Developer volumes and how much they lift

Four numbers carry a disproportionate share of the coloring domain, and they are worth knowing cold rather than reasoning out under time pressure. Everything else on this page follows from what the volume number actually measures.

10 volume	Deposit only, no lift — demi-permanent color and toning
20 volume	One to two levels — the standard for permanent color and gray coverage
30 volume	Two to three levels
40 volume	Three to four levels
The oxidizer	Hydrogen peroxide — the volume number is its concentration
Ammonia's job	Alkalizing: raises pH and swells the cuticle. It is not the oxidizer
Equal parts 20 and 40	30 volume — mixing averages, it does not add
The bowl	Non-metallic; metal reacts with the mixture and changes the result
Storing peroxide	Cool, dark, capped, in its original container

Where the point is lost: Two lightener facts sit alongside these and get tested just as often. Powder lightener is an off-scalp product — it is the strongest form and it dries as it works, which is why it belongs in foils. And a lightener that has dried on the hair has stopped lifting altogether; leaving it on longer achieves nothing, so drying out is a technique failure, not a sign of progress.