What Size Crochet Hook for Worsted Yarn?

The short answer: 5.5 to 6.5 mm

The Craft Yarn Council's Standard Yarn Weight System sorts yarn into eight weights, numbered 0 to 7. Worsted is in weight 4, called Medium, together with afghan and aran yarns. For that weight the CYC lists a crochet hook range of 5.5 to 6.5 mm, written in US sizes as I-9 to K-10 1/2.

Many yarn labels print the same weight number inside a small skein symbol, so a label with a 4 on it points you to the same range. If the label also gives its own recommended hook, use that as your first try, because the spinner chose it for that exact yarn.

• Yarn weight: 4, Medium (worsted, afghan, aran).
• Hook range: 5.5 to 6.5 mm (US I-9 to K-10 1/2).
• Gauge range: 11 to 14 single crochet over 4" (10 cm).

Hook sizes for the weights around worsted

It helps to see worsted next to its neighbours. DK and light worsted sit one step finer in weight 3, and chunky yarns sit one step heavier in weight 5. The table copies the CYC guideline ranges, so you can see where the hook ranges touch: 5.5 mm is the top of weight 3 and the bottom of weight 4, and 6.5 mm is the top of weight 4 and the bottom of weight 5.

US letters and millimetres side by side

US hooks use a letter, often with a number, while UK and most other patterns give the size in millimetres. The letters are not a measurement, and brands have not always matched them exactly, so the CYC advises relying on the millimetre size, which is an accurate measurement. The table lists the CYC conversions for the hooks you are most likely to use with worsted.

Notice the in-between sizes such as 5.25 mm (US I) and 5.75 mm (US J). If a pattern names one of those and you only own the standard sizes, pick the nearer millimetre size and check your gauge.

Let your gauge pick the exact hook

Gauge is how many stitches and rows fit in a set width, usually 4" (10 cm). The CYC guideline gauge for weight 4 is 11 to 14 single crochet (sc, called double crochet or dc in UK patterns) over 4". Your pattern will give one exact figure, and matching it matters more than staying inside the standard hook range.

Two people with the same hook and the same yarn can get different gauges, because tension differs from hand to hand. That is why the CYC tells crocheters to work a gauge swatch and compare it with the pattern before starting.

How to check gauge in five steps

A swatch takes a few minutes and saves a whole project. Work it in the stitch the pattern uses for its gauge, which is often single crochet for worsted patterns.

• Chain enough stitches for a swatch a little wider than 4", for example 20 chains for a pattern gauge of 14 sc.
• Work rows in the gauge stitch until the swatch is a little taller than 4".
• Lay it flat without stretching and measure 4" across the middle, away from the edges.
• Count the stitches inside those 4". Half stitches count as halves.
• Compare with the pattern and change hook if needed, then swatch again.

Worked example: going up one hook size

Say a worsted pattern asks for 12 sc = 4" with a 6 mm hook, and your swatch with that hook shows 14 sc in 4". Your stitches are smaller than the designer's, so the piece would come out too small. Try the next size up, 6.5 mm (US K-10 1/2), and swatch again.

If your swatch shows fewer stitches than the pattern, for example 10 sc instead of 12, the stitches are too big. Go down one size, to 5.5 mm (US I-9), and check again. The rule is short: more stitches than the pattern, bigger hook; fewer stitches, smaller hook.

What to check before you start

Read the yarn label for its weight number and suggested hook, read the pattern for its gauge and hook, and swatch with the pattern's hook first. Keep the millimetre size in your notes, not just the letter, so you can buy the same hook again. When you are ready, the crochet pattern generator on this site uses your yarn weight and gauge to work out stitch counts for hats, blankets and amigurumi, with every assumption shown.

Common questions

Can I use a 5 mm hook with worsted yarn?

You can if it gives the gauge your pattern asks for. A 5 mm (US H-8) hook is below the CYC weight 4 range of 5.5 to 6.5 mm, so it usually makes a firmer, denser fabric. Swatch first and compare.

Is aran yarn the same as worsted?

The Craft Yarn Council puts worsted, afghan and aran in the same weight, 4 Medium, with the same 5.5 to 6.5 mm hook range. Single yarns still vary in thickness, so read each label and check gauge when you swap one for another.

What does the number 4 on a yarn label mean?

It is the CYC yarn weight number. Weight 4 means Medium, the group that includes worsted, afghan and aran yarns.

Why do US hooks have letters and numbers?

US hooks are named with a letter, often followed by a number, such as I-9 or K-10 1/2. The CYC chart pairs each one with a millimetre size, and advises using the millimetre size because it is an accurate measurement.