You can use baking soda and buttermilk instead of baking powder at a ratio of 1/4 tsp baking soda + 1/2 cup buttermilk per 1 tsp baking powder (replace 1/2 cup of the recipe's liquid). That is the whole swap in one sentence, and it works because baking powder is baking soda plus acid, so the swap is always soda + an acid in the right ratio. Baking powder is not a single ingredient: it is baking soda mixed with a dry acid and a little starch, so whenever you replace it you have to rebuild that partnership yourself — soda for the lift, and an acid to wake it up. Get the ratio right and the cake rises exactly as the recipe intended. Get it wrong and you will taste it: too much soda leaves a soapy, metallic tang and a yellowish crumb, too little leaves a flat, dense bake.
This swap comes up most often at the worst moment — halfway through a recipe, with the oven already hot and the tin already greased. The good news is that the replacement is quick to measure and the chemistry is forgiving if you move fast. Leavening starts working the moment the soda meets the acid and the liquid, so once you have mixed the batter, get it into the oven without delay. The guidance below walks through the ratio in a table, three real recipes where the swap is used, what changes in the finished bake, and the cases where you should not substitute at all.
The ratio, in a table
| Baking powder called for | Baking soda | Buttermilk (and liquid to remove) |
|---|---|---|
| 1/2 tsp | 1/8 tsp | 1/4 cup (remove 1/4 cup other liquid) |
| 1 tsp | 1/4 tsp | 1/2 cup (remove 1/2 cup other liquid) |
| 2 tsp | 1/2 tsp | 1 cup (remove 1 cup other liquid) |
| 1 tbsp (3 tsp) | 3/4 tsp | 1 1/2 cups (remove 1 1/2 cups other liquid) |
Measure the soda carefully — a quarter of a teaspoon looks like almost nothing, and it is easy to tip in twice as much by accident. Use proper measuring spoons, level them off with a knife, and do not guess with a dinner spoon. The ratio above is the sourced ratio for this pair: 1/4 tsp baking soda + 1/2 cup buttermilk per 1 tsp baking powder (replace 1/2 cup of the recipe's liquid). All amounts in this guide are approximate, and the ratio comes from CookScale's substitution data rather than from a single brand's packaging. If the recipe matters — a cake for a celebration, a batch you are selling, or anything where a small error shows — weigh the ingredients on a kitchen scale and taste as you go.
One timing rule matters more than anything else with this swap. Baking powder is double-acting: it releases gas once when it meets liquid and again when it meets heat. Baking soda with an acid is mostly single-acting: the fizz starts as soon as the wet and dry ingredients meet. That is why a batter made with this substitute should go straight into a fully preheated oven. If you let it sit on the counter while you answer the door, a good share of the lift escapes into the air and no amount of extra baking time will bring it back.
Worked examples
Example 1: Buttermilk pancakes from a plain recipe
A pancake recipe calls for 1 tablespoon of baking powder and 1 1/2 cups of milk. Use 3/4 teaspoon of baking soda and 1 1/2 cups of buttermilk instead of the milk, and the swap is a genuine improvement: the buttermilk gives the soda its acid, and the pancakes come out tangier and more tender than the original. Remove no other liquid — the buttermilk replaces the milk one for one. Cook on a medium pan and flip when the bubbles stop filling in.
Example 2: A buttermilk scones rescue
A scone recipe calls for 2 teaspoons of baking powder and 1 cup of milk. Use 1/2 teaspoon of baking soda and 1 cup of buttermilk in place of the milk, and the scones will be lighter and flakier than the original, with a gentle tang. Work the dough as little as possible — cold butter, cold buttermilk, a hot oven — and cut the scones straight down without twisting the cutter, so the layers rise evenly.
Example 3: A tender buttermilk chocolate cake
A chocolate cake calls for 1 teaspoon of baking powder and 1/2 cup of milk. Use 1/4 teaspoon of baking soda and 1/2 cup of buttermilk instead of the milk, and the cake will be noticeably more tender: buttermilk's acid softens the crumb and deepens the chocolate colour. The cake will also stay moist for an extra day, which is a real advantage for a cake baked the day before it is eaten.
What changes in the result
This swap changes the flavour more than any other in the leavening family, and almost always for the better: buttermilk adds a gentle tang and a tenderness that plain milk does not. The crumb becomes softer, the cake stays moist longer, and chocolate tastes darker and richer. The trade-off is the liquid accounting — the buttermilk replaces half a cup of the recipe's liquid for every teaspoon of baking powder, so in a recipe with very little liquid this swap may not fit at all. If the recipe has less liquid than the swap needs, use the cream-of-tartar version instead.
Flavour is the other thing to watch. Baking soda that has not fully reacted leaves a soapy, slightly metallic taste, and it can also yellow the crumb and darken the crust. That is the main reason the ratio is so small — a quarter of a teaspoon of soda does the work of a full teaspoon of baking powder, because baking powder is mostly filler and acid. If you double the soda 'to be safe', you will not get double the rise; you will get a bitter cake that spreads too much and then sinks. Trust the ratio, and if the bake comes out flat, the cause is usually stale soda or a slow oven, not too little leavening.
When not to substitute
Do not swap leavening in a recipe without acid — baking soda alone will taste soapy and will not rise. Do not use this swap in a recipe that has little or no liquid to replace — a shortbread, a dry biscuit dough, a recipe that calls for only a few tablespoons of milk — because the buttermilk has nowhere to go and the batter will be too wet. Do not use soured milk that has actually gone off: cultured buttermilk is a controlled ferment, spoiled milk is not the same thing.
There are also recipes where the swap is technically possible but not worth it. A delicate sponge, a chiffon cake or a recipe that depends on a very fine, even crumb will show the difference: the rise is a little less even and the crumb a little coarser. In those recipes, if you can wait, buy the right leavening. If you cannot wait, use the swap, expect a slightly rustic result, and serve it warm with something that flatters it — a custard, a compote, or a dusting of icing sugar covers a multitude of small imperfections.
Practical guidance
Test your baking soda before you trust it: drop half a teaspoon into a little vinegar or lemon juice. If it fizzes hard straight away, it is alive. If it barely bubbles, it is stale and no ratio will save the bake. Store baking soda and cream of tartar in airtight containers away from damp, and replace opened baking soda every few months — it absorbs smells from the fridge, which is useful there and ruinous in a cake. Mix the soda with the dry ingredients and the acid with the wet ingredients, then combine them at the last moment, so the reaction happens in the batter rather than in the bowl. And keep the oven door shut for the first two-thirds of the baking time: opening it early lets the heat out just as the crumb is setting, and a cake that has lost its lift will not recover.
If you do not have buttermilk, you can sour milk yourself: add 1 tablespoon of lemon juice or white vinegar to 1 cup of milk and leave it for ten minutes until it thickens slightly. It is not identical to cultured buttermilk, but it works as the acid for this swap. Buttermilk freezes well in small containers, so freeze the leftover half-carton for the next bake.
Frequently asked questions
Can I use this swap if the recipe has no milk in it?
Only if the recipe has enough other liquid to replace. The rule is 1/2 cup of buttermilk per teaspoon of baking powder, and you must remove 1/2 cup of the recipe's liquid. If the recipe has less than that — a dry biscuit dough, a shortbread — the swap does not fit, and you should use the cream-of-tartar or vinegar version instead, which adds almost no liquid.
What is the difference between buttermilk and soured milk?
Cultured buttermilk is milk fermented with specific bacteria, which gives it a thick texture and a clean, tangy flavour. Soured milk is ordinary milk with lemon juice or vinegar added, which curdles it and makes it acidic but not thick. Both work as the acid for this swap, but cultured buttermilk gives a better texture and flavour. Do not use milk that has gone off on its own — that is spoilage, not culturing.
Can I use this substitute in every recipe that calls for baking powder?
In most everyday baking, yes — muffins, pancakes, quick breads, scones and simple cakes all take the swap well, as long as you follow the ratio of 1/4 tsp baking soda + 1/2 cup buttermilk per 1 tsp baking powder (replace 1/2 cup of the recipe's liquid). Be more cautious with delicate sponges, chiffon cakes and anything where a very fine, even crumb is the whole point of the recipe. In those, the swap works but the texture is slightly coarser, so use it when you have no choice and accept a slightly more rustic result.
Why does my cake taste soapy after substituting?
A soapy or metallic taste almost always means too much baking soda, or soda that never met enough acid. Baking soda needs an acid to neutralise it; without one, the unreacted soda stays alkaline and tastes bitter and soapy. Check that you used 1/4 tsp baking soda + 1/2 cup buttermilk per 1 tsp baking powder (replace 1/2 cup of the recipe's liquid) and not a heaped spoon, and make sure the acid was actually in the batter. If the recipe has no acid at all, this swap is the wrong one — use the version that brings its own acid.
How do I know if my baking soda is still good?
Drop half a teaspoon into a tablespoon of vinegar or lemon juice. Fresh soda fizzes vigorously at once; stale soda barely bubbles. Baking soda goes flat over months, especially once the box is open, so test it before you bake with it. Cream of tartar keeps much longer, but keep it dry and sealed. If either one fails the test, replace it — a flat leavener is the most common reason a substituted bake does not rise.