Thickening Calculator
How much cornstarch, flour, arrowroot, or potato starch to thicken a soup, stew, or chili — scaled to how much liquid you're actually working with.
Why the amount changes by thickener, not just by liquid
Cornstarch, arrowroot, and potato starch are all pure starches with roughly twice the thickening power of all-purpose flour by volume, so a recipe written for 2 Tbsp of flour needs only about 1 Tbsp of cornstarch to reach the same consistency. Tapioca starch sits in between — slightly weaker than cornstarch, but still stronger than flour — and it also gives a glossier, slightly stretchy texture that some cooks like in fruit-based dishes but avoid in savory soups.
Always mix into a slurry first
Whisk the starch or flour into an equal amount of cold liquid (water or broth) before adding it to a hot pot — dumping dry starch straight into a simmering liquid almost always creates lumps that never fully smooth out. Once whisked into a slurry, stir it into the pot and simmer for 1–2 minutes; starches need to actually reach a simmer to fully thicken, so a slurry that's been added but not given enough heat can look thin at first and then continue thickening as it cooks.
Flour needs longer, starches thicken fast and can break
A flour-thickened soup should simmer at least 5–10 minutes to cook out the raw flour taste. Pure starches (cornstarch, arrowroot, potato starch) thicken almost immediately and can actually thin back out if they're boiled too hard or too long, or if the dish is very acidic (like a tomato-heavy chili) — arrowroot in particular holds up better than cornstarch under prolonged heat or acidity if you're planning to simmer a long time after thickening.