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Natural dyeing

Dye bath calculator

Turns a dry fibre weight into the mordant, dyestuff and water quantities a recipe assumes you will work out yourself — and produces a line to copy into your notebook, which is the part that makes a good result repeatable.

The question this answers: “The recipe says 15% alum and 50% madder. Fifteen per cent of what, and how much do I actually weigh out for this skein?”

Weighed dry and before scouring.

Of fibre weight. Usually 10–15% for wool and silk.

Enough that the fibre moves freely. Twenty is a comfortable default.

Result

Alum mordant
12 g
12% of 100g dry fibre.
Dyestuff — medium shade
50 g
50% weight of fibre. Dried dyestuff; fresh plant material needs several times this.
Bath volume
20 litres
Enough for the fibre to move freely. More is safer than less.
For your notebook
100g fibre · 12% alum · 50% dye · 20L
Copy this alongside a snipped sample. Six months later it is the difference between a craft and a series of accidents.

Worth knowing about this answer

  • Weigh the fibre dry and before scouring. Wet fibre holds several times its own weight in water, and scouring removes a little material — both of which move the figure that everything else is a percentage of.
  • These dye percentages are working ranges from the craft, not a standard. The same dyestuff varies with harvest, age and supplier, so treat the first bath as a test and write down what you actually got.
Show the working
  1. Mordant100 g × 12%12 g
  2. Dyestuff100 g × 50%50 g
  3. Water100 g ÷ 100 × 20 L20 L

Alum mordant: 12 g. Dyestuff — medium shade: 50 g. Bath volume: 20 litres. For your notebook: 100g fibre · 12% alum · 50% dye · 20L

How it works

Everything is a straight percentage of the dry fibre weight: mordant grams are fibre weight times the mordant percentage, dyestuff grams are fibre weight times the strength percentage, and bath volume scales linearly from the litres-per-100g ratio. The arithmetic is deliberately trivial — the value is that it is done against the right number, which is dry weight of fibre and not the weight of the finished item or the volume of the pot.

What it assumes

  • Fibre is weighed dry and before scouring. Wet fibre holds several times its own weight in water and scouring removes a little material, so weighing at any other point changes the figure every other quantity depends on.
  • Dyestuff percentages are working ranges from the craft rather than anybody’s published standard, and they assume dried material. Fresh plant matter needs several times the weight for the same result, and the multiple varies by plant.
  • Alum percentages in the 10–15% range assume protein fibres — wool, silk, alpaca. Cellulose fibres such as cotton and linen need a different and longer mordanting process that this does not model.
  • Water volume affects evenness rather than depth of colour. The dye exhausts onto the fibre rather than remaining in solution, so a larger bath dilutes nothing.

Where it stops being reliable

  • It cannot predict a colour, and nothing can. The result depends on the fibre, the mordant, the minerals in your water, the temperature the bath was held at and the particular batch of dyestuff — which is exactly why the notebook line is one of the outputs.
  • It models a single alum mordant bath. Tannin-and-alum sequences for cotton, iron and alkali modifiers, and indigo vats all work differently and none of them is this arithmetic.
  • It says nothing about safety. Mordants are chemicals with their own safety data sheets, and the handling guidance is on the natural dyeing page rather than in a calculator.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Natural dyeing.