Cornstarch to Flour: the Thickener Conversion
Out of cornstarch — or thickening gluten-free and out of flour? For sauces, gravies and soups the rule is simple: 1 tbsp cornstarch = 2 tbsp all-purpose flour. Cornstarch has twice the thickening power of flour, so use half as much — Argo, Bob's Red Mill, America's Test Kitchen and Utah State Extension all publish the same 2:1 rule. Enter what the recipe calls for:
Seen "1 tbsp flour = 1½ tsp cornstarch" and wondered if it's a different rule? It isn't — 1½ tsp is exactly half a tablespoon, so it's the same 2:1 ratio restated in teaspoons. Why does pure starch beat flour? Flour is only about 75% starch (America's Test Kitchen); the rest is protein and other solids that dilute its thickening power.
Flour to cornstarch conversion chart
The recipe thickens with flour and you're reaching for cornstarch — use half as much:
| Flour called for | Flour (g) | Cornstarch instead | Cornstarch (g) |
|---|---|---|---|
| 1 tsp | 2.5 g | ½ tsp | 1.2 g |
| 2 tsp | 5 g | 1 tsp | 2.3 g |
| 1 tbsp | 7.5 g | 1½ tsp | 3.5 g |
| 2 tbsp | 15 g | 1 tbsp | 7 g |
| 3 tbsp | 23 g | 1½ tbsp | 11 g |
| ¼ cup | 30 g | 2 tbsp | 14 g |
| ⅓ cup | 40 g | 2 tbsp + 2 tsp | 19 g |
| ½ cup | 60 g | ¼ cup | 28 g |
| 1 cup | 120 g | ½ cup | 56 g |
Cornstarch to flour conversion chart
The recipe thickens with cornstarch and you only have flour — use twice as much:
| Cornstarch called for | Cornstarch (g) | Flour instead | Flour (g) |
|---|---|---|---|
| 1 tsp | 2.3 g | 2 tsp | 5 g |
| 1½ tsp | 3.5 g | 1 tbsp | 7.5 g |
| 2 tsp | 4.7 g | 1 tbsp + 1 tsp | 10 g |
| 1 tbsp | 7 g | 2 tbsp | 15 g |
| 2 tbsp | 14 g | ¼ cup | 30 g |
| 3 tbsp | 21 g | ¼ cup + 2 tbsp | 45 g |
| ¼ cup | 28 g | ½ cup | 60 g |
How much thickener per cup of liquid?
Starting from scratch rather than converting a recipe? The extension-service dosing for a medium-bodied sauce or gravy is 1 tbsp cornstarch — or 2 tbsp flour — per 1 cup of liquid (University of Illinois Extension; Argo's gravy recipe and Bob's Red Mill's white sauce use exactly these proportions):
| Consistency (per 1 cup liquid) | All-purpose flour | Cornstarch |
|---|---|---|
| Thin (soup-like) | 1 tbsp (7.5 g) | 1½ tsp (3.5 g) |
| Medium (gravy, white sauce) | 2 tbsp (15 g) | 1 tbsp (7 g) |
| Thick (binding, croquettes) | 3 tbsp (23 g) | 1½ tbsp (11 g) |
The medium row is the extension-verified figure; the thin and thick rows follow the classic white-sauce ladder (1 tbsp flour per cup for thin, 3 for thick — the Betty Crocker convention) with cornstarch at half throughout.
By weight: it's 47%, not 50%
The 2:1 rule is a volume rule. A tablespoon of cornstarch weighs 7 g while a tablespoon of all-purpose flour weighs 7.5 g (King Arthur's chart: 112 g per cup of cornstarch vs 120 g per cup of flour) — so by weight, replace flour with about 47% as much cornstarch, not 50%: 100 g flour → ≈47 g cornstarch, and in reverse 100 g cornstarch → ≈214 g flour. The calculator above does this automatically when you pick grams.
Technique: slurry vs roux
Cornstarch — cold slurry, end of cooking. Stir the cornstarch into a little cold liquid until completely smooth (hot liquid makes it clump), stir the slurry into the simmering pot, then — Argo's instruction — bring it to a full boil, stirring constantly, and boil 1 minute so the granules swell to full capacity. Then reduce the heat: prolonged hard cooking or rough whisking will thin it back out.
Flour — roux, or slurry plus a longer simmer. The classic route is a roux: equal parts flour and fat (butter), cooked together before the liquid goes in — King Arthur's tested gravy cooks the roux about 4 minutes (light golden, nutty-smelling) and then simmers the gravy ~15 minutes to thicken fully and lose any raw-flour taste. The shortcut is beurre manié — equal parts flour and softened butter kneaded to a paste, whisked into boiling liquid a spoonful at a time, 2–3 minutes' simmer each (America's Test Kitchen).
Which thickener should you use?
Reach for cornstarch when you want speed (no roux), a glossy, translucent finish — stir-fry sauces, fruit sauces, pudding — or a gluten-free result. Reach for flour for classic opaque, matte gravies and cream sauces, anything that simmers a while, and sauces you'll reheat: America's Test Kitchen notes cornstarch-thickened sauces break down more quickly than flour-thickened ones.
Honest caveats, all documented: acid (lemon, lime, vinegar) weakens or defeats cornstarch (Argo); freezing makes both weep — ATK tested flour and cornstarch gravies and both separated when thawed (a full boil plus hard whisking rescues them; Clemson Extension's advice is to freeze the broth unthickened, or use waxy rice flour); and neither belongs in home canning — extension services say to thicken after opening, not before processing.
The pie-filling exception
Don't carry the 2:1 rule into fruit pie. King Arthur's pie-thickener guide doses per fruit, and the flour-to-cornstarch ratio there runs from about 3.5:1 for apples down to 1:1 for peaches — sugar, acidity and an hour in the oven change how each starch behaves. The 2:1 rule is for sauces, gravies and soups; for pie, follow a fruit-specific chart.
Other thickeners, per 1 tbsp of cornstarch
| Thickener | Instead of 1 tbsp cornstarch | Notes |
|---|---|---|
| All-purpose flour | 2 tbsp | Unanimous: Argo, Bob's Red Mill, ATK, Utah State Extension |
| Arrowroot | 1–1½ tbsp | Sources genuinely disagree: Utah State says equal; ATK says 1½×. ATK: avoid with dairy (turns slimy) |
| Potato starch | 1–1½ tbsp | Same split (Utah State 1:1; ATK 1–1½×). Add late; pull off the heat once thick |
| Tapioca starch / flour | 1 tbsp | ATK: equal volume — and it holds up in a slow cooker, unlike flour or cornstarch |
| Quick (granular) tapioca | 2 tbsp | Utah State. Not the same thing as tapioca starch — the granules need a 15–30 min rest |
Need a different conversion?
Just weighing, not swapping? 1 cup of cornstarch is 112 g and 1 cup of all-purpose flour is 120 g — the flour & starch chart covers arrowroot, tapioca and every flour on the site. Fun mirror image: the cake flour substitute runs the same two ingredients the other way — cornstarch stirred into flour to weaken it for tender cakes. More swaps: butter to oil and sugar to honey. And teaspoons in a tablespoon spells out the spoon math used here (1 tbsp = 3 tsp).
Frequently asked questions
Can I use flour instead of cornstarch to thicken a sauce?
Yes — use twice as much. Flour has about half the thickening power of cornstarch, so replace every tablespoon of cornstarch with 2 tablespoons of all-purpose flour. Argo, Bob's Red Mill, America's Test Kitchen and Utah State University Extension all publish this same 2:1 rule. Flour also needs more cooking than cornstarch to lose its raw taste, and the sauce will finish opaque rather than glossy.
How much cornstarch equals 1/4 cup of flour?
2 tablespoons. That's Argo's own worked example of the rule: cornstarch has twice the thickening power of flour, so use half as much. Spoon by spoon, 1 tablespoon of flour is replaced by 1 1/2 teaspoons of cornstarch.
How much cornstarch does it take to thicken 1 cup of liquid?
1 tablespoon of cornstarch per cup of liquid for a medium-bodied sauce or gravy — the University of Illinois Extension figure, and exactly the proportion in Argo's gravy recipe and Bob's Red Mill's white sauce. Doing it with flour instead, use 2 tablespoons per cup. Use about half that for a thin sauce and about one-and-a-half times for a thick one.
Is 1 tablespoon of flour really equal to 1 1/2 teaspoons of cornstarch?
Yes — and it's the same rule, not a different one. 1 1/2 teaspoons is exactly half a tablespoon, so '1 tbsp flour = 1 1/2 tsp cornstarch' is the standard 2:1 ratio restated in teaspoons. Some sites present the two phrasings as if they were competing ratios; they are identical.
Do you have to boil a cornstarch-thickened sauce?
Yes — briefly. Argo's instruction is to bring the mixture to a full boil, stirring constantly, and boil for 1 minute, at which point the starch granules have swelled to full capacity; Bob's Red Mill says the same. The popular warning to 'never boil cornstarch' garbles the real caveats, which are significant overcooking and rough stirring — not the one-minute boil itself.
Why did my cornstarch sauce thin out again?
Argo documents three causes: significant overcooking (thickened mixtures can thin as they cool), excessive or rough stirring (it physically breaks the swollen starch cells), and acidic ingredients like lemon juice or vinegar (they reduce the starch's thickening ability or stop it thickening at all). Freezing does it too — ice ruptures the starch network.
Is the cornstarch-to-flour ratio the same by weight?
Almost, but not exactly. A tablespoon of cornstarch weighs 7 g and a tablespoon of all-purpose flour 7.5 g (King Arthur's chart: 112 g vs 120 g per cup), so halving by volume means multiplying the weight by 7/15 — about 47%, not 50%. Replace 100 g of flour thickener with about 47 g of cornstarch; going the other way, 100 g of cornstarch becomes about 214 g of flour.
Which makes better gravy — cornstarch or flour?
Flour is the traditional choice: a roux-based gravy is opaque and matte, tastes rounder, and America's Test Kitchen notes flour-thickened sauces hold up longer than cornstarch ones. Cornstarch is faster (no roux), gluten-free, and gives a glossy, translucent finish — the look of many Chinese-style sauces and fruit fillings. For a classic opaque gravy, flour; for speed, shine or gluten-free, cornstarch.
Can I freeze gravy thickened with flour or cornstarch?
Both weep. America's Test Kitchen froze and thawed both kinds and found each one separates as the starch structure breaks down — the fix is to bring the thawed gravy to a full boil and whisk hard, which re-emulsifies it. Clemson Extension's cleaner answer: freeze the broth unthickened and make the gravy fresh, or thicken with waxy rice flour, which survives freezing.
Does the 2:1 ratio work for pie fillings?
No — pie is the exception. King Arthur's pie-thickener guide sets the amounts per fruit, and the flour-to-cornstarch ratio swings from about 3.5:1 for apples to roughly 1:1 for peaches, because sugar, acidity and a long bake all change how each starch performs. Use the 2:1 rule for sauces, gravies and soups, and a fruit-specific chart for pie.
Is cornstarch gluten-free?
Yes — Argo states its cornstarch is gluten-free (it's pure starch from corn, no wheat). That makes the swap in this calculator the standard way to make gravy gluten-free: use half as much cornstarch in place of the flour. Arrowroot, potato starch and tapioca starch are gluten-free alternatives too.
What about arrowroot, potato starch or tapioca instead?
Per 1 tablespoon of cornstarch: arrowroot 1 to 1 1/2 tablespoons (Utah State Extension says equal amounts; America's Test Kitchen says 1 1/2× — a genuine disagreement; ATK also warns arrowroot turns slimy with dairy), potato starch 1 to 1 1/2 tablespoons (same split), tapioca starch an equal amount (ATK — and it survives slow cooking better than flour or cornstarch), granular quick tapioca 2 tablespoons (Utah State).