How to Calculate Weight of Fibre (WOF) for Dyeing

You may have seen the abbreviation WOF used in textile dyeing and wondered what it means. Because textiles vary drastically in thickness, fibre type, and mass, dyers use a universal system called Weight of Fibre – WOF. WOF (Weight of Fibre) is the standard formula used to calculate dye quantities in textile dyeing.

Here’s how to calculate the Weight of Fibre (WOF) when dyeing fibres, yarns and fabrics with natural and synthetic dyes.

Whether you are working with natural dyes, reactive Procion MX powders, or vibrant Acid dyes, calculating your WOF helps you achieve predictable and repeatable colours. It works perfectly whether you are dyeing a delicate silk scarf, hanks of wool yarn for knitting, or loose unspun fibres like wool tops, fleece, and silk sliver for spinning and felting.

What is Weight of Fibre (WOF)

WOF is a percentage system that determines how much dye powder, extract, or mordant you need based entirely on the dry weight of the material you intend to dye.

For example, a recipe might call for 2% WOF of a specific dye powder. This means the weight of your dye powder should equal exactly 2% of your textile’s dry weight.

Step 1: Weigh Your Dry Fibre, Yarn or Fabric

Always weigh your material while it is completely dry – before scouring, mordanting, soaking, or wetting it down.

• Use a digital kitchen or studio scale.
• Record the weight in grams (g) for the most accurate results, i.e. the hank of yarn in the image above weighs 100g

Step 2: Check Your Recipe Percentage

Different dye types – whether natural extracts or synthetic formulas – require different percentages to achieve pale, medium, or deep shades. As a general guide:

• Procion MX Dyes (for Cotton, Linen, Plant Fibres): Typically require 0.5% WOF for pale pastel shades, 2% WOF for medium tones, and up to 4% to 6% WOF for deep blacks or dark navy.
• Acid Dyes (for Protein Fibres such as Wool and Silk, and for Nylon): Generally use 1% WOF for light shades,  2% WOF for medium depths, and 4% WOF for highly saturated, dark shades.
• Natural Dyeing: Concentrated natural dye extracts such as sticklac and cochineal are often used at 2–5% WOF, while the unprocessed sticklac and cochineal natural dyes will need a higher ratio. Some natural dyes need only a low ratio to WOF to give a full deep colour while others will need up to 100% WOF. This is where experimenting and note taking is essential – results from dyeing with different natural dyes will vary considerably.
• Aluminium Lactate (Mordant for fixing Natural Dyes on Cotton, Linen, Plant Fibres): Typically used at 7% to 10% WOF for optimal fixation on plant fibres.

Step 3: Run the Calculation

The maths is incredibly simple. Take your dry material weight, multiply it by the recommended WOF percentage, and divide by 100.

(Dry Material Weight in Grams × WOF Percentage) ÷ 100 = Weight of Dye Needed

Let’s look at two practical examples using our 100g total project weight:

• Example A: Dyeing unspun wool fibre with Acid Dyes for a vibrant, medium shade (at 2% WOF)
  • (100g wool tops × 2) ÷ 100 = 2g of Acid Dye powder.
• Example B: Mordanting cotton with Aluminium Lactate (at 8% WOF)
  • (100g material × 8) ÷ 100 = 8g of Aluminium Lactate.

Quick Adjustments for Creative Control

Once you understand the baseline formula, you can easily tweak the numbers to change your visual results:

• For Pastel Shades: Drop your synthetic or natural dye percentages down significantly (e.g., using just 0.25% to 0.5% WOF) to keep your loose fibres or yarns soft and delicate.
• For Deep, Saturated Shades: Increase the percentage to the maximum recommended limit for that specific dye range, keeping in mind that fibres eventually reach a point where increasing the dye concentration produces little additional depth of shade.

What About the Water?

You might notice that the WOF formula doesn’t mention water volume. That is because water doesn’t change the depth of your colour – it simply acts as the vehicle to carry the dye molecules to your fibre.

As a general rule of thumb, a useful starting point for many dye baths is around a 20:1 to 30:1 liquor ratio. This means you want 20ml (approximately 20g) or 30ml (approximately 30g) of water for every 1g of dry fibre.

How to adjust your water volume:

• The Crucial Rule: Your fibre, yarn, or fabric must be able to swim freely in the pot. If it is crammed in tightly, the dye cannot circulate, resulting in patchy, uneven, or accidental tie-dye effects.
• For Smooth, Level Colours (Procion MX & Acid Dyes): Stick to the 30:1 rule or add slightly more water. More water will not dilute the final colour, but it *will* ensure a beautifully even result.
• For Texture and Shading (Low-Immersion Dyeing): If you intentionally want a variegated, multi-tonal effect on your spinning fibres or yarn hanks, reduce the liquor ratio so the material is only partially submerged and movement is restricted, encouraging variegated effects.
• Natural dyes are generally more sensitive to liquor ratio than synthetic dyes, and excessively large volumes of water may produce paler shades. However, there is no fixed maximum ratio, and additional water is often required when extracting colour from raw materials such as bark, roots or leaves.

*Note: Remember to follow the dyeing instructions for these dyes:

• Procion MX dyes require soda ash and Acid dyes require an acidic environment, typically provided by citric acid or white vinegar, to bond with the fibre.
• Natural Dyes: Protein fibres such as wool and silk are commonly mordanted with alum, while cellulose fibres such as cotton and linen are often treated with aluminium lactate before dyeing with natural dyes.

Further FAQs

• Do you weigh fibre wet or dry?
  Always calculate WOF from the dry weight.
• Can you mix fibres together when calculating WOF?
  Yes, although different fibres may absorb the same dye at different rates and depths of shade.
• Does adding more water make the colour paler?
  No. Provided the fibre can move freely, water mainly affects levelness rather than colour depth.
• What does 2% WOF mean?
  It means using 2g of dye for every 100g of dry fibre.
• Can I use the Weight of Fibre (WOF) formula when dyeing fleece, wool tops or spinning fibres?
  Yes. The Weight of Fibre (WOF) calculation is the same for loose fibres, fleece, tops and sliver as it is for yarn and fabric. Simply weigh the material dry and apply the recommended percentage.

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George Weil supply a comprehensive range of dyes and dye assistants for wool, silk, cotton, linen and synthetic fibres. Browse the Dyes & Dye Assistants section to find the most suitable products for your next project.