Select your current hair colour, the shade you want to reach, and the lightening (bleach) level you plan to use to get an estimated processing time range and an associated risk level.
Before you start: patch test and professional advice
Home hair colouring carries a real risk of allergic reaction and hair damage. Run a skin allergy patch test at least 48 hours before applying any dye or bleach, following the instructions on the product packaging. For a major colour change, such as going from dark brown or black to blond, a professional colourist is strongly recommended rather than a single at-home session.
This is a general, indicative estimate built from timing ranges widely used in home hair colouring guidance, not a professional colour formulation or protocol. Never exceed the maximum processing time stated on the product packaging, even if this estimate suggests otherwise, and stop immediately and rinse if you feel burning, itching, or unusual discomfort.
How this processing time estimate is built
The estimate combines two things: how far your target shade sits from your current colour on the standard light-to-dark hair colour scale, and the lightening level you plan to use. A same-level or darker colour is a simple deposit of pigment, typically processed in 20 to 30 minutes. Lightening several levels at once needs a stronger bleach and a longer processing time, up to a widely used ceiling of around 45 minutes for over-the-counter bleach products, past which the risk of scalp irritation and hair breakage rises sharply without a proportional gain in lift.
Allergen labelling on hair dye in the UK
UK cosmetics regulation, carried over from the EU framework after Brexit and enforced by the Office for Product Safety and Standards, requires oxidative hair dyes to list known contact allergens such as para-phenylenediamine (PPD) and its relatives, and to carry a warning that the product must not be used to dye eyelashes or eyebrows. Several individual dye substances have been restricted or banned over the years following safety reviews, which is why checking the ingredient list against your own known sensitivities, and patch testing every time, matters even with a product used safely before.
Semi-permanent, permanent, and bleach: what actually differs
Semi-permanent colour sits on the hair shaft without opening the cuticle, fades over several washes, and cannot lighten hair at all. Permanent colour uses ammonia or an alternative alkaline agent to open the cuticle and deposit pigment with peroxide, which allows it to lighten hair by a level or two while colouring. Bleach uses a stronger peroxide concentration with an alkaline booster to strip the natural pigment from the hair shaft itself, which is the only way to achieve a large jump toward blond, but it is also the step that causes the most structural damage to the hair.
What this tool cannot tell you
It cannot account for your hair's previous chemical history, its natural porosity, or the exact strength of the specific product you are using, all of which change real processing time. It does not replace a strand test, the instructions printed on your product, or a consultation with a professional colourist, particularly for a first-time large colour change, chemically treated hair, or a known history of allergic reaction.