Baking powder vs bicarbonate of soda
One is a complete leavening system; the other is a single alkaline compound that needs an acid supplied by the recipe.
The short answer
Baking powder contains its own acid and works in any batter. Bicarbonate of soda does not, and needs an acidic ingredient present — which is why they are not interchangeable in either direction without adjusting the recipe.
Where they differ
| Dimension | First | Second |
|---|---|---|
| What it contains | Bicarbonate of soda, one or more acid salts, and a starch to keep them apart in the tin. | Sodium bicarbonate alone. |
| Needs an acid in the recipe | No. It brings its own. | Yes. Without one, most of it stays unreacted. |
| Strength by weight | Weaker, because it is substantially diluted by its acid and starch. | Stronger, because it is undiluted. |
| When it releases gas | Most modern powders are double-acting: some on wetting, the rest on heating. | On contact with acid and moisture, and it continues on heating. |
| Effect of using too much | Over-inflation, coarse crumb and collapse. | The same, plus a soapy, metallic taste and a darker colour from the unreacted alkali. |
Differences that do not matter
These are real distinctions that are unlikely to change your decision. They are listed so you can stop thinking about them rather than left out so the comparison looks decisive.
- Shelf stability. Loses potency with age and humidity. Loses potency with age and humidity.
The substitution question, answered honestly
Baking powder can usually replace bicarbonate of soda, but not gram for gram — more is needed because it is diluted, and the recipe’s own acid is then left unreacted, which changes both flavour and colour.
Bicarbonate of soda cannot replace baking powder unless an acid is added as well, and how much depends entirely on what else is in the batter.
BakeHQ deliberately does not publish a single conversion ratio for either direction. Every ratio circulating online assumes a formulation it does not state, and applying it to a different one gives a result no better than a guess.
Related
Compares
One side of the comparison readers most often arrive with.
The other side, and the one whose dependence on a recipe acid causes most of the confusion.