon of black treacle rubbed through 200 g of caster makes light brown and two makes dark. Rub until there are no streaks: an unmixed pocket of treacle is the only way this one fails.
Common questions
How do I make self-raising flour from plain flour?
Two teaspoons of baking powder and a quarter teaspoon of salt for every 150 g of plain flour. Whisk it through properly, because a pocket of baking powder is a bitter spot in the finished bake. Note that the quantity follows the WEIGHT of the flour: American lists give it per cup, which is a volume, and a cup of flour is anywhere between 120 and 150 g depending on how it got in there.
Can I use plain flour instead of self-raising?
Only with the raising agent added, which is what the substitution above is. Self-raising flour is not a different kind of flour, it is plain flour with baking powder and salt already mixed in, so using plain on its own leaves the recipe with nothing to lift it. The reverse is worth knowing too: using self-raising where a recipe wanted plain adds lift that the recipe did not ask for, which is how a pastry ends up puffy.
How do I make buttermilk at home?
A tablespoon of lemon juice or white vinegar stirred into 250 ml of milk, left to stand for ten minutes until it thickens and looks curdled. What matters is understanding why: the acid is the point, not the tang. Buttermilk is in the recipe to react with the bicarbonate of soda, and that reaction is what raises the bake. Plain milk leaves the bicarbonate with nothing to work against, so it does not rise and it tastes faintly soapy.
What can I use instead of eggs in baking?
It depends entirely on what the egg was doing, and that is the question every substitution list skips. Where the egg is binding, a tablespoon of ground flaxseed in three tablespoons of water works well and gives a slightly heavier, nuttier result. Where the egg is doing the LIFTING, in a genoise, a meringue or a souffle, there is no honest one-line replacement, because nothing in a domestic cupboard whips to many times its own volume and sets around the air. Aquafaba genuinely replaces egg white in a meringue and is not a substitute for a whole egg.
Can I use oil instead of butter in a cake?
Yes, at about 80 g of oil to 100 g of butter, since butter is roughly 80 percent fat and the rest is water. The result is a different cake rather than a near miss. Butter is solid, so beating it with sugar traps air and that air is part of the lift; oil is liquid and cannot do it. You get a denser, more even crumb that stays soft for days longer, which is exactly why carrot cake and most muffins use oil deliberately. In a recipe where the butter was melted anyway, it is a clean swap.
How do I make baking powder?
Half a teaspoon of cream of tartar to a quarter teaspoon of bicarbonate of soda gives you one teaspoon of baking powder, and it is worth knowing that this IS baking powder: the commercial stuff is the same two things with a little starch added to stop them reacting in the tin. Mix it immediately before you use it, because without that starch it will start working on cupboard humidity.
Can I use baking powder instead of bicarbonate of soda?
Three times as much, and it is a trade rather than a swap. Baking powder is bicarbonate already diluted with its own acid, so you need more of it to get the same amount of base, and that brings three times the acid and salt along with it. In a small quantity nobody notices. In anything calling for a full teaspoon of bicarbonate you can taste it, and the browning changes too, because bicarbonate makes a dough alkaline and that is what gives biscuits their colour and their spread.
What is the difference between vanilla extract and vanilla essence?
Extract is made from real vanilla beans, essence is a synthetic imitation, and they do not taste the same. If you are out of extract the honest advice is to leave it out and carry on, because in almost every recipe vanilla is a background note rather than structure, so its absence costs a little roundness and nothing else. Substituting essence gives you a flavour that is present and wrong, which is usually worse than one that is absent.
How do I make brown sugar from white sugar?
One tablespoon of black treacle or molasses rubbed into 200 g of caster or granulated sugar for light brown, two for dark. That is genuinely what brown sugar is, so nothing is lost. Rub it through with your fingers until there are no dark streaks left, which takes about a minute: a pocket of unmixed treacle shows up as a dark spot in the finished bake and it is the only way this one goes wrong.