Ran out of potato starch mid-recipe? Cornstarch will do the job at 1:1 (thickening) — but only if you also adjust for what the swap takes away or adds. Starches are the quiet workhorses of the kitchen: a spoonful turns a thin liquid into a glossy sauce, holds a pie filling together, or lightens a cake crumb. Most of them swap one-to-one for thickening, but they do not all behave the same way once heated — some need a rolling boil to activate, some turn cloudy, some go slack if you boil them too long, and one of them (flour) needs twice as much and a longer cook to lose its raw taste. This page gives the ratio, shows it in a table, and walks through three real dishes so you can see the swap working in a pan, not just on paper.
The ratio, in a table
The table below converts potato starch to cornstarch for the amounts you are most likely to meet. The rule is always the same: 1:1 (thickening).
| Potato starch called for | Cornstarch to use |
|---|---|
| 1 tsp | 1 tsp |
| 2 tsp | 2 tsp |
| 1 tbsp | 1 tbsp |
| 2 tbsp | 2 tbsp |
| 3 tbsp | 3 tbsp |
| 1/4 cup | 1/4 cup |
| 1/3 cup | 1/3 cup |
| 1/2 cup | 1/2 cup |
By weight, one cup of cornstarch is about 112 g (spooned and levelled, King Arthur Baking). Our density table does not list a grams-per-cup figure for potato starch, so no weight is given for it here — measure it by volume. All weights on this page are approximate; verify with a kitchen scale when the result matters.
Worked examples
Example 1: Gravy
A recipe for gravy calls for 2 tbsp of potato starch to thicken the pan juices. Using the ratio, that becomes 2 tbsp of cornstarch. Make a slurry with cold water, whisk it into the simmering gravy, and give it a full minute at a gentle bubble — cornstarch needs more heat than potato starch to activate. The gravy will be smooth and savoury, a touch less silky than the original.
Example 2: Soup
A recipe for a soup calls for 1 tbsp of potato starch to give it body. Using the ratio, that becomes 1 tbsp of cornstarch. Mix it with cold water, stir it into the simmering soup, and give it a minute to thicken. The soup will be slightly cloudier than the potato-starch version, so this swap suits a hearty soup better than a clear consommé.
Example 3: Stir-fry sauce
A recipe for a stir-fry sauce calls for 1 tbsp of potato starch in a slurry. Using the ratio, that becomes 1 tbsp of cornstarch. Mix it with cold water, add it to the simmering sauce, and stir until it thickens. The sauce will be smooth and it will hold up to a proper boil, which is cornstarch's advantage over potato starch.
What changes in the result
Cornstarch in place of potato starch is a straightforward swap: the two thicken at about the same strength, and the ratio is one-to-one. The main difference is the finish — potato starch gives a clearer, slightly silkier gel, while cornstarch is a touch cloudier — and the heat: potato starch thickens at a lower temperature and breaks down if it is boiled hard, while cornstarch needs a proper simmer and tolerates a boil. For an everyday gravy or soup, the difference barely matters, and cornstarch is the easier of the two to find. For a clear sauce or a delicate soup where the finish shows, potato starch is the better thickener, and the swap is a small step down in clarity.
When not to substitute
Do not make this swap where the clear, silky finish of potato starch is the point of the dish. A clear soup, a delicate fruit sauce, or a glossy glaze will look cloudier with cornstarch, and the difference will show. Do not use cornstarch either in a dish that must be kept warm for a long time at a low heat: potato starch holds its thickness well at a gentle warmth, while a cornstarch sauce can go thin if it is held too long. And do not use it in a recipe where potato starch is doing more than thickening — in some gluten-free baking, potato starch also adds moisture and structure, and cornstarch will not do that job.
Practical guidance
Always mix a starch into a little cold liquid before it meets anything hot — a slurry made with cold water, stock or milk is the single best defence against lumps. Add the slurry to a simmering liquid, stir constantly, and give it a full minute at a gentle bubble so the starch granules swell and the sauce reaches its final thickness; a sauce thickened with starch will look thinner while it is still hot than it will be on the plate. Do not boil a starch-thickened sauce hard for a long time: most starches thin out again if they are overheated or stirred too vigorously. If you are thickening something acidic, such as a fruit filling or a tomato sauce, add the starch late and keep the heat moderate, because acid weakens starch gels. Store starches in airtight jars in a cool, dry cupboard; they keep almost indefinitely. All amounts on this page are approximate; weigh the starch on a kitchen scale when the sauce or filling must set exactly.
Frequently asked questions
How much cornstarch do I use instead of 1 tbsp of potato starch?
Use 1 tbsp — the ratio is 1:1 for thickening. Our density table lists cornstarch at about 112 g per cup, but it does not list a grams-per-cup figure for potato starch, so measure this swap by volume.
Will the sauce be cloudier?
A little. Potato starch gives a clearer, slightly silkier gel than cornstarch, which can go a touch cloudy. The difference is subtle in a gravy and more noticeable in a clear soup or a glossy glaze.
Does cornstarch need more heat?
Yes — cornstarch must come to a proper simmer before it thickens, while potato starch thickens at a lower temperature. Give the sauce a full minute at a gentle bubble after you add the slurry.
Can I boil a cornstarch sauce?
Yes — cornstarch tolerates a proper boil, while potato starch breaks down if it is boiled hard. That makes cornstarch the better choice for a sauce that must be boiled or reheated.
Can I use cornstarch in gluten-free baking instead of potato starch?
Not as a straight swap. In gluten-free baking, potato starch also adds moisture and structure, and cornstarch will not do that job. This swap is for thickening sauces and soups, not for baking.
Scaling the swap up or down
A starch swap scales linearly: multiply both sides of the ratio by the same factor and the thickening power stays the same, whether you are thickening a single pan of gravy or a large pot of soup. The thing to watch is the slurry. Do not make one giant slurry for a very large pot — make it in two batches, so it stays smooth and lump-free, and add it in stages, stirring and waiting a minute between additions, because a starch-thickened sauce keeps thickening for a minute after the slurry goes in. If you add the whole amount at once to a big pot, you can overshoot and end up with a sauce that is too thick to pour. For a very small amount, measure the starch with measuring spoons rather than by eye: half a teaspoon too much in a single serving is a bigger error than a tablespoon too much in a large pot. All amounts on this page are approximate; weigh the starch on a kitchen scale when the sauce or filling must set exactly.
If the swap goes wrong — and how to store the substitute
If the sauce goes lumpy, the starch met hot liquid before it was mixed into a slurry — take the pan off the heat, whisk in a little cold liquid, and strain the sauce through a sieve to rescue it. If the sauce is too thin, it probably did not reach a proper simmer: bring it back to a gentle bubble and give it a full minute, because most starches need heat to activate. If it is too thick, whisk in a splash of cold liquid a little at a time until it loosens. If a starch-thickened sauce thins out on reheating, it was probably boiled too hard — reheat it gently and do not boil it again. Store starches in airtight jars in a cool, dry cupboard; they keep almost indefinitely.