Knitting needle size converter
Match knitting needle sizes across millimeter, US, old UK and Japanese labels: US 8 is 5 mm, the same needle as UK 6. When no label shares the exact diameter, you get the nearest size, clearly marked. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Value
- 8
- From
- us
- To
- mm
- Value
- 5
- Input
- 8
- Mm
- 5
- Us
- 8
Knitting patterns and yarn labels rarely agree on one needle notation. A sweater chart printed in the United States may call for US size 8 while a British vintage leaflet lists UK 6 and a metric pattern simply says 5 mm. Buying the wrong diameter changes gauge, fabric drape, and finished measurements, so knitters, shop staff, and pattern publishers need a reliable map between millimetres, US sizes, old UK numbered sizes, and Japanese sizes. This knitting needle size converter accepts a size value plus source and target systems, converts through diameter in millimetres using fixed commercial ladders, and returns the matching size (or the nearest discrete size when systems step differently).
How knitting needle systems relate through millimetres
Modern needles are manufactured to a physical diameter in millimetres. Metric patterns already speak that language: 4 mm is four millimetres across the shaft, full stop. US sizes are labels layered on top of those diameters—US 6 is 4 mm, US 8 is 5 mm, US 10 is 6 mm—using a ladder that is conventional rather than a single arithmetic formula.
Old UK sizes run the other way: larger UK numbers are thinner needles, so UK 8 is 4 mm while UK 0 is 8 mm, with double-zero and triple-zero sizes for still thicker shafts. Japanese sizes step in regular 0.3 mm increments from JP 0 at 2.1 mm upward. Because the three labelling systems do not share the same step sizes, an exact round-trip is only guaranteed when both labels share the same millimetre diameter on the tables.
This converter therefore always goes source → millimetres → target. When you convert into a discrete system from millimetres, an exact table hit is preferred; if no label shares that diameter, the nearest size by absolute millimetre distance is returned and the exact flag is false so callers can decide whether to warn the knitter.
Where a needle size converter helps
Independent yarn shops use a knitting needle size converter at the counter so staff can answer “what is UK 10 in millimetres?” without flipping a paper chart while the customer waits. Online pattern houses normalize mixed US and metric needle callouts before generating print PDFs or translation packs for eight markets.
Inventory tools map supplier SKUs that use Japanese sizes onto a single millimetre field in the warehouse database. Automation scripts that score pattern difficulty can reject impossible needle jumps once sizes share a common millimetre scale. Keep the ladders pinned in tests, treat exact false as a soft nearest match, and you get reproducible conversions for shop kiosks, pattern pipelines, and edge workers without maintaining a separate spreadsheet per locale.
What you can do with it
Yarn shop counter conversions
Convert a customer’s US or UK needle request into millimetres instantly so staff can pull the correct metric-marked set from the wall.
Pattern localization pipelines
Normalize mixed needle notations in imported patterns to a single mm field, then emit US, UK, or JP labels for each market edition.
Inventory and SKU mapping
Map supplier Japanese or old UK size codes onto millimetre diameters for unified stock, reorders, and product pages.
FAQ
Which knitting needle systems does the converter support?
Millimetres (metric diameter), US sizes, old UK numbered sizes including 00 and 000, and Japanese sizes.
How do I convert US size 8 to millimetres?
Send value 8, from us, to mm. The result is 5 mm (exact), with a UK crosswalk of size 6 on the same diameter.
What if my millimetre value is not on the US ladder?
The converter returns the nearest US size by absolute millimetre distance and sets exact to false so you can show a nearest-match warning.
What happens with an unknown unit or size?
Unknown unit names and discrete sizes missing from the source ladder are rejected and not billed.
Why do UK needle numbers get smaller as needles get thicker?
Old UK sizes run the opposite way to US and metric sizes: a higher number means a thinner needle. UK 8 is 4 mm, UK 0 is 8 mm, and sizes 00 and 000 are thicker still.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/conv/knitting-needle-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"value":8,"from":"us","to":"mm"}'const res = await fetch("https://api.kit.forhosting.com/conv/knitting-needle-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"value": 8,
"from": "us",
"to": "mm"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/knitting-needle-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"value": 8,
"from": "us",
"to": "mm"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/knitting-needle-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"value":8,"from":"us","to":"mm"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"value":8,"from":"us","to":"mm"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/knitting-needle-size", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"value": 8,
"from": "us",
"to": "mm"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.knitting_needle_size",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
The same pure logic powers the free browser tool and the API path, so a yarn-store kiosk and a catalog script never disagree on what US 8 means in millimetres.
How knitting needle systems relate through millimetres
When you convert from a discrete system (US, UK, or JP), the size must appear on that ladder or the call fails with invalid_input instead of inventing a diameter.
Input contract, response fields, and error behaviour
Example: value 8, from us, to mm yields value 5, mm 5, exact true, uk 6, and a formula stating that US 8 equals 5 mm. POST a JSON body with three fields. value is the size or diameter to convert: a positive finite number for millimetres, or a known ladder size for US, UK, and Japanese systems. UK double-zero and triple-zero sizes may be sent as the strings 00 and 000. from and to name the source and target systems; canonical ids are mm, us, uk, and jp, with aliases such as metric, millimeter, usa, american, gb, british, old_uk, japanese, and japan. Missing fields, non-finite numbers, unknown unit names, non-positive millimetre diameters, and sizes that do not appear on the source ladder all fail with invalid_input and are not billed. On success the body echoes input (the resolved source size), from and to in canonical form, value as the primary result in the target system, mm as the diameter used for the conversion, exact as a boolean for whether the target label matches that diameter exactly, plus us, uk, and jp crosswalk sizes for the same diameter, and a short formula string for display. Pair the response with your own gauge calculator if you also need cast-on math; this endpoint only converts needle labels and diameters.
Technical questions
What does the knitting needle size converter cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.