Ran out of cornstarch mid-recipe? Arrowroot 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 cornstarch to arrowroot for the amounts you are most likely to meet. The rule is always the same: 1:1 (thickening).
| Cornstarch called for | Arrowroot 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: Fruit pie filling
A recipe for a cherry pie filling calls for 2 tbsp of cornstarch to thicken the fruit. Using the ratio, that becomes 2 tbsp of arrowroot. Mix the arrowroot into the sugar and fruit, and bake until the filling bubbles — the filling will be clear and glossy, with the fruit showing through, where a cornstarch filling can look cloudy.
Example 2: Clear fruit glaze
A recipe for a fruit glaze calls for 1 tbsp of cornstarch. Using the ratio, that becomes 1 tbsp of arrowroot. Dissolve the arrowroot in cold water, add it to the simmering fruit juice, and stir until it turns clear and shiny. Take it off the heat as soon as it thickens — do not boil it hard, or the glaze will thin out again.
Example 3: Stir-fry sauce
A recipe for a stir-fry sauce calls for 1 tbsp of cornstarch in a slurry. Using the ratio, that becomes 1 tbsp of arrowroot. Mix it with cold water, add it to the simmering sauce, and stir until it turns glossy. The sauce will be shinier than the cornstarch version — but keep the heat gentle once it has thickened.
What changes in the result
Arrowroot in place of cornstarch gives a clearer, glossier sauce that thickens at a lower temperature. It is the thickener of choice for fruit sauces and clear glazes, because the gel it makes is transparent and shiny where cornstarch can go slightly cloudy. The thickening power is the same, so the ratio is one-to-one, and the method is the same — a slurry in cold liquid, stirred into a simmering sauce. The main difference is what happens if you boil it: arrowroot breaks down and thins out if it is boiled hard for a long time, where cornstarch tolerates a proper boil. So use arrowroot for a gentle simmer, and keep cornstarch for a hard boil.
When not to substitute
Do not use arrowroot where the sauce must be boiled hard for a long time. A gravy that needs a rolling boil, a sauce that will be reheated repeatedly, or a filling that must be baked for a long time will thin out with arrowroot, because the starch breaks down under prolonged heat. Do not use it in a dairy sauce that must be boiled either — arrowroot can go slimy in milk if it is overheated. And do not use it where cost matters: arrowroot is more expensive than cornstarch, so save it for the dishes where its clarity shows.
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 arrowroot do I use instead of 1 tbsp of cornstarch?
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.
Does arrowroot make a glossier sauce?
Yes — arrowroot is prized for the clear, shiny gel it makes, while cornstarch can go slightly cloudy. That makes arrowroot the better choice for fruit sauces, clear glazes and pie fillings where the finish shows.
Can I boil an arrowroot sauce?
Only gently. Arrowroot breaks down and thins out if it is boiled hard for a long time, while cornstarch tolerates a proper boil. Bring the sauce to a gentle simmer, take it off the heat once it has thickened, and do not keep boiling it.
Does arrowroot work in dairy sauces?
Carefully. Arrowroot can go slimy in milk if it is overheated, so keep the heat gentle and take the sauce off as soon as it thickens. For a dairy sauce that must be boiled, cornstarch is the safer thickener.
Is arrowroot gluten-free?
Yes — arrowroot is naturally gluten-free, and it is also corn-free, which makes it the usual substitute when corn is the problem. Check the packet for cross-contamination if that matters for your guests.
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 tastes of raw flour or raw starch, it was not cooked long enough — flour needs several minutes of cooking, and even a pure starch needs a full minute at a simmer, so give it time and taste before serving. If the sauce turns cloudy when it should be clear, the wrong starch was used: flour and cornstarch cloud a sauce, while arrowroot and tapioca stay clear, so choose the starch for the finish you want. If a fruit filling leaks in the oven, the starch did not get hot enough to set — bake until the filling bubbles in the middle, not just at the edges. Keep starches sealed and dry, and label the jar, because several white starches look identical.