Blocking is a finishing process used to shape crochet fabric. For planning, the important result is the size of the fully dry piece after the treatment you intend to use. A neat sample is helpful, but a measured sample tells you whether the dimensions changed.
Choose the treatment from the material
Read the yarn's care instructions and any finishing directions in the pattern before selecting a method. Wet, spray and steam treatments are not interchangeable for every fibre. Heat can permanently alter some materials; do not experiment with an iron on your finished project. Ask the yarn manufacturer about an unclear label and test an offcut or spare sample first.
For a sample that can be washed, use the permitted washing method, remove excess water gently as directed and arrange it on a suitable clean surface to dry. Shape it without forcing it to meet a hoped-for measurement. Measure only once it is fully dry and released from any supports.
Keep a before-and-after measurement pair
Mark the orientation and measure the same part of the fabric both times. Record width and length separately. A table with “before”, “after” and “method” is enough. Also note the hook, yarn and stitch so you can reproduce the sample if something unexpected happens.
Suppose the whole sample measures 12 × 12 cm before care and 12.6 × 12.3 cm afterwards. Width increased by 5%, while length increased by 2.5%. The new area is 154.98 cm², up from 144 cm²: about 7.6% larger. These numbers illustrate why one percentage cannot always describe both directions.
For a weight-based estimate, pair the fully dry sample's mass with that same complete sample's finished area. Do not use the unfinished dimensions or the area of a smaller central counting window. The yarn calculator explains this distinction beside its inputs.
Compare motifs before assembly
If a pattern calls for finishing separate motifs before joining, use that sequence on your sample pair. Measure several finished motifs rather than selecting only the neatest one. If their sizes vary substantially, investigate the stitch counts and tension before trying to pull them all to a single target.
Join two representative motifs with the intended method and measure the pair. A planning diagram cannot predict how tightly you will seam the pieces. Use the observed joining contribution in the blanket planner, and record whether your square size was measured before or after finishing.
Do not use finishing to hide a counting error
A missing stitch or an incorrect repeat needs a construction check. Blocking may change the appearance of an edge, but it cannot add the omitted stitch. If the shape returns when released or after the next wash, the forced measurement was not a reliable basis for the project.
Reference for finishing methods and care precautions: Lion Brand: blocking. The measurement example is illustrative, not a physical yarn test.