Updated 28 July 2026 · Written by Daniel Wright
A sauce stays thin when there is too much liquid and no thickening agent, or when a starch thickener has not reached a boil. Most sauces thicken either by simmering to reduce the water, or by adding a starch like flour or cornstarch that must heat to activate. Give it time to reduce, or stir in a slurry.
Thin sauce = too much liquid or an inactive thickener. Fix: simmer to reduce, or stir in a cornstarch slurry and bring it to a boil.
Why Sauces Stay Thin
- Too much liquid and not enough time simmering to reduce it.
- A cornstarch or flour thickener that never reached a boil to activate.
- Adding cold starch directly, which clumps instead of thickening evenly.
How to Thicken a Sauce
- Reduce it: simmer uncovered and let excess water evaporate to concentrate the sauce.
- Cornstarch slurry: mix 1 tablespoon cornstarch with 1 tablespoon cold water, stir in, and bring to a boil.
- Roux or beurre manie: cook equal parts flour and butter, then whisk in.
- Simmer gently in a wide saucepan so liquid evaporates faster.
Frequently Asked Questions
Why won’t my cornstarch thicken the sauce?
Cornstarch only thickens once it reaches a boil, so bring the sauce up to a simmer after adding the slurry. Also avoid adding acidic ingredients until after it has thickened, as they can weaken it.
How can I thicken sauce without cornstarch?
Simmer to reduce it, or use a flour-based roux, mashed potato, a little tomato paste, or a beurre manie of butter and flour whisked in.
