Knitting Needle Size Conversion: US, UK, and Metric

Three Systems, Zero Agreement

Knitting needles are sized in three incompatible systems used in different parts of the world:

• Metric (mm) – the universal measurement used in most countries. A 4 mm needle is exactly 4 mm in diameter.
• US sizing – a numerical system from 0 to 50. Larger numbers = larger needles. Not directly related to millimetres.
• UK/Canadian sizing – also numerical, but running in the opposite direction: larger numbers = smaller needles. UK size 14 is a very fine needle; UK size 000 is very large.

Older British patterns use UK sizes; American patterns use US sizes; modern patterns from most countries use metric. You need the conversion table.

Complete Needle Size Conversion Table

Metric (mm)	US Size	UK/Canadian Size
1.5 mm	000	–
1.75 mm	00	–
2.0 mm	0	14
2.25 mm	1	13
2.5 mm	–	12
2.75 mm	2	–
3.0 mm	–	11
3.25 mm	3	10
3.5 mm	4	–
3.75 mm	5	9
4.0 mm	6	8
4.5 mm	7	7
5.0 mm	8	6
5.5 mm	9	5
6.0 mm	10	4
6.5 mm	10.5	3
7.0 mm	–	2
7.5 mm	–	1
8.0 mm	11	0
9.0 mm	13	00
10.0 mm	15	000
12.0 mm	17	–
15.0 mm	19	–
19.0 mm	35	–
25.0 mm	50	–

Note: The metric sizes are the most reliable reference. US sizing is not perfectly standardized across manufacturers – some brands differ slightly at larger sizes.

Crochet Hook Sizes (Bonus Reference)

Metric	US Letter/Number
2.25 mm	B/1
2.75 mm	C/2
3.25 mm	D/3
3.5 mm	E/4
3.75 mm	F/5
4.0 mm	G/6
4.5 mm	7
5.0 mm	H/8
5.5 mm	I/9
6.0 mm	J/10
6.5 mm	K/10.5
8.0 mm	L/11
9.0 mm	M/N/13
10.0 mm	N/P/15

How to Identify an Unlabeled Needle

If you have needles without size markings (common with vintage sets), a needle gauge tool – a small metal or plastic card with precisely-sized holes – lets you identify the diameter by finding the largest hole the needle passes through cleanly. These cost a few dollars and belong in every knitter's kit.

Alternatively, a digital caliper (found in any hardware store) measures the diameter directly in millimetres with high precision.

When to Use a Different Size Than Specified

A pattern's needle size is a starting point, not a fixed rule. Your gauge (stitches per 10 cm) is the real target. If your gauge with the specified needle size doesn't match the pattern:

• Too many stitches per 10 cm (knitting too tightly): go up one needle size
• Too few stitches per 10 cm (knitting too loosely): go down one needle size

For a knitting project where fit matters, always swatch before starting and adjust needles until your gauge matches the pattern.

Quick Conversion Tool

The Knitting Needle Converter on this site converts any needle size between US, UK, and metric instantly.

Summary

Metric millimetres are the universal reference for knitting needle sizes. US sizes count up (bigger number = bigger needle); UK sizes count down (bigger number = smaller needle). Always convert to metric when in doubt – your ruler doesn't lie. When a pattern specifies needle size, treat it as a starting point and adjust to match the stated gauge.