Developer comes in different strengths, the common ones being 10 volume, 20 volume, 30 volume and 40 volume. It’s possible to buy peroxide in stronger concentrations but they’re much too strong to use on hair and should be avoided. Developer strength is sometimes represented as a percentage. 6% and 20 vol. are the same strength.

What strength/volume developer should I use?

10 volume is ideal for use when you don’t want the natural hair colour to be lightened by the dyeing process. It’s used in toners and colour glazes. These types of dyes are known as “deposit-only”.

20 volume peroxide is commonly used with semi-permanent and permanent hair colours. It can have a lightening effect on natural, undyed hair of 1 or 2 shades when used with permanent colour.
When mixed with bleach powder, 20 volume developer will lighten virgin hair by about 5 levels.

30 volume developer is used for lifting out colour. Its main use is in bleaching and it can lighten around 7 levels depending on the bleach powder used.

40 volume is occasionally used to lighten very dark, stubborn hair as part of the bleaching process. It lightens quickly and its effects are harsher than lower concentrations. 40 volume is not recommended for on-the-scalp use. Using 40 volume developer can cause hair damage and chemical burns if not used correctly. Instead, using multiple applications using lower volume peroxide will allow more controlled lightening.