Screw gauge size converter
Turn a US machine-screw number into the diameter a drawing or catalog quotes: a #6 screw is 0.138 in, 3.5052 mm or 138 mils across, and a measured diameter converts back to its number. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Value
- 6
- Source unit: us
- us
- From unit
- us
- Us screw
- 6
- Us screw label
- #6
- Diameter in
- 0.138
A screw gauge size converter turns a US machine-screw number into the major diameters fabricators, CAD notes, and hardware catalogs actually quote. Enter a Unified numbered size such as six with from equal to us, or enter a measured major diameter in millimetres, inches, mils, or centimetres; the tool returns the continuous screw number, a readable hash label, and diameter in inches, millimetres, centimetres, and mils using the exact relation diameter in inches equals zero point zero six zero plus zero point zero one three times n. Numbered sizes from zero through sixteen cover the everyday machine-screw series before fractional inch stock takes over product sheets.
What US screw numbers mean and how major diameter is defined
North American machine screws in the Unified series use a small integer gauge, written with a hash prefix such as number six or number ten, before the thread designation UNC or UNF. That gauge is not an arbitrary catalog label: for the standard numbered sizes the major diameter of the external thread follows the closed linear formula diameter in inches equals zero point zero six zero plus zero point zero one three times the screw number n.
Substituting n equals zero yields zero point zero six zero inches, n equals six yields zero point one three eight inches, and n equals ten yields zero point one nine zero inches—the same rows that appear on countless fastener charts and drawing notes. Because the relation is linear and continuous, half sizes and reverse conversion from a measured diameter are well-defined even when a stock list only prints integers.
This tool reports major diameter only; it does not encode threads per inch, pitch diameter, minor diameter, head style, drive type, or material strength. Metric coarse labels such as M3 or M4 are a different naming system and are out of scope here—convert those with a metric screw chart after you know the millimetre major diameter.
Wood-screw tables sometimes list slightly different outer diameters for the same number; treat the results as the Unified machine-screw major diameter reference used in mechanical design, then confirm specialty wood or sheet-metal product sheets when your application is not a machine screw.
Practical tips, limits, and related searches
Keep numbered sizes inside zero through sixteen so results stay on the chart band this landing advertises; larger fasteners are usually specified as fractional inch major diameters such as one quarter inch rather than a hash number. When you only need wire geometry for electrical conductors, use the separate American Wire Gauge converter—screw numbers and AWG share the word gauge but are unrelated sizing systems. Clearance-hole and tap-drill tables need pitch and class of fit in addition to major diameter; this tool only answers the size-to-diameter question.
This landing targets people searching for a screw gauge size converter, US screw number to millimetres, machine screw diameter in inches, or hash six screw size online; it is not a torque calculator, not a metric M-series encoder, and not a wood-screw sales chart.
What you can do with it
Drawing notes and BOM exports
Map hash-numbered machine screws on US drawings to millimetre major diameter for mixed SI/imperial bills of materials and CNC notes.
Hardware kitting and receiving
Convert a caliper reading in mm or mils back to the nearest continuous US screw number before matching bins and purchase orders.
FAQ
What formula converts a US screw number to diameter?
Multiply by 25.4 for millimetres or by 1000 for mils.
Which units are supported?
us (screw number), mm, in, mils, and cm.
What range of screw numbers is allowed?
US numbers from 0 through 16 inclusive. Diameters that invert outside that band are also rejected.
Does this include thread pitch or TPI?
No. It only converts major diameter and the continuous screw number. Pitch, UNC/UNF series, and tap drills need a dedicated thread table.
Is the screw gauge converter free?
The browser tool is free and runs locally.
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/screw-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"value":6,"from":"us"}'const res = await fetch("https://api.kit.forhosting.com/conv/screw-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"value": 6,
"from": "us"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/screw-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"value": 6,
"from": "us"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/screw-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"value":6,"from":"us"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"value":6,"from":"us"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/screw-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": 6,
"from": "us"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.screw_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.
Limits
screw_min | 0 |
screw_max | 16 |
max_abs | 1000000 |
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
Unknown unit names and sizes outside the published band are rejected with a clear invalid_input error so automation never invents a phantom diameter.
How to call the API and what comes back
When from is mm, in, mils, or cm (with common spellings such as millimetre, inch, thou, and centimetre), value is a positive major diameter in that unit and the continuous screw number is recovered by inverting the same formula. Absolute magnitudes must stay within the published limit of one million. Send a JSON body with two required fields: value, a finite number, and from, a unit string. When from is us (aliases include gauge, screw number, hash, and machine screw), value is the screw number n between zero and sixteen inclusive. On success the response echoes rounded value and from, sets us_screw and us_screw_label for the gauge, diameter_in, diameter_mm, diameter_cm, and diameter_mils for linear size, screw_min and screw_max for the published band, formula to the diameter and inverse expressions, message to a one-line English summary, steps to an ordered array of substitution lines, and supported_units to the five canonical unit ids. Failed validation—missing fields, non-finite numbers, non-positive diameters, sizes outside the zero-to-sixteen band, or an unknown unit string—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and related searches
Prefer sending from as one of the five canonical tokens us, mm, in, mils, or cm when you control the client; free-form aliases exist for human forms but automation should stay on the short ids. Round-trip checks are easy: convert number six from us, read diameter_mm, then call again with that millimetre value and from equal to mm and confirm us_screw returns six within the published rounding. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when you deliberately probe unknown units or range edges.
Integration use cases
Unit tests for fastener libraries
Generate known-good #0/#6/#10 diameter fixtures plus deliberate unknown-unit and out-of-range rejections for catalog converters.