Crochet Hook Size Converter - Free Online Tool

Convert between Metric, US, and UK hook sizing systems instantly

Crochet hook sizes vary depending on which country your pattern comes from and when it was written. A UK vintage pattern from the 1970s uses a completely different numbering system from a modern American pattern, and both differ from the metric millimeter standard endorsed by the Craft Yarn Council. If you have ever stood in front of your hook collection wondering whether your 4.0 mm hook is the same as a G-6, or tried to decode a UK size 8, this converter is built exactly for you. The three most commonly encountered hook sizing systems are: Metric (millimeters), which measures the actual diameter of the hook shaft and is the universal standard recommended by all major craft authorities; the US letter-number system, where most hooks carry both a letter and a number designation (like H-8 or K-10½) based on conventions that evolved from the early twentieth century; and the UK/Canadian old-number system, a legacy scale that runs roughly inverse to size — a UK size 14 is a very small 2.0 mm hook, while a UK size 0 corresponds to a large 8.0 mm hook. Many UK and Canadian books published before the 1980s use this old numbering, so it remains relevant for anyone working from vintage or inherited patterns. Understanding which sizing system your pattern uses is the first step to successful crochet. Hook size directly affects gauge — the number of stitches and rows per inch — which in turn determines whether your finished project will match the dimensions stated in the pattern. Using a hook even half a millimeter too small or too large can cause a garment to come out significantly smaller or larger than intended. For decorative items like blankets and dishcloths the difference may be acceptable, but for fitted garments, hats, or toys with specific sizing requirements, using the correct hook is essential. Beyond the three main systems, crochet hooks come in two distinct categories that use entirely separate size ranges. Standard aluminum, bamboo, and plastic hooks cover the range from about 2.0 mm to 30 mm or beyond, and their US size numbers increase as the hook gets larger. Steel crochet hooks, used for thread crochet and fine lace work, operate on a reverse numbering system — a steel #14 is the smallest common size at approximately 0.75 mm, while a steel #00 is the largest steel hook at 3.5 mm. This reversal trips up many beginners, so our converter clearly separates standard and steel hook types with an explicit note about the inverse logic. Each hook size is also associated with a recommended yarn weight category from the Craft Yarn Council's standardized scale, which runs from #0 (Lace) through #7 (Jumbo). For example, a 5.0 mm hook (H-8 in the US system) is recommended for Light (#3) weight yarns, while a 6.0 mm hook (J-10) pairs well with Medium (#4) worsted weight yarn. These recommendations are guidelines — some crocheters intentionally use larger hooks for a looser drape, or smaller hooks for denser fabric — but they provide an excellent starting point, especially for beginners. Our converter displays the recommended yarn weight for every hook size in the result. For crafters who work across multiple fiber arts, we also include a knitting needle cross-reference. Because knitting needles and crochet hooks are both measured by shaft diameter in millimeters, a 4.0 mm crochet hook and a US size 6 knitting needle have exactly the same diameter. This cross-reference is helpful when estimating gauge from knitting patterns, or when shopping for tools at a store that only labels by one system. One important caveat: actual hook diameters can vary slightly between manufacturers even for hooks bearing the same nominal size label. The Craft Yarn Council explicitly recommends relying on the millimeter measurement rather than the letter or number designation because manufacturing tolerances differ across brands. If a hook's markings have worn off — common with heavily used aluminum hooks — a needle gauge tool with labeled holes is the most reliable way to identify the true size. Our converter includes this manufacturer variance disclaimer alongside every result to help users understand when to measure rather than assume.

Understanding Crochet Hook Sizing

What Are the Three Hook Size Systems?

Three sizing systems coexist in crochet today. The Metric system measures the hook shaft diameter in millimeters and is the internationally recognized standard — when a pattern specifies both a letter and a millimeter size, the mm is always more reliable. The US system assigns letter-number combinations (B-1, G-6, K-10½, etc.) that developed historically and are still printed on most hooks sold in North America. The UK/Canadian old-number system, used in vintage British and Canadian publications, runs on a scale where lower numbers indicate larger hooks — the opposite of the US system. Modern UK patterns have adopted metric sizing, but the old system still appears in reprinted vintage patterns. For thread and lace work, a separate category of steel hooks uses US numbers from #00 (largest, 3.50 mm) down to #14 (smallest, 0.75 mm), in an inverse order compared to standard hooks.

How Does the Lookup Work?

Unlike formula-based calculators, hook size conversion is a table lookup. Each hook size has a fixed, standardized mapping between systems, based on the Craft Yarn Council reference table and cross-validated against multiple industry sources including Vogue Knitting and Purple Kitty Yarns. You select the system you know (mm, US letter-number, or UK number) and the converter retrieves the corresponding entries in the other two systems. If you enter a metric value that falls between standard sizes — for example, 2.6 mm, which has no standard designation — the converter returns the nearest size above and below with a note explaining that no standard hook exists at that exact diameter. This nearest-match logic is important because hand-measuring an unmarked hook with a gauge tool will sometimes yield a value that sits between standard sizes.

Why Does Hook Size Matter for Gauge?

Gauge — the number of stitches and rows per unit of measurement — is the foundation of accurate crochet. It is determined by the combination of hook size, yarn weight, yarn fiber content, and the individual crocheter's tension. When a pattern states a gauge such as '14 sc x 16 rows = 4 inches using H-8 / 5 mm hook and worsted weight yarn,' it means the designer achieved those dimensions using exactly that hook and yarn combination at their personal tension. A crocheter who works tight may need a larger hook to match gauge; one who works loosely may need a smaller hook. Correct gauge ensures that a size Medium sweater will actually fit a Medium, that a baby blanket will cover the right area, and that amigurumi pieces will assemble correctly. Hook size conversion matters not just for finding the right hook in your collection, but for understanding whether the stated US size in a pattern is the right size for you personally.

Limitations and Manufacturer Variance

The size tables used in this converter reflect the Craft Yarn Council standard, which is the most widely adopted reference in the US. However, several important limitations apply. First, manufacturer variance is real: hooks labeled as the same size by different companies can have actual diameters that differ by up to 0.25 mm. Clover, Boye, Susan Bates, and Furls hooks of the same nominal US size may not be interchangeable without a gauge swatch. Second, some metric sizes (2.0, 2.5, 3.0, 7.0, 12.0 mm) have no standard US letter-number equivalent and are indicated with a dash in conversion charts. Third, the UK old-number system has gaps and variations across older publications — not every UK number has a universally agreed-upon mm equivalent. Fourth, Tunisian and Afghan hooks use the same size designations as standard hooks but are physically longer; no separate conversion is needed, but the tool selection differs. Always work a gauge swatch before beginning any project where fit or final dimensions matter.

Crochet Hook Size Formulas & Conversions

Metric to US Letter/Number

US Size = Lookup Table (mm → Letter/Number)

Crochet hook sizes do not follow a linear formula. Each US letter/number designation maps to a specific millimeter diameter via an industry-standard lookup table maintained by the Craft Yarn Council. For example, B/1 = 2.25 mm, G/6 = 4.0 mm, K/10½ = 6.5 mm.

UK Old Number to Metric

UK Size = Inverse Scale (larger UK number → smaller mm)

The UK/Canadian old-number system runs inversely: UK 14 = 2.0 mm (smallest common size) and UK 000 = 10.0 mm (largest). There is no mathematical formula — sizes are mapped through a historical lookup table based on wire gauge standards.

Gauge Relationship to Hook Size

Gauge (stitches per inch) ∝ 1 / Hook Diameter (mm)

Larger hooks produce fewer stitches per inch (looser fabric), while smaller hooks produce more stitches per inch (denser fabric). The exact gauge depends on yarn weight, fiber content, stitch type, and individual crocheter tension, but hook diameter is the primary variable.

Steel Hook Numbering (Reverse)

Steel #14 = 0.75 mm, Steel #00 = 3.50 mm

Steel crochet hooks use an inverted numbering system compared to standard hooks. Higher steel numbers indicate smaller hook diameters. Steel #14 is the smallest at 0.75 mm, and steel #00 is the largest at 3.50 mm. Used exclusively for thread and lace crochet.

Crochet Hook Size Reference Charts

Standard Crochet Hook Size Conversion Chart

Complete conversion table for standard (non-steel) crochet hooks showing US letter/number, metric millimeter, and UK/Canadian old-number equivalents. Sizes range from the smallest standard hook (B/1 at 2.25 mm) to the largest commonly available (S at 19 mm).

US Size	Metric (mm)	UK/Canadian	Recommended Yarn Weight
B/1	2.25	13	#1 Super Fine
C/2	2.75	12	#1 Super Fine
D/3	3.25	10	#2 Fine
E/4	3.50	9	#2 Fine
F/5	3.75	—	#2 Fine
G/6	4.00	8	#3 Light
7	4.50	7	#3 Light
H/8	5.00	6	#3 Light
I/9	5.50	5	#4 Medium
J/10	6.00	4	#4 Medium
K/10½	6.50	3	#5 Bulky
L/11	8.00	0	#5 Bulky
M/13	9.00	00	#6 Super Bulky
N/15	10.00	000	#6 Super Bulky
P/16	11.50	—	#6 Super Bulky
Q	15.75	—	#7 Jumbo
S	19.00	—	#7 Jumbo

Worked Examples

Convert US Size H/8 to Metric

You have a US pattern that calls for a size H/8 crochet hook and need to know the metric equivalent to purchase a hook labeled in millimeters.

Look up US size H/8 in the standard conversion table.

Find the corresponding metric value: H/8 = 5.00 mm.

The UK/Canadian equivalent is size 6.

The recommended yarn weight is #3 Light (DK/Light Worsted).

A US H/8 crochet hook is 5.00 mm in diameter, equivalent to UK/Canadian size 6, and is recommended for Light (#3) weight yarn.

Find UK Equivalent of a 5 mm Hook

You have a 5.00 mm crochet hook and are following a vintage UK pattern that specifies hook sizes using the old UK/Canadian numbering system.

Enter 5.00 mm in the metric input field.

The converter looks up the exact match in the conversion table.

5.00 mm corresponds to US H/8 and UK/Canadian size 6.

Verify the yarn weight recommendation matches the pattern's specified yarn: #3 Light.

A 5.00 mm hook is UK/Canadian size 6. If your vintage pattern calls for a UK size 6 hook, your 5.00 mm hook is the correct tool.

Find Hook Size for Medium (#4) Worsted Yarn

You have a skein of worsted weight yarn labeled as #4 Medium and need to know which crochet hook sizes are recommended.

Use the 'Find Hook by Yarn Weight' feature and select Medium (#4).

The recommended hook range for #4 Medium yarn is 5.50 mm to 6.00 mm.

In US sizes, this corresponds to I/9 (5.50 mm) through J/10 (6.00 mm).

In UK sizes, this is size 5 (5.50 mm) through size 4 (6.00 mm).

For Medium (#4) worsted weight yarn, use a hook between 5.50 mm (US I/9, UK 5) and 6.00 mm (US J/10, UK 4). Always swatch to verify gauge.

How to Use This Converter

Choose Your Hook Type

Select 'Standard' for regular aluminum, bamboo, or plastic hooks used with yarn. Select 'Steel (Thread)' if you are working with crochet thread or lace-weight thread and need to look up a steel hook. Steel hooks have a separate size range and use reverse numbering logic.

Select Your Known System

Choose the sizing system printed on your hook or stated in your pattern. Use 'Metric (mm)' if your hook has a millimeter diameter stamped on it. Use 'US Letter/Number' for American patterns (B-1, G-6, H-8, etc.). Use 'UK/Canadian' when working from British or Canadian vintage patterns that use the old number system.

Pick Your Size and Read the Conversions

Select the specific hook size from the dropdown. The converter instantly displays the equivalent size in all three systems, the recommended yarn weight category, and the equivalent knitting needle size. A size comparison bar chart shows where your selected hook falls relative to other common sizes.

Use 'Find Hook by Yarn Weight' for Reverse Lookup

If you know the yarn weight but not the hook size, use the 'Find Hook by Yarn Weight' section. Select a yarn weight category (Lace, Super Fine, Medium, Bulky, etc.) and the tool shows the full recommended hook range in all three systems — useful when you only have a yarn label with a weight category and no pattern.

Frequently Asked Questions

Why does my hook say 'G-6' and '4.0 mm' — which measurement should I trust?

Always trust the millimeter measurement. The Craft Yarn Council, Vogue Knitting, and other industry authorities all recommend relying on metric sizing because letter-number designations vary between manufacturers. A 'G-6' hook from one brand may measure 4.0 mm, while another brand's G-6 measures 4.25 mm. The millimeter diameter is a physical measurement that is independent of any manufacturer's labeling convention. When following a pattern, use the stated mm size as your guide and verify with a gauge swatch. If your hook's markings have worn off, a needle gauge tool — a flat plate with labeled holes — will tell you the actual millimeter diameter.

What is the difference between standard and steel crochet hooks?

Standard hooks (also called aluminum, bamboo, or plastic hooks) are used with yarn — from fingering weight all the way up to jumbo chunky yarn — and range from about 2.0 mm to 25 mm or larger. Steel crochet hooks are much finer, ranging from 0.75 mm (steel #14, the smallest) to 3.5 mm (steel #00, the largest), and are designed for use with crochet thread to create fine lace, doilies, and appliqué. The key difference besides diameter is the numbering direction: standard US hook numbers increase as the hook gets larger, but steel hook numbers are inverted — a steel #14 is smaller than a steel #1. This reversal is a common source of confusion for beginners moving from thread crochet to yarn work or vice versa.

Why do some metric sizes have no US equivalent?

The US letter-number system was not designed to cover every possible millimeter diameter — it evolved incrementally as manufacturers settled on common sizes. As a result, several metric sizes (including 2.0 mm, 2.5 mm, 3.0 mm, 7.0 mm, and 12.0 mm) have no standard US letter-number equivalent. If a pattern specifies one of these sizes, it will typically list only the metric measurement. When purchasing hooks, check the mm label rather than the US size label for these intermediate sizes. Our converter marks these gaps with a dash in the US column and, when you enter one of these measurements, returns the nearest standard sizes above and below with a clear note.

I have a vintage UK pattern that calls for a size 8 hook — what size is that in mm?

A UK size 8 in the old UK/Canadian system corresponds to 4.0 mm (US G-6). This is one of the most commonly encountered UK sizes in vintage patterns from the mid-twentieth century. The UK old-number system runs roughly inverse to diameter at the lower end — UK size 14 is only 2.0 mm, while UK size 3 is 6.5 mm. Note that the system has some gaps: there is no standard UK equivalent for several metric sizes like 3.75 mm (US F-5), and the old system stops being documented for sizes larger than UK 000 (10.0 mm). For large-hook or super-bulky projects in vintage UK books, the millimeter measurement is usually provided alongside the number. Use our UK column in the converter to look up any size from UK 14 down to UK 000.

How do I know which hook size to use if I only have a yarn label?

Most commercially sold yarn labels include a recommended hook or needle size, usually printed as a small crochet hook icon with a size range. This range is the manufacturer's starting recommendation for typical medium-tension crochet. However, the recommended size is a guideline, not a rule. If your swatch shows too many stitches per inch (fabric is too dense), go up one hook size. If your swatch shows too few stitches per inch (fabric is too loose), go down one hook size. Our 'Find Hook by Yarn Weight' feature provides the full standard range for each yarn weight category, so you can see all available sizes in mm, US, and UK formats. Always work at least a 4×4 inch swatch and compare to your pattern's stated gauge before committing to a full project.

Can I use a crochet hook and a knitting needle of the same mm size interchangeably for yarn weight purposes?

Yes — for the purpose of selecting yarn weight and estimating gauge, a crochet hook and a knitting needle of the same millimeter diameter are equivalent. A 5.0 mm crochet hook (US H-8) and a US size 8 knitting needle both have a 5.0 mm shaft diameter and are both recommended for light to medium weight yarn. This cross-compatibility is useful when reading knitting patterns to understand yarn weight, or when shopping for hooks in a store that carries both. The actual gauge you achieve will still differ between knitting and crochet because the stitch structures are different — knitting stitches are generally taller and more open than single crochet stitches. But for yarn weight categorization and hook selection, the millimeter diameter is the shared reference point across both crafts.