You are mid-recipe, one tin is empty, and the other one is right there. The question is reasonable and the answer is more interesting than yes or no.
They are not two brands of the same product. One of them is an ingredient that needs a partner. The other is a pre-mixed kit that brings its own partner with it. That difference decides what goes wrong, and it goes wrong differently in each direction.
What each one actually is
Baking soda is a single chemical, a base. On its own in a batter it does essentially nothing useful. Give it an acid and water and it reacts, producing carbon dioxide, and that gas is the rise.
So a recipe using baking soda always has an acid somewhere in the ingredient list doing the other half of the job: buttermilk, yogurt, sour cream, brown sugar, molasses, cocoa in its natural form, lemon juice, vinegar, or fruit. If you scan an ingredient list and cannot find the acid, look again, because it is there, and it is load-bearing.
Baking powder is baking soda with an acid already mixed in, in dry powdered form, plus a starch to keep the two apart in the tin. Add liquid and the reaction starts on its own. It needs nothing from the rest of your recipe, which is why it is used in recipes with no acidic ingredient.
Most baking powder sold is double-acting, meaning it releases some gas when it gets wet and more when it gets hot. That second release is why a batter can sit briefly without losing all its lift.
Direction one: baking soda where the recipe wanted baking powder
This is the swap that fails badly, and it fails in two ways at once.
Not enough gas. The recipe has no acid in it, because it was not written to have one. Baking soda with no acid to react with produces very little gas, so the bake comes out flat and dense. What that looks like, and how it is told apart from the other causes of a heavy bake, is on why a cake comes out dense and heavy and why muffins come out flat.
A taste problem. Unreacted baking soda has a flavor: soapy, metallic, chemically bitter. Because there is nothing for it to react with, it is all still there when you eat it. This is often more noticeable than the texture failure and it cannot be fixed afterwards.
The workaround, and its limits. You could add an acid to give the soda something to react with. That is genuinely how the chemistry works, and it is also a rewrite of the recipe rather than a substitution: adding an acidic liquid changes the total liquid, and adding a dry acid changes the flavor. It is a thing an experienced baker does knowingly. It is not a swap.
Direction two: baking powder where the recipe wanted baking soda
This fails less catastrophically and it still fails.
Baking powder is much weaker than baking soda by volume, because most of what is in the tin is the acid and the starch rather than the active base. So substituting it one for one leaves you with far less lift than the recipe expected.
And the acid in the recipe now has nothing to do. The buttermilk, the yogurt, the brown sugar are still acidic, and their acidity was part of the leavening plan. Removing the base leaves that acidity unbalanced, which affects flavor and can affect the crumb.
You can end up with a bake that is both under-risen and oddly tangy.
The number this page will not give you
Every page answering this question hands you a conversion ratio, usually expressed as some multiple of powder for a given amount of soda.
We are not publishing one, and the reason is the point. The figures in circulation are stated without a source and without a scope. They vary between pages. They take no account of the fact that baking powders differ in formulation between brands and between countries, or of the fact that the acid already in your recipe does not disappear when you change the base.
A ratio implies that these are two forms of one ingredient with a conversion factor between them. They are not. One is a component and the other is a kit, and the swap changes the acid balance of the whole recipe, not just the amount of gas.
If a page gives you a number, ask what it was tested on and against which brand of powder. That question does not usually have an answer.
What to do instead
If you are missing baking powder, the honest answer is that you cannot cleanly replace it with soda, because you would also have to introduce an acid, in the right amount, in the right form, without changing the liquid balance. Some bakers do this and it is a considered adjustment rather than a substitution.
If you are missing baking soda, the same applies in reverse and with less margin.
If a recipe calls for both, and many do, that is deliberate rather than redundant. The soda is neutralizing a specific amount of acid and providing some lift, and the powder is providing the rest of the lift independently. Removing either one changes both the rise and the flavor balance.
Check the tin before you blame the recipe
Both products lose potency, and an old tin is one of the most common reasons a bake fails to rise while everything appears to have been done correctly.
Both have a date on the container, and the date is the honest guide rather than any general shelf-life figure a page might give you. If a bake came out flat and nothing about your method changed, an old leavener is worth ruling out before anything else, alongside the causes on why a cake sinks in the middle.
What to take away
- Soda is a base that needs an acid. Powder brings its own.
- Soda for powder gives you a flat bake and a soapy taste.
- Powder for soda gives you an under-risen bake and an unbalanced flavor.
- A recipe calling for both is using them for two different jobs.
- The conversion ratios in circulation have no source and no scope. Treat them accordingly.