Felted Product Size Control — Core Principle

The main reason for size inaccuracy in wet felting is failing to measure the actual shrinkage rate of the wool beforehand and reverse-calculating an enlarged pattern based on that rate.
There’s no universal formula, but there is a precise and controllable process, especially effective for different wool types.

1. Understand First: Why Does Size Always Go Wrong?

1. Wool type differences:
   Fine wool (like Merino) shrinks more (15%–30%), while coarse or blended wool shrinks less (5%–15%).

2. Felting intensity & water temperature:
   The more vigorously you rub and the greater the temperature contrast (hot rinse + cold press), the higher the shrinkage rate.

3. Felting degree:
   “Firm and sculpted” felt requires more rubbing and thus shrinks 5%–10% more than “soft and flexible” felt.
   Even different soaps and detergents can affect the shrinkage.

2. The Key Step: Test the “Custom Shrinkage Rate” (Must Do!)

Different wools — even different batches from the same brand — can shrink differently.
You must test a small sample first. It’s very simple:

1. Cut a square test swatch: e.g. 20 cm × 20 cm (mark size clearly, stitch edges to prevent fraying).

2. Felt it using your normal process: same wool layers, water temperature, rubbing force, and felting degree (e.g. felt until fibers lock and surface is elastic).

3. Measure after drying: if it becomes 16 cm × 16 cm, calculate:

   $$\text{Shrinkage Rate} = \frac{\text{Original Size} - \text{Finished Size}}{\text{Original Size}} \times 100\%$$

   Example: (20 – 16)/20 × 100% = 20% shrinkage.

3. Accurate Pattern Calculation — Reverse Enlargement Formula (Core Technique)

Once you know the shrinkage rate, you can reverse-calculate the template size from your target finished size:

$$\text{Pattern Size} = \frac{\text{Target Finished Size}}{1 - \text{Shrinkage Rate}}$$

Example:
You want a slipper sole 25 cm long, with 20% shrinkage:
25 ÷ (1 – 0.2) = 31.25 cm (apply the same logic for width).

4. Shrinkage Reference by Wool Type (Baseline — Still Needs Testing)

• Fine Merino wool (high shrinkage): 15%–30% (good for firm hats, slippers).

• Coarse or blended wool (low shrinkage): 5%–15% (good for soft bags, scarves).

• Roving or loose fibers: shrink 3%–5% more than carded batts (higher loft, more room to contract).

5. Practical Tips to Control Shrinkage (Reduce Error)

1. Match test to final conditions:
   Same wool layers, thickness, and rubbing style (e.g. if slippers use 3 layers, the test swatch should too).

2. Add a safety margin:
   After calculating pattern size, add 1–2 cm extra (to allow for over-felting; you can trim later).

3. Felt gradually:
   First lightly felt until fibers hold, measure once, and then increase pressure gradually (avoid over-shrinking).

4. Directional shrinkage control:
   Wool shrinks less along fiber direction (5%–8%) and more across/diagonal (10%–15%).
   Use this when drawing patterns — e.g. for slippers, align fibers lengthwise and crosswise strategically.

By first testing the shrinkage and then scaling the pattern, you can keep size errors within 1 cm — far more reliable than guessing by feel!