A 1 lb box of cornstarch holds about 4.1 cups. The arithmetic is simple: 1 lb is 454 g, and one cup of cornstarch weighs 112 g spooned and levelled, so 454 divided by 112 is 4.1 cups. Cornstarch is sold in a box rather than a bag, and in 1 lb rather than 5 lb, because it is used in tiny amounts — a tablespoon to thicken a sauce, a quarter cup in a cake, a teaspoon in a stir-fry. A 1 lb box is a large amount of cornstarch for most households: at a tablespoon per recipe, a box covers more than sixty recipes, and it will last for years.
Four point one cups is about sixteen batches of shortbread at a quarter cup each, or sixty-five sauces at a tablespoon each, or eight cakes at half a cup each. Cornstarch is a pure starch with no flavour of its own, and its job is to thicken and to lighten: it thickens sauces and gravies, it makes shortbread and cakes more tender by replacing some of the flour, and it gives fried food a crisp coating. Knowing the box holds 4.1 cups matters less for planning — you will not run out — and more for understanding how little you need: a recipe that calls for a cup of cornstarch is using a quarter of the box, which is a lot of cornstarch, and you should double-check the recipe before you commit.
Box of cornstarch conversion table
The table below shows how many cups you get from fractions of a 1 lb box, and how many cups of cornstarch weigh how many grams. All figures use 112 g per cup, spooned and levelled.
| Amount | Cups (approx.) | Weight |
|---|---|---|
| 1/4 bag | 1.02 cups | 114 g |
| 1/2 bag | 2.05 cups | 227 g |
| 3/4 bag | 3.07 cups (about 3 1/8 cups) | 340 g |
| Full bag | 4.10 cups (about 4 1/8 cups) | 454 g |
| Cups | Grams (approx.) |
|---|---|
| 1 cup | 112 g |
| 2 cups | 224 g |
| 3 cups | 336 g |
| 4 cups | 448 g |
| 5 cups | 560 g |
| 10 cups | 1,120 g |
Cornstarch is used in tablespoons, not cups: 1 tablespoon is about 7 g. A 1 lb box holds about 65 tablespoons, so a single box covers a year or more of everyday cooking for most households.
Worked examples
Thickening a stir-fry sauce
A stir-fry sauce for four people uses 1 tablespoon of cornstarch, mixed with 2 tablespoons of cold water to make a slurry, then stirred into the hot sauce. One 1 lb box holds 4.1 cups, which is about 65 tablespoons, so it covers sixty-five stir-fries — more than a year of weekly stir-fry nights. Mix the slurry first and add it to the sauce off the heat, then return the pan to the heat and stir until the sauce thickens and turns glossy. If you add dry cornstarch straight to a hot sauce, it will clump, and no amount of stirring will fix it.
Shortbread with a tender crumb
A batch of shortbread uses 1/4 cup of cornstarch mixed with 3/4 cup of flour, butter and sugar, which gives the biscuits a melt-in-the-mouth texture that flour alone cannot match. One 1 lb box holds 4.1 cups, so it covers sixteen batches of shortbread — about 240 biscuits. The cornstarch replaces some of the flour, which reduces the gluten and makes the shortbread more tender. Cream the butter and sugar until pale, add the flour and cornstarch, and mix only until the dough comes together.
A fruit pie filling that sets cleanly
A fruit pie filling uses 1/4 cup of cornstarch to thicken the juices from 4 cups of fruit, which gives a filling that slices cleanly and does not run. One 1 lb box holds 4.1 cups, so it covers sixteen pies — years of pie baking. Toss the cornstarch with the fruit and sugar and let it sit for fifteen minutes before filling the pie. Bake the pie until the filling bubbles in the centre, which tells you the starch has activated. Let the pie cool completely before slicing, because a cornstarch filling sets as it cools, and slicing it hot will give you a runny mess.
How the figure is worked out
Think of the answer as a conversion, not a measurement. The package weight is a standard 1 lb (454 g) box of cornstarch. One cup of this ingredient weighs a known number of grams, so the cups in the package are the package weight divided by the cup weight. The method matters as much as the number: a spooned and levelled cup is the standard, and a scooped cup packs in more, which is why two bakers can get different cup counts from the same bag. The cup weight used here is 112 g per cup, spooned and levelled, sourced from King Arthur Baking.
Practical guidance
Cornstarch must be mixed with cold liquid before it goes into a hot dish — this is the single most important rule. A tablespoon of cornstarch stirred into two tablespoons of cold water makes a slurry, and the slurry stirs smoothly into a hot sauce, thickening it in seconds. Dry cornstarch added straight to a hot liquid clumps instantly, and the lumps will not dissolve no matter how hard you stir. The same rule applies to gravies, soups and pie fillings: always make a slurry first. And do not boil a cornstarch-thickened sauce for long — prolonged boiling breaks the starch down and the sauce goes thin again. Bring it to a simmer, let it thicken, and take it off the heat.
Measure cornstarch by the tablespoon, not by the cup. A tablespoon is about 7 g, and a cup is 112 g, but nobody measures cornstarch by the cup because the amounts are so small. Spoon it into the measuring spoon and level it off — do not heap it, because a heaped tablespoon can be half as much again, and in a sauce that is the difference between glossy and gloopy. Store cornstarch in its box in a cool, dry place, and it will keep for years — it is a pure starch with no oils to go rancid. Keep it dry and keep it away from strong smells, which it absorbs. If cornstarch gets damp it will clump, and damp cornstarch should be thrown away.
Frequently asked questions
How many tablespoons are in a box of cornstarch?
About 65 tablespoons. A 1 lb box is 454 g, and a tablespoon of cornstarch weighs about 7 g, so 454 divided by 7 is about 65. At a tablespoon per recipe, a single box covers more than a year of everyday cooking for most households.
Can I use flour instead of cornstarch to thicken a sauce?
Yes, but you need twice as much — a tablespoon of cornstarch thickens about as much as 2 tablespoons of flour — and the result will be cloudier and less glossy. Flour also needs to cook longer to lose its raw taste. Cornstarch is the better thickener for a clear, glossy sauce; flour is the better choice for a gravy or a roux-based sauce.
Why did my cornstarch sauce go lumpy?
Because the cornstarch was added dry to a hot liquid. Cornstarch must be mixed with cold water first to make a slurry, and the slurry stirred into the hot sauce. Dry cornstarch in hot liquid clumps instantly and the lumps will not dissolve. Next time, make the slurry first and add it off the heat, then return the pan to the heat to thicken.
How long does cornstarch keep?
Years, in a cool, dry place. Cornstarch is a pure starch with no oils to go rancid, so it keeps far longer than any grain flour. Keep it dry — damp cornstarch clumps and should be thrown away — and keep it away from strong smells, which it absorbs.
Is cornstarch gluten-free?
Yes, naturally. Cornstarch is made from corn, which does not contain gluten. It is a staple thickener in gluten-free cooking. Check the label for certification if you need guaranteed gluten-free starch, because it can be processed in facilities that handle wheat.