Crochet Hook Size Converter: US, UK & Metric Guide

You find a pattern that calls for a 4.0 mm hook, but your hooks are labelled with US letters. Or the pattern says “size 8” and you are not sure which system it means. Crochet hook sizing has no single global standard. US patterns use letters and numbers (B/1, G/6, H/8), UK patterns use a numbered system that counts down as hooks get bigger, and most modern hooks and patterns also print the diameter in millimetres. This crochet hook size converter shows the matching size in all three systems, plus a general yarn weight label.

How the Crochet Hook Size Converter Works

This converter does not use an arithmetic formula. It is a lookup. Each hook size is stored as one row in a chart, with the metric size as the key. The other columns hold the US label, the UK number and a general yarn weight label. Picking a size in any system finds that row and shows the rest.

1. Choose the system you know. Pick Metric, US or UK, then select your size from the list. Example: your pattern says US H/8.
2. Find the matching row. The tool finds the chart row for that size. H/8 sits on the 5.0 mm row. The lists only offer sizes that are in the chart, so you normally get an exact match. If a value ever falls between rows, the nearest metric size is shown and flagged.
3. Read across the row. The row for 5.0 mm gives US H/8, UK 6 and a worsted yarn label. There is no sum to do, so no rounding is involved. A dash means the chart has no entry in that system for that size.
4. Use the result. Buy or pick the hook by its millimetre size, then crochet a gauge swatch with your yarn. If the swatch is off, move up or down a size.

3 Real-Life Examples

US vs UK vs Metric Hook Sizes Explained

Three sizing systems are in everyday use:

• Metric (mm): The diameter of the hook shaft in millimetres. It is the most precise of the three and the easiest to compare across brands.
• US sizes: Letters and numbers such as B/1, C/2, D/3, E/4, F/5, G/6, H/8, I/9, J/10, K/10.5, L/11, M/N-13 and N/P-15. They generally rise as the hook gets larger.
• UK sizes: A numbered system running from 14 (fine) down to 000 (very large). The numbers fall as the hook gets larger, the opposite of US numbers. Older UK and Canadian patterns often use these numbers, while many newer hooks print only the mm size.

Why Hook Sizes Differ by Country

The systems grew up separately in different markets, and no one system replaced the others. Metric labelling became the common language because it states the actual shaft diameter. Many hooks are now stamped in mm, often with the US size printed beside it.

In practice, check the mm size when you buy a hook, not just the US or UK label. Charts from different sources do not always agree, especially for the large sizes. For example, the Craft Yarn Council lists P-16 at 11.5 mm, while this chart lists O/P-16 at 12 mm. A measured diameter settles the question.

Hook Size and Yarn Weight Pairing Guide

The table below follows the Craft Yarn Council Standard Yarn Weight System. These are typical ranges, not rules.

The yarn weight label in the converter is a simpler, general guide and can sit about one band away from the official ranges above. For example, the converter labels 5.0 mm as Worsted, while the Craft Yarn Council table puts 5.0 mm in the Light (DK) band. Your yarn label and your gauge swatch are the better guide.

How Hook Size Affects Your Crochet Project

Hook size and yarn weight are the two biggest choices that set your fabric. A hook that is too small gives a tight, stiff fabric and a smaller finished piece. A hook that is too large gives an open, floppy fabric that may not hold its shape. That is why patterns state a hook size alongside a gauge, the number of stitches and rows in a set width and height.

For garments and fitted items, crochet a gauge swatch before you start. If your swatch comes out too small, try a larger hook. If it comes out too big, try a smaller one. The pattern’s hook size is a starting point, because people crochet at different tensions.

Choosing the Right Hook for Your Project

Common Mistakes with Crochet Hook Sizes

• Assuming US and UK numbers match: They do not. A pattern that says “size 10” could mean 6.0 mm (US J/10) or 3.25 mm (UK 10). Check which system the pattern uses.
• Skipping the gauge swatch: Starting a garment without checking gauge is a common reason for a poor fit.
• Using one hook for every yarn: A 5 mm hook suits many worsted yarns but gives a very loose fabric with fingering yarn. Match the hook to the yarn.
• Trusting the label alone: Brands can mark the same letter with slightly different mm values. If you are unsure, check the hook with a hook gauge tool.

Important Notes

• The chart holds standard sizes only. It has no half sizes or in-between values, and it starts at 2.0 mm, so very fine steel thread hooks are not covered.
• Some sizes have no entry in one system. In this chart 2.0, 2.5, 3.0 and 7.0 mm have no US label, and 2.75, 3.5, 12, 15, 16, 19 and 25 mm have no UK number.
• Charts from different sources can disagree, mainly for large hooks. Treat the millimetre size as the reference and check the stamp on your hook.
• The yarn weight label is a general guide and can differ from official yarn weight ranges. Use your yarn label and a gauge swatch to confirm.
• The tool converts sizes only. It cannot predict your gauge, stitch pattern results or yarn amounts.

References and Further Reading

• Craft Yarn Council: Standard Yarn Weight System, with hook ranges for each yarn weight category.
• Craft Yarn Council: Crochet hook sizes, a metric to US hook size list.
• Craft Yarn Council: Steel crochet hook and crochet thread sizes, for very fine thread work.

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