Quick answer
For basic sauce or filling thickening, start with 2 tablespoons all-purpose flour for 1 tablespoon cornstarch. Arrowroot, tapioca starch, and potato starch are other useful options, but they behave differently with heat, acid, freezing, and prolonged cooking.
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Quick comparison
| Substitute | Starting point | Best for |
|---|---|---|
| All-purpose flour | 2 tbsp per 1 tbsp cornstarch | Gravies, sauces, cooked fillings |
| Arrowroot | Often near 1:1; recipe dependent | Glossy sauces and fruit fillings |
| Tapioca starch | Check recipe/application | Pie fillings, chewy textures |
| Potato starch | Check recipe/application | Thickening and some gluten-free baking |
Why the ratio changes
Cornstarch is a purified starch, while wheat flour also contains protein and other components. Flour therefore has less thickening power spoon-for-spoon and produces a more opaque result. Other purified starches can thicken efficiently but differ in texture and stability.
For baking: do not automatically use the sauce-thickening ratio. Cornstarch in cake flour blends, cookies, shortbread, and coatings serves different purposes.
For fruit pie filling
The ideal thickener depends on the fruit, amount of juice, acidity, and whether the pie will be frozen. Follow a tested pie formula when clarity and sliceability matter rather than swapping starches blindly.