Ran out of arrowroot mid-recipe? Cornstarch will do the job at 1:1 (thickening, slightly less glossy) — 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 arrowroot to cornstarch for the amounts you are most likely to meet. The rule is always the same: 1:1 (thickening, slightly less glossy).
| Arrowroot 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 arrowroot, 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 arrowroot 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 arrowroot to activate. The gravy will be smooth and savoury, a touch less glossy than the arrowroot version.
Example 2: Fruit sauce
A recipe for a fruit sauce calls for 1 tbsp of arrowroot. Using the ratio, that becomes 1 tbsp of cornstarch. Whisk the cornstarch into cold water first, add it to the simmering fruit, and stir until it thickens. The sauce will be slightly cloudier than the arrowroot version, so this swap suits a berry sauce better than a clear apple glaze.
Example 3: Custard
A recipe for a custard calls for 1 tbsp of arrowroot as the thickener. Using the ratio, that becomes 1 tbsp of cornstarch. Whisk the cornstarch into the cold milk before heating, and stir constantly until the custard coats the back of a spoon. The custard will be smooth and neutral.
What changes in the result
Cornstarch in place of arrowroot gives a slightly less glossy finish, but the thickening power is the same. Arrowroot is prized for the clear, shiny gel it makes in a sauce, and cornstarch is a touch cloudier and a touch less brilliant — the difference is subtle in a gravy and more noticeable in a clear fruit sauce. Cornstarch also needs more heat to activate: it must come to a proper simmer before it thickens, where arrowroot thickens at a lower temperature. On the other hand, cornstarch is more forgiving when a sauce must be boiled, and it is cheaper and easier to find, so for an everyday gravy or custard the swap is a practical one.
When not to substitute
Do not make this swap where the clear, glossy finish of arrowroot is the point of the dish. A clear fruit glaze, a glossy fruit pie filling, or a delicate sauce that must stay transparent will look cloudier with cornstarch, and the difference will show. Do not use cornstarch either in a sauce that must stay clear after it cools: arrowroot holds its clarity, while a cornstarch sauce can go opaque as it sets. And do not use it in a dish for someone who cannot eat corn — arrowroot is the usual substitute precisely because it is corn-free.
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 arrowroot?
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 arrowroot, so measure this swap by volume.
Will the sauce be less glossy?
A little. Arrowroot is prized for the clear, shiny gel it makes, and cornstarch is a touch cloudier and less brilliant. The difference is subtle in a gravy and more noticeable in a clear fruit sauce or glaze.
Does cornstarch need more heat?
Yes — cornstarch must come to a proper simmer before it thickens, while arrowroot thickens at a lower temperature. Give the sauce a full minute at a gentle bubble after you add the slurry.
Can I use cornstarch if I cannot eat corn?
No — that is the one case where this swap is wrong. Arrowroot is the usual substitute precisely because it is corn-free. If corn is the problem, use tapioca flour or potato starch instead.
Is cornstarch cheaper than arrowroot?
Usually, yes — cornstarch is cheaper and easier to find than arrowroot, which is why it is the practical choice for an everyday gravy or custard where the glossy finish does not matter.
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.