Cornstarch vs. Flour for Thickening
Updated August 15, 2026
Cornstarch thickens with about twice the power of flour: 1 tablespoon of cornstarch does the work of 2 tablespoons of flour per cup of liquid. Cornstarch gives a glossy, translucent finish in seconds; flour gives an opaque, sturdy one that survives long simmering and reheating.
| Thickener | Amount | Character | Watch out |
|---|---|---|---|
| Cornstarch (slurry) | 1 tbsp + cold water | Glossy, clear, fast | Breaks down with long boiling or freezing |
| Flour (roux or slurry) | 2 tbsp | Opaque, stable, reheats well | Needs minutes of simmering to lose rawness |
| Beurre manié | 1 tbsp butter kneaded with 1 tbsp flour | Rich, last-minute | Whisk in bits into simmering liquid |
| Arrowroot / tapioca | ≈ cornstarch amounts | Glossy, freeze-stable | Arrowroot goes slimy with dairy |
| Reduction (no starch) | — | Purest flavor | Takes time; concentrates salt too |
The slurry rule
Starch dumped into hot liquid seizes into lumps instantly. Always make a slurry: whisk the cornstarch (or flour) into a few tablespoons of COLD water until smooth, then stream it into the simmering pot while stirring. Cornstarch needs to reach a brief boil to thicken fully — then back off: extended hard boiling un-thickens it as the starch granules rupture. Flour is the reverse: it wants several minutes of simmering after thickening to cook off the raw-flour taste.
Match the thickener to the dish
Cornstarch's glossy translucence is the look of stir-fry sauces, fruit pie fillings, and Chinese soups. Flour's matte opacity is gravy, chowder, and béchamel — and it holds through tomorrow's reheat, where cornstarch sauces thin out. Freezing tips the choice further: cornstarch weeps on thawing, tapioca and arrowroot survive it. If a dish will be frozen, flour or tapioca; if it's glossy and eaten tonight, cornstarch.
Quick answers
- How much cornstarch thickens 1 cup of liquid?
- 1 tablespoon (as a cold-water slurry) for sauce thickness; 2 for a pie-filling set. Flour needs double those amounts.
- Can I swap cornstarch and flour 1:1?
- No — halve when going flour→cornstarch, double when going the other way, and adjust technique: cornstarch as slurry to a brief boil, flour with a longer simmer.
- Why did my cornstarch sauce go thin again?
- Over-boiling ruptured the starch, or acid and time wore it down. Rethicken with a fresh half-slurry and stop at the first sign of thickness.