Thickener Conversion and Starch Power, Compared Gram for Gram
Thickeners are not interchangeable by weight, which is why a recipe that says "one tablespoon of cornstarch" cannot simply be reread as "one tablespoon of flour". Each starch has its own thickening power per gram, and the differences are large: cornstarch, arrowroot and potato starch thicken roughly equally, tapioca starch about 70% as much, and plain all-purpose flour about half as much. The calculator above converts a quantity of one into another on that basis, and handles xanthan gum and gelatin separately, for reasons set out below.
The powers are relative figures for a water-based sauce, measured against cornstarch at 1.0. They are close enough to cook with and not precise enough to treat as exact, because the liquid, the acidity, the fat content and how long you simmer all shift the result. Use the conversion as a starting point, then judge the sauce by eye at the end — a starch-thickened sauce tells you what it needs.
Relative Thickening Power: The One Conversion Worth Memorising
If you remember one number from this page, remember this one: one tablespoon of cornstarch is worth about two tablespoons of plain flour. It follows directly from the power figures — cornstarch at 1.0 against flour at 0.5 — and it is the swap that comes up most often in a home kitchen, because most published recipes assume one or the other and your cupboard may hold only one of them. Every other conversion on this page is this same idea applied to a different starch.
| Thickener | Power per Gram | 1 tbsp Equals | Note |
|---|---|---|---|
| Cornstarch | 1.0 | The reference | The default; neutral and glossy |
| Arrowroot | 1.0 | 1 tbsp cornstarch | Clearest of the starches |
| Potato starch | 1.0 | 1 tbsp cornstarch | Most freeze-stable |
| Tapioca starch | 0.7 | about 0.7 tbsp cornstarch | Use roughly 1½ tbsp for 1 tbsp cornstarch |
| Plain all-purpose flour | 0.5 | 2 tbsp flour per 1 tbsp cornstarch | Needs fat and a longer cook |
How to Add a Starch: The Slurry
Starch cannot go into a hot sauce dry. Dropped into liquid it clumps instantly, the outside of each grain gelatinises and seals the dry starch inside, and you end up with lumps that no amount of whisking will break up. The fix is a slurry: whisk one part starch into two parts cold water until it is completely smooth, with no dry pockets at the bottom of the bowl, then stir that into the sauce. Off the heat, or at a bare simmer, is the safe moment to add it, and you stir while it goes in.
Then cook it. Cornstarch granules begin to gelatinise at around 95 °C, which is just below a simmer, and they need one to two minutes at that temperature to thicken fully and to lose the raw, chalky starch taste. Plain flour is a different job: it needs a longer simmer, and in practice it wants cooking out before it ever meets the liquid, which is what a roux does. A flour-thickened sauce that tastes of raw paste has not been simmered long enough.
Working Quantities for 250 ml of Liquid
For 250 ml, which is one cup, of liquid, the doses below are the practical starting points for cornstarch. Plain flour wants about double those quantities, which follows from the power table. These are starting points rather than fixed rules: a sauce that will be reduced further needs less starch, and one that will be served cold needs a little more, because a starch-thickened sauce sets firmer as it cools.
| Target | Cornstarch per 250 ml | Plain Flour per 250 ml |
|---|---|---|
| Thin sauce or gravy | 1 teaspoon | 2 teaspoons |
| Medium sauce or gravy | 1 tablespoon | 2 tablespoons |
| Pie filling | 2 tablespoons | 4 tablespoons |
A pie filling sits at the heavy end because it has to hold its shape once sliced and cooled, which is a different demand from a sauce that runs across a plate. Fruit fillings also bring their own liquid, so the figure is a starting point rather than a fixed dose.
Why Xanthan and Gelatin Are Not on the Conversion Scale
The starch powers work as a conversion because every starch thickens by the same mechanism: the granules absorb water, swell and gelatinise, and they do it at broadly comparable rates. Xanthan gum and gelatin thicken by entirely different means. Xanthan is a polysaccharide that builds a three-dimensional network in the liquid, and it does so in tiny quantities — roughly 0.1% to 0.5% of the liquid's weight, which for 250 ml is well under a gram. Gelatin sets into a gel through protein chains linking as it cools, and its dose depends on the bloom strength written on the packet or leaf rather than on any ratio against another thickener.
That is why the calculator gives them as a dose per volume of liquid and refuses to convert them. A direct ratio conversion would be wrong by a large factor, in the direction of making something inedible rather than merely under-thick. Xanthan is added while the liquid is moving, usually in a blender, because it hydrates instantly and clumps if it meets still water; it works cold, holds through freezing and thawing, and turns unmistakably slimy if you overshoot. Gelatin's defining property is that it is thermo-reversible: a gelatin-thickened sauce melts again when you reheat it. That makes it exactly right for a terrine, a panna cotta or a set mousse, and exactly wrong for a gravy.
What Each Thickener Does in the Pan
Choosing between them is less about how much they thicken than about how the finished sauce looks, how it behaves when it cools or freezes, and what it does under heat and acid. The table below is the deciding information.
| Thickener | Character | Freezing | Watch Out For |
|---|---|---|---|
| Cornstarch | Neutral, glossy, translucent | Goes cloudy, can separate | Breaks down under long high heat |
| Arrowroot | The clearest; glossy and translucent | Poor | Slimy with dairy; loses power with acid or over-stirring |
| Potato starch | Neutral, slightly heavier body | The most freeze-stable starch | Thickens at a lower temperature; loses power if boiled hard |
| Tapioca starch | Glossy and stretchy | Moderate | Turns stringy if over-whisked |
| Plain flour | Opaque, the traditional roux base | Moderate | Raw taste if not cooked out; needs fat, about equal weights of flour and butter |
| Xanthan gum | Neutral when used sparingly | Stable | Slimy in excess; a synthetic mouthfeel if overdone |
| Gelatin | Silky, sets to a gel | Depends on the set | Melts again on reheating |
Read the table as a set of trade-offs rather than a ranking. Cornstarch is the default because it is neutral and predictable, and it is the right answer for most pan sauces and gravies. Potato starch is the one to reach for when the dish is going into the freezer, which is why it suits a stew you are batch-cooking. Arrowroot gives the clearest result of all and is worth the trouble for a fruit glaze or a clear sauce, provided there is no cream in it and not much acid. Tapioca is the classic choice for a fruit pie, where its slight stretch suits the filling. Flour is what you use when you want a roux, an opaque sauce and a gravy that tastes of the roast.
Fixing a Sauce That Went Wrong
If a sauce is too thick, add liquid — stock, water, milk or wine — a little at a time and stir it through before judging, because starch takes a moment to disperse. The trap here is that a starch-thickened sauce continues to thicken as it cools, so a sauce that looks right in the pan can be glue on the plate. Judge it slightly looser than you want it at the table, and if it will be reheated, loosen it a little more still.
If a sauce will not thicken at all, there are three common causes and they are worth checking in order. The first is that the starch went into a boiling liquid without a slurry and clumped, so the thickening power is sitting in lumps at the bottom rather than in the sauce. The second is acidity: a strongly acidic sauce, or one that has had a lot of lemon juice or vinegar added after thickening, can thin a starch back out, and arrowroot is the most sensitive of all. The third is fat: starch needs to hydrate, and if it has been coated in fat — or added to a sauce that is mostly fat — it cannot absorb water and will not do its job.
Frequently Asked Questions
No. Plain flour has about half the thickening power of cornstarch per gram, so you need roughly two tablespoons of flour for every tablespoon of cornstarch. Going the other way, use about half the cornstarch a flour-based recipe calls for. Cornstarch also thickens clear and glossy where flour gives an opaque sauce, so the swap fixes the texture but not the appearance.
Because it met hot liquid while still dry. The outside of each grain gelatinises immediately and traps dry starch inside, and those lumps will not whisk out afterwards. Mix the starch with an equal or larger volume of cold water first — one part starch to two parts water — whisk until completely smooth, and add it off the heat or at a bare simmer while stirring.
No. Cornstarch granules begin to gelatinise at around 95 °C, so a cold mixture will stay thin however much you stir in. This is why a slurry is only a way of delivering the starch evenly, not a way of thickening at room temperature. If you need cold thickening — a cold dessert, a vinaigrette — use xanthan gum or gelatin instead, both of which work without heat.
It is clearer, not better. Arrowroot gives the most transparent, glossy result of any starch, which matters in a fruit glaze or a clear sauce. Against that, it turns slimy when it meets dairy, loses its thickening power in acidic mixtures and if the sauce is stirred too much, and it freezes badly. Cornstarch is more forgiving in every one of those situations, which is why it is the everyday default.
It usually works badly. Cornstarch sauces go cloudy and can separate into a spongy solid and a watery liquid after freezing and thawing. Potato starch is the most freeze-stable of the starches and is the better choice for anything destined for the freezer, which is why it suits stews. Xanthan gum is also freeze-and-thaw stable, but it brings its own texture with it.
Acid attacks the starch. A strongly acidic sauce, or one that has had a lot of lemon juice or vinegar stirred in after thickening, can break the gel back down and thin the sauce out. Arrowroot is the most sensitive, cornstarch less so but still affected. If a dish needs both acid and body, add the acid at the end and taste before adding more, or thicken with a starch that tolerates it better.
Roughly 0.1% to 0.5% of the liquid's weight, which for 250 ml is well under a gram, so you need a scale that reads fractions of a gram or a very light hand with a pinch. Sprinkle it into liquid that is already moving — a blender is the reliable way — because it hydrates on contact and clumps in still water. It works cold and survives freezing and thawing, but it turns slimy quickly if you overshoot, and it cannot be undone.
Because gelatin is thermo-reversible: it sets as it cools and melts again when it is warmed. That behaviour is exactly what you want in a terrine, a panna cotta or a set mousse, and exactly what you do not want in a gravy, which has to survive reheating. The dose also depends on the bloom strength printed on the leaf or powder, so a recipe written for one strength will set differently with another. For a sauce that must stay thick when hot, use a starch.