How much roux do I need for the sauce thickness I want?

Almost every white sauce recipe gives one ratio and calls it 'a white sauce', which is why people end up with cauliflower cheese that runs across the plate and lasagne that is soup. There are four useful consistencies and they are a long way apart. Knowing which one a dish needs is more useful than knowing any single recipe.

What you'll need

• Kitchen scales
• Butter
• Plain flour
• Heavy-based pan and a whisk

Step by step

1. Work in weights, not spoons.Equal weights of butter and plain flour is the ratio. A tablespoon of butter and a tablespoon of flour are not the same weight, which is where a lot of unreliable sauces come from. Scales take ten seconds.
2. Pick the consistency from what the sauce has to do.Per 500ml of liquid: 20g+20g for a cream soup or for loosening something already wet; 25g+25g for a pouring sauce over fish or vegetables; 40g+40g for cauliflower cheese, macaroni cheese or lasagne; 60g+60g to bind a croquette or a fish-pie filling.
3. Melt the butter and stir in the flour, then cook it.At least two minutes over a medium heat for a white roux, stirring, until it smells biscuity rather than of raw flour. This is the step that decides whether the finished sauce tastes floury.
4. Add the liquid gradually, whisking each addition smooth.The first splash makes a thick paste — whisk it completely smooth before adding more. Adding all the liquid at once is the usual cause of lumps. Warm liquid makes it easier but cold liquid works if you go gradually.
5. Bring it to a simmer and hold it for two or three minutes.The sauce thickens as the starch granules swell, which happens between about 60 and 80°C. It will not reach its final thickness until it has simmered, so do not judge it before then and do not add more roux early.
6. Judge the consistency at serving temperature, not at the boil.Every starch-thickened sauce thickens considerably as it cools. A sauce that looks right at a rolling simmer will be stiff on the plate. Aim slightly thinner than the finished result you want.
7. Adjust with liquid rather than more roux.Too thick is easy — whisk in more warm liquid. Too thin needs either more simmering, or a beurre manié whisked in at the end, which is raw butter and flour kneaded together.

Tips

• A darker roux thickens less. Browning breaks down some of the starch, so a nut-brown roux for a gravy needs roughly a quarter to a third more than a white one for the same body.
• The same ratios work with any liquid — milk, stock, the cooking liquid from a poached fish, or a mixture.
• Cheese does not thicken a sauce much, but it does add fat and salt. Make the béchamel slightly thinner than you want if a lot of cheese is going in.
• A roux can be made in bulk and refrigerated or frozen in a block. Cut off a piece and whisk it into hot liquid.
• For a lasagne, err towards the coating consistency. The pasta absorbs liquid as it bakes and a thin béchamel disappears.

Common mistakes

• Measuring butter and flour by volume — They have very different densities. Equal spoonfuls give you far more flour than fat by weight, and the sauce tastes pasty.
• Judging the thickness before it has simmered — The starch has not finished swelling. Add more roux at that point and the sauce sets like wallpaper paste ten minutes later.
• Adding all the milk in one go — The roux disperses into the liquid in clumps that seal on the outside and stay dry inside. That is what a lump is.

Questions people ask