If you have ever stood in a yarn aisle holding a pattern that says "4 mm hook, worsted weight" and wondered whether the skein in your hand is actually worsted, this guide is for you. By the end, you will be able to read any hook label, match a yarn to the right hook, and know when substituting is safe and when it will wreck your gauge. Reading takes about ten minutes; the first swatch takes twenty.
What you need before you start
- One or two yarn skeins whose labels you can still read (the band is your best data source).
- A hook or two, ideally in sizes near what the yarn label suggests.
- A ruler or tape measure.
- A small notebook or your phone for jotting numbers.
- A pattern, if you have one, so you can see what it actually calls for.
Step 1: Learn the two numbering systems for hooks
Crochet hooks come in metric sizes (millimetres) and in older letter or number systems (US sizes like G, H, I, or UK numbers). Metric is the reliable one. A 5 mm hook is 5 mm everywhere; a "H" hook is 5 mm in the US but not a fixed size in every legacy chart you will find online.
Why it matters: patterns written in different decades and countries mix systems, and a hook that is one size off changes your fabric. Practical tip: when a pattern lists both, trust the metric number. If it lists only a letter, check a reputable conversion chart before you start.
Approximate time: five minutes to write your own conversion card for the sizes you own.
Step 2: Understand yarn weight, not yarn thickness
Yarn "weight" does not mean how heavy the skein is. It is a category, from 0 (lace) up to 7 (jumbo), defined by the Craft Yarn Council's standard system. Each category has a recommended hook range and a typical gauge.
The categories most people actually use:
- 1 Fine (sock/fingering): often around 2.25-3.5 mm hooks.
- 3 Light (DK/light worsted): often around 4-5 mm.
- 4 Medium (worsted/aran): often around 5-6.5 mm.
- 5 Bulky (chunky): often around 6.5-9 mm.
- 6 Super Bulky: often around 9-15 mm.
Why it matters: two yarns can look similar on the shelf and behave completely differently. A "worsted" from one brand can sit closer to a DK from another. Practical tip: treat the label's hook range as a starting point, never a rule. If you want a quick sanity check on the terms, the glossary of key terms is a good place to confirm definitions before you buy.
Step 3: Read the yarn label like a spec sheet
Most ball bands give you four useful things: the weight category number, a recommended hook size, a suggested gauge (for example, 16 stitches and 20 rows to 10 cm), and fibre content with care instructions.
Why it matters: the suggested gauge is the single most useful number on the band, because it tells you what the manufacturer expects your fabric to look like. Practical tip: photograph the band before you lose it. If you buy more of the same yarn months later, you can compare dye lots too.
Approximate cost note: labels are free, but ignoring them costs you a frogged project.
Step 4: Swatch, and measure honestly
Chain enough stitches to make a swatch at least 15 cm wide, work in the stitch your pattern uses, then measure the middle 10 cm. Do not measure the edges; they curl and lie.
Why it matters: your tension is personal. Two crocheters with the same yarn and hook can differ by a full stitch per inch, which over a sweater is several sizes. Practical tip: if your swatch has more stitches than the pattern's gauge, go up a hook size. Fewer stitches, go down. Re-swatch; do not guess.
Approximate time: 20-30 minutes per swatch, plus a wash and dry if the fibre needs it.
Step 5: Match hook to yarn for the fabric you want
The recommended hook range gives you a fabric that is neither stiff nor holey. Within that range, the hook you choose changes the result:
- Smaller hook: denser, stiffer fabric. Good for amigurumi, bags, and anything that needs to hold shape.
- Larger hook: drapey, open fabric. Good for shawls, summer tops, and lace.
Why it matters: the same yarn can make a sturdy toy or a floaty wrap depending entirely on hook size. Practical tip: if a pattern says "gauge is not critical" (scarves, blankets), you still want a fabric you like the feel of. Make a 10 cm square and squish it.
Step 6: Substitute yarn without wrecking the project
Substitution is fine when the new yarn is in the same weight category and you match the gauge, not the label. If you swap a worsted for a DK, expect to change hook size and possibly stitch count.
Why it matters: yarn gets discontinued, and colours sell out. Knowing how to substitute keeps you working instead of waiting. Practical tip: compare the metres per 100 g on both labels. A close match in length per weight usually means a close match in thickness. For projects where the numbers really matter (blankets, garments), running the figures through the free calculators saves you from buying the wrong quantity.
Step 7: Calculate how much yarn you actually need
You can estimate from a swatch: weigh your swatch, then scale it to the finished area. If your swatch is 10 cm by 10 cm and weighs 8 g, and your blanket is 100 cm by 120 cm, that is 120 squares of 100 cm², so roughly 960 g. Add 10-15% for joins, borders, and mistakes.
Why it matters: running out mid-project with a discontinued dye lot is genuinely painful. Practical tip: buy one extra skein and keep the receipt. Verify your own numbers against the pattern's estimate; neither is exact.
Pro tips
- Keep a "gauge card" for every yarn you use: yarn name, hook size, stitches per 10 cm. Future you will thank present you.
- If you crochet tightly, try a hook one size up before you blame the pattern.
- Metal, aluminium, and ergonomic hooks in the same size can feel different in your hand; the millimetres are the same, the comfort is not.
- Wash and block your swatch the way you will wash the finished item, especially with cotton or wool.
- When in doubt between two hook sizes, make two swatches. It is faster than frogging a sleeve.
- Check the ready-to-use checklists before a big project so nothing gets forgotten at the yarn shop.
Common pitfalls
- Trusting the letter on the hook. Legacy US and UK sizing does not line up perfectly. Always confirm the millimetre figure.
- Skipping the swatch. "It's only a scarf" becomes "why is my scarf 40 cm too long" surprisingly fast.
- Assuming all worsted is the same. It is not. Brand to brand, the difference can be a full hook size.
- Measuring the edge of the swatch. Curled edges make your gauge look wrong. Measure the middle.
- Buying exactly the estimated yarn quantity. Add a buffer. Dye lots change, and running short is worse than having a spare skein.
Useful resources
The Craft Yarn Council publishes the standard yarn weight system and hook conversion charts, and most major yarn brands repeat those charts on their own sites. Your yarn label and pattern are the two most reliable sources you already own. For everything else, this site's practical guides and articles cover the day-to-day questions, and the site search will find a specific term fast.
FAQ
Is yarn weight the same as yarn thickness? Roughly, but not exactly. Weight is a category, and two yarns in the same category can still differ slightly in thickness. Gauge is the real test.
Can I use a bigger hook than the label recommends? Yes, if you want a looser, drapier fabric. Just know your stitch count per 10 cm will drop, so recalculate for garments.
What if my gauge is off by one stitch? Over a large project, one stitch per 10 cm adds up fast. Change hook size and re-swatch rather than adjusting mid-project.
Do I need to swatch for amigurumi? Yes. Amigurumi depends on tight, consistent fabric, so a small swatch tells you whether your hook gives you the density you need.
How do I convert a UK pattern to US terms? The hook sizes and stitch names differ. Use a reputable conversion chart for both, and check the pattern's own notes first.
One last thing
Hook and yarn sizing feels fiddly for about one project, then it becomes second nature. Swatch once, write the numbers down, and you will stop guessing and start finishing. Grab your yarn, pick a hook, and make that first square today.