Instantly convert knitting needle and crochet hook sizes between US, UK, metric, and Japanese systems.
Crochet hook sizes vary by country. US sizes use letters and numbers, while metric sizes use millimeters. Use this converter to find the equivalent hook size for any international standard. It also covers all knitting needle sizes across US, UK, metric, and Japanese systems.
Why You Need a Knitting Needle Size Converter
You find a gorgeous Japanese pattern that calls for 8号 needles, or a vintage British pattern listing No. 6 — what needle do you actually grab from your case? Needle sizing systems vary dramatically by country, and using the wrong size can throw off your entire gauge and finished dimensions.
With patterns now shared globally through Ravelry, YouTube, and social media, crafters regularly encounter unfamiliar sizing systems. A reliable converter eliminates confusion and ensures you start every project with exactly the right tool in hand, no matter where the pattern originated.
What Are Knitting Needle Sizing Systems?
Knitting needle sizes refer to the diameter of the needle shaft, which directly controls stitch size and fabric gauge. The metric system measures this diameter in millimeters and serves as the universal reference point. All other systems are country-specific naming conventions mapped to these millimeter values.
The US system uses numbers that generally increase with size, the Japanese system uses a similar ascending numbered scale, and the old UK system uses numbers that run in reverse — a UK 14 is a tiny 2.0mm needle, while a US 14 is a hefty 10.0mm needle. This reversal catches many knitters off guard.
Modern patterns increasingly list metric sizes alongside regional numbers, but older and vintage patterns often use only the local system. Understanding these mappings is essential for anyone working from international or historical pattern sources.
How Needle Size Conversion Works
Needle conversion uses standardized lookup tables maintained by needle manufacturers and craft organizations. Each system maps its numbered or named sizes to specific millimeter diameters. For example, US 8 equals 5.0mm, which equals UK 6, which equals Japanese 棒針 8号.
The critical detail to understand is that UK sizing runs backward compared to US and metric. UK 14 is 2.0mm while US 14 is 10.0mm — the exact opposite ends of the size spectrum. This reversal has caused countless gauge disasters for knitters working from British patterns with American needles.
Some sizes do not have exact equivalents across all systems. For instance, US 11 is 8.0mm, but the nearest UK size jumps from 7.5mm to 8.0mm without a standard number. The converter flags these gaps so you can choose the closest available option.
What is this?
A size converter for knitting needles and crochet hooks that translates between US, metric (mm), UK, and Japanese sizing systems instantly.
Who needs it?
Knitters and crocheters working from international patterns, or anyone whose pattern calls for a needle or hook size in an unfamiliar system.
Bottom line
Select or type any needle or hook size and see the equivalent in every major sizing system — no more guessing whether a 4.0 mm hook is a US G/6.
Needle and Hook Size Converter
How to Convert Needle Sizes
Needle Size Conversion Results
Type any size — metric, US number, UK, or Japanese — to find all equivalents. Try "8" or "5mm".
Metric (mm)
US
UK (old)
Japanese
Yarn Weight
Size
2 mm
US 0
14
JP 0
Lace
2.25 mm
US 1
13
–
Lace / Fingering
2.5 mm
US 1.5
–
JP 1
Fingering
2.75 mm
US 2
12
JP 2
Fingering
3 mm
US 2.5
11
JP 3
Fingering / Sport
3.25 mm
US 3
10
JP 4
Sport / DK
3.5 mm
US 4
–
JP 5
DK
3.75 mm
US 5
9
–
DK
4 mm
US 6
8
JP 6
DK / Worsted
4.5 mm
US 7
7
JP 7
Worsted
5 mm
US 8
6
JP 8
Worsted / Aran
5.5 mm
US 9
5
JP 9
Aran
6 mm
US 10
4
JP 10
Aran / Bulky
6.5 mm
US 10.5
3
JP 11
Bulky
7 mm
–
2
JP 12
Bulky
7.5 mm
–
1
JP 13
Bulky
8 mm
US 11
0
JP 14
Bulky / Super Bulky
9 mm
US 13
00
JP 15
Super Bulky
10 mm
US 15
000
–
Super Bulky
12 mm
US 17
–
–
Jumbo
15 mm
US 19
–
–
Jumbo
19 mm
US 35
–
–
Jumbo
25 mm
US 50
–
–
Jumbo
💡 Quick Reference
Metric (mm) is the universal standard — when in doubt, go by mm.
UK old sizes run backwards — UK 14 is tiny (2mm), UK 000 is huge (10mm).
Japanese sizes start at 0 (2mm) and go up. Common in Japanese knitting books.
Crochet hooks use both letters and numbers in the US — patterns may use either.
How to Use the Needle & Hook Size Converter
Enter a needle or hook size in any system — US numbered, UK old-system numbered, or metric millimeters — and the converter returns the equivalent in all three systems instantly. US sizes run from 0 to 50 for knitting needles. UK sizes run in the opposite direction, with smaller numbers for larger needles. Metric sizes are measured in millimeters and range from 2.0mm through 25mm for standard needles.
The converter also handles crochet hook sizes, including lettered US hooks (B through S) and their metric equivalents. Select the tool type — knitting needle or crochet hook — to see the correct conversion table for your needs.
Understanding Your Results
Metric millimeter sizes are the universal standard across manufacturers worldwide. When a US size and metric size appear to conflict, trust the millimeter measurement. Some manufacturers round differently — a US 8 needle is technically 5.0mm, but you may encounter needles labeled US 8 that measure 5.1mm or 4.9mm with calipers.
Vintage UK needles follow an older sizing system that was officially replaced by metric in the 1970s. Patterns from before that era may reference sizes that do not map cleanly to modern equivalents. If you are working from a vintage pattern and your gauge is off, check the actual millimeter diameter of your needle against the converter output.
Pro Tips
From 30+ years of fiber arts experience
✓Always verify your needle size with a physical needle gauge tool. Manufacturer tolerances vary, and the printed size on a needle is not always accurate to the nearest 0.25mm.
✓Japanese needle and hook sizes use their own numbering system that differs from US sizes even when the numbers look similar. A Japanese size 8 is not the same as a US size 8.
✓Crochet hook letter designations are not standardized across all brands. A Boye H hook and a Clover H hook may differ slightly in diameter. Check the millimeter size printed on the hook itself.
✓Interchangeable needle sets often skip half-sizes. If a pattern calls for a 4.5mm and your set jumps from 4.0mm to 5.0mm, you will need to buy that size separately.
How to Read This Chart
This converter maps knitting needle sizes across four major systems: US numbered sizes, UK old-system numbered sizes (which run in reverse of US), metric millimeter measurements, and Japanese numbered sizes. Each row shows how one size appears across all four systems — for example, US 8 equals 5.0mm, UK 6, and Japanese size 8. Metric millimeters are the universal standard, so use those as your reference point when systems conflict. US sizes run from 0 to 50; UK sizes run in the opposite direction (larger numbers = smaller needles); metric sizes go from 2.0mm to 25mm; Japanese sizes use a distinct numbering system that doesn't directly correlate with US numbers despite overlapping values. For crochet hooks, separate tables cover US letter designations (B through S), metric, and UK sizes.
Industry Standards
Knitting needle sizing standards evolved from multiple regional systems that developed independently. ISO 4035 serves as the international standard, measuring in millimeters. The US system runs from 0 (smallest) to 50 (largest). The UK old-system, officially obsolete since metric adoption in the 1970s, runs in reverse — UK 14 is 2.0mm while US 14 is 10.0mm, the exact opposite ends of the spectrum. Japanese needles use their own numbered system that originated from US sizing but diverges enough to require conversion. ISO standardization in the 1970s–80s moved most manufacturers toward metric labeling, though US, UK, and Japanese systems persist in vintage needles and region-specific patterns.
Real-World Variations
In practice, needle tolerance varies between manufacturers — a needle labeled 5.0mm may measure 4.9mm or 5.1mm depending on quality control. European manufacturers typically maintain tighter tolerances than budget-brand Asian manufacturers. Vintage UK needles can be troublesome because old UK system numbers don't map perfectly to modern metric standards. Some needle materials (bamboo, wood) vary fractionally in diameter with humidity and temperature. Japanese needles are often labeled with both Japanese and metric sizes but occasionally contain errors in the metric conversion. Always verify sizing with a physical needle gauge tool before starting any critical project.
Explore Related Fiber Arts Tools
Yarn Weight Chart — See recommended needle and hook sizes for each yarn weight
Gauge Calculator — Check your gauge after selecting your needle or hook size
Check your yarn label for the recommended needle size. Our converter shows the equivalent size across US, metric (mm), UK, and Japanese systems so you can match any pattern regardless of origin.
How do I convert US needle sizes to mm?
US sizes don’t follow a linear scale. For example, US 6 = 4.0mm, US 7 = 4.5mm, US 8 = 5.0mm. Our converter handles all 23 sizes from US 0 (2.0mm) to US 50 (25.0mm).
What size crochet hook do I need?
It depends on your yarn weight. Lace yarn uses 1.5–2.25mm hooks, worsted uses 5.0–6.0mm hooks, and bulky uses 6.5–9.0mm hooks. Our chart shows recommendations for every weight.
Are UK and US needle sizes the same?
No. UK sizes run backwards — a UK 14 is 2.0mm (tiny) while a US 14 is 10.0mm (large). Our converter prevents this common confusion by showing all systems side by side.
What’s the difference between US and metric crochet hooks?
US crochet hooks use letter sizes (B through S) while metric uses millimeters. For example, US H/8 = 5.0mm. Our converter maps all 24 common hook sizes across both systems.
Can I substitute a different needle size?
Yes, but it will change your gauge. Going up a size creates a looser, drapier fabric. Going down creates a tighter, firmer fabric. Always swatch first when substituting.
What size knitting needle do I need for worsted weight?
Worsted weight yarn (CYC category 4) typically uses US 7–9 needles (4.5–5.5mm). The exact size depends on the pattern and your personal tension. Check your pattern for the recommended size, then swatch to confirm your gauge matches.
How do I convert US needle sizes to metric?
US needle sizes don’t follow a linear scale. Common conversions: US 4 = 3.5mm, US 6 = 4.0mm, US 7 = 4.5mm, US 8 = 5.0mm, US 10 = 6.0mm, US 15 = 10.0mm. Use our converter for the complete chart of all 23 sizes.
What needle size is best for beginners?
US 8 (5.0mm) needles with worsted weight yarn are the most beginner-friendly combination. The stitches are large enough to see clearly and count easily, and worsted is the most widely available yarn weight with the most pattern options.
🎙️
Ready to start your project?
You've done the planning — now keep track while you craft. MyCrochetKit is a free voice-activated row counter that lets you say "next" to count rows hands-free. Track multiple projects, save your progress, and never lose count again.