Ran out of all-purpose flour mid-recipe? Cornstarch will do the job at 2 tbsp flour per 1 tbsp cornstarch (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 all-purpose flour to cornstarch for the amounts you are most likely to meet. The rule is always the same: 2 tbsp flour per 1 tbsp cornstarch (thickening).
| All-purpose flour called for | Cornstarch to use |
|---|---|
| 1 tbsp | 1/2 tbsp |
| 2 tbsp | 1 tbsp |
| 3 tbsp | 1 1/2 tbsp |
| 1/4 cup | 2 tbsp |
| 1/3 cup | 2 tbsp + 2 tsp |
| 1/2 cup | 1/4 cup |
By weight, one cup of all-purpose flour is about 120 g (spooned and levelled, King Arthur Baking) and one cup of cornstarch is about 112 g (spooned and levelled, King Arthur Baking). The two weigh very differently per cup, so measure this swap by volume, not by weight, unless you are converting the whole recipe to grams — and then use the numbers above. All weights on this page are approximate; verify with a kitchen scale when the result matters.
Worked examples
Example 1: Quick pan sauce
A recipe for a pan sauce calls for 2 tbsp of flour to thicken the pan juices. Using the ratio, that becomes 1 tbsp of cornstarch — half as much. Mix the cornstarch into cold water, add it to the simmering sauce, and stir for a minute until it thickens. The sauce will be glossy and clear, and it will be ready in a fraction of the time a flour sauce would take.
Example 2: Stir-fry sauce
A recipe for a stir-fry sauce calls for 1 tbsp of flour. Using the ratio, that becomes 1/2 tbsp of cornstarch. Mix it with cold water, add it to the simmering sauce, and stir until it turns glossy. The sauce will cling to the vegetables nicely and it will be clearer than the flour version.
Example 3: Fruit sauce
A recipe for a fruit sauce calls for 3 tbsp of flour to thicken the fruit. Using the ratio, that becomes 1 1/2 tbsp of cornstarch. Mix the cornstarch into the sugar first, add it to the fruit, and cook until the sauce thickens. The sauce will be clearer and glossier than the flour version, with the fruit showing through.
What changes in the result
Cornstarch in place of flour needs only half as much, and it changes the sauce in two ways: it makes it clearer, and it thickens faster. Flour turns a sauce opaque and needs several minutes of cooking to lose its raw taste; cornstarch thickens in a minute and leaves a clear, glossy sauce. The swap works well in a sauce that must be clear or glossy, and it is a good choice when the sauce needs thickening at the last minute. The trade-off is that a cornstarch sauce can go thin if it is boiled hard for a long time, where a flour sauce holds its thickness — so use cornstarch for a quick finish, and keep flour for a long, slow simmer.
When not to substitute
Do not use cornstarch where the flour is doing more than thickening. In a roux-based sauce, a stew that simmers for hours, or a dish where the flour also adds body and flavour, cornstarch will not do the same job — it thickens, but it does not add the rich, toasty depth of a cooked roux. Do not use it in a sauce that must be boiled hard for a long time either: cornstarch thins out if it is overheated, while a flour sauce holds up. And do not use twice as much: cornstarch is twice as powerful as flour, so using the same amount will make the sauce far too thick.
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 flour?
Use 1/2 tbsp — the ratio is 2 tbsp of flour per 1 tbsp of cornstarch, so cornstarch needs only half as much. One cup of all-purpose flour is about 120 g and one cup of cornstarch is about 112 g, so the weights are close, but measure by volume for this swap.
Will the sauce be clearer?
Yes. Cornstarch leaves a sauce clear and glossy, while flour turns it opaque. That makes cornstarch the better choice for a fruit sauce or a stir-fry sauce, and a poorer one for a hearty gravy where opacity is part of the character.
Does cornstarch thicken faster?
It does — a cornstarch slurry thickens a sauce in about a minute, while a flour slurry needs several minutes of cooking to lose its raw taste. That makes cornstarch the better choice for a last-minute thickening.
Can I use cornstarch in a long-simmered stew?
Not as a straight swap. A cornstarch sauce can go thin if it is boiled hard for a long time, while a flour sauce holds its thickness. For a stew that simmers for hours, keep the flour, or add the cornstarch at the end.
Is cornstarch gluten-free?
Yes — cornstarch contains no gluten, while flour does. That makes this swap a good choice for a gluten-free table, as long as the rest of the recipe is gluten-free too.
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.