About Satin Fern
Small, free calculators for colouring your hair at home or at the colour bar, and for growing it out.
What it is
Satin Fern answers the questions that come up around a bowl of hair colour and a tape measure: how much developer goes with this tube, what 30 volume means, how long each step has been processing, when hair will reach the length you want, and what “bra-strap length” actually measures. Each tool shows its working and names the sources behind its numbers, such as the FDA, the NHS, DermNet, the EU cosmetics regulation, the makers' own mixing guides and published measurements of hair growth and body proportions.
How it works
Everything runs in your browser. The formulas are part of the page, so nothing you type is sent anywhere, and once a page has loaded it keeps working without a connection. The dye timers are kept in your browser's local storage on your device and nowhere else. There are no accounts, cookies or ads.
What it is not
It is not medical advice, and it does not replace the instructions that come with your colour. Brands differ in their ratios, strengths and timings, and the leaflet in the box always wins. Hair colour can cause serious allergic reactions: do the skin allergy alert test the maker describes before every colouring, and see a pharmacist or doctor about any reaction. Where the evidence only supports a range, such as hair growth or how much colour long hair needs, the tools show a range or label the number as an estimate.
Check our working
Every tool page ends with its sources, linked where they are online, so you can check each figure against the original.